Electrical Energy

An electrically powered ventilator uses voltage from an electrical line outlet. In the United States, this line voltage is normally 110 to 115 V alternating current (AC) (60 Hz). In addition to powering the ventilator, this AC voltage may be reduced and converted to direct current (DC). This DC source can be used to power delicate electronic control circuits. Some ventilators, notably transport ventilators, have rechargeable batteries to be used as a source of power if AC is unavailable. In the home care setting, battery backup for electrically powered ventilators is an essential lifesaving feature in the event of a power outage.

Pneumatic Power

A pneumatically powered ventilator uses compressed gas as its power source. Most modern intensive care unit (ICU) ventilators are pneumatically powered. Ventilators powered by compressed gas usually have internal pressure-reducing valves so that the normal operating pressure is lower than the source pressure. Ventilators can operate without interruption from hospital-piped gas sources, which are usually regulated to 50 psi (pounds per square inch) but are subject to periodic fluctuations.

Most pneumatically powered ICU ventilators still require electrical power to support their control functions (see the following section on control mechanisms). However, a few pneumatically powered ventilators can function without electrical power. These devices are ideal in situations where electrical power is unavailable (e.g., during certain types of patient transport) or as a backup to electrically powered ventilators in case of power failures. They are also particularly useful where electrical power is undesirable, such as near magnetic resonance imaging (MRI) equipment.

Drive Mechanism

The drive mechanism of the ventilator converts the input power to useful work. The characteristic flow and pressure patterns the ventilator produces are determined in part by the type of drive mechanism it contains. Drive mechanisms can be either (1) a direct application of compressed gas via a pressure-reducing valve or (2) an indirect application via an electrical motor or compressor. Descriptions of these devices are provided in textbooks devoted to respiratory care equipment.⁶

Output Control Valve

The output control valve regulates the flow of gas to the patient. It may be a simple on/off exhalation valve, as in the Newport E100i (Newport NMI Ventilators; Newport Medical Instruments, Newport Beach, CA). Alternatively, the output control valve can shape the output waveform, as in the Maquet SERVO-i (Maquet, Bridgewater, NJ). Commonly used output control valves include the pneumatic diaphragm, electromagnetic poppet/plunger valve, and proportional valve.⁶

Control Circuit

To manipulate pressure, volume, and flow, a ventilator must have a control circuit. A control circuit is a system of components that measures and directs the output of the ventilator to replace or assist the breathing efforts of the patient. A ventilator control circuit may include mechanical, pneumatic, electrical, electronic, or fluidic components. Most modern ventilators combine two or more of these subsystems to provide user control.

Mechanical control circuits use devices such as levers, pulleys, and cams. These types of circuits were used in the early manually operated ventilators illustrated in history books.⁸ Pneumatic control is provided using gas-powered pressure regulators, needle valves, jet entrainment devices, and balloon-valves. Some transport ventilators use pneumatic control systems.

Electrical control circuits use only simple switches, rheostats (or potentiometers), and magnets to control ventilator operation. Electronic control circuits use devices such as resistors, capacitors, diodes, and transistors and combinations of these components in the form of integrated circuits. The most sophisticated electronic systems use preprogrammed microprocessors to control ventilator function.

Fluidic logic-controlled ventilators, such as the Bio-Med MVP-10 (Bio-Med Devices, Stanford, Connecticut) and Sechrist IV-100B (Sechrist, Anaheim, California), also use pressurized gas to regulate the parameters of ventilation. However, instead of simple pressurized valves and timers, these ventilators use fluidic logic circuits that function similar to electrical circuit boards.⁹ Fluidic control mechanisms have no moving parts. In addition, fluidic circuits are immune to failure from surrounding electromagnetic interference, as can occur around MRI equipment.

Control Variables

A control variable is the primary variable that the ventilator manipulates to cause inspiration. There are only three explicit variables in the equation of motion that a ventilator can control: pressure, volume, and flow. Because only one of these variables can be the independent variable, the others are dependent variables. In other words, only one variable can be directly controlled at a time, and a ventilator must function as a pressure, volume, or flow controller. Time is implicit in the equation of motion and in some cases serves as a control variable. Figure 42-4 illustrates the criteria for determining what variable the ventilator controls at any given time.

Figure 42-5 illustrates the important variables for volume-controlled modes. It shows that the primary variable we wish to control is the patient’s minute ventilation. A particular ventilator may allow the operator to set minute ventilation directly. More frequently, minute ventilation is adjusted by means of a set tidal volume and frequency. Tidal volume is a function of the set inspiratory flow and the set inspiratory time. Inspiratory time is affected by the set frequency and, if applicable, the set inspiratory-to-expiratory (I : E) ratio. The mathematical relationships among all these variables are shown in Table 42-1.

TABLE 42-1

Equations Relating the Important Parameters for Volume-Controlled and Pressure-Controlled Ventilation

Mode	Parameter	Symbol	Equation
Volume-controlled	Tidal volume (L)	VT
	Mean inspiratory flow (L/min)
Pressure-controlled	Tidal volume (L)	VT	VT = ΔP × C × (1 − e−t/τ)
	Instantaneous inspiratory flow (L/min)
	Pressure gradient (cm H2O)	ΔP	ΔP = PIP − PEEP
Both modes	Exhaled minute ventilation (L/min)
	Total cycle time or ventilatory period (seconds)	TCT	TCT = TI + TE = 60 ÷ f
	I : E ratio	I : E
	Time constant (seconds)	τ	τ = R × C
	Resistance (cm H2O/L/sec)	R
	Compliance (L/cm H2O)	C
	Elastance	E
	Mean airway pressure (cm H2O)
Primary variables	Pressure (cm H2O)	P
	Volume (L)	V
	Flow (cm H2O/L/sec)
	Time (sec)	τ
	Inspiratory time (sec)	TI
	Expiratory time (sec)	TE
	Frequency (breaths/min)	f
	Base of natural logarithm (≈2.72)	e

With pressure-controlled modes, the goal is also to maintain adequate minute ventilation. However (as the equation of motion shows), when pressure is controlled, tidal volume and minute ventilation are determined not only by the ventilator’s pressure settings but also by the elastance and resistance of the patient’s respiratory system. This additional variable makes minute ventilation (and gas exchange) less stable in pressure-controlled modes than in volume-controlled modes. Figure 42-6 shows the important variables for pressure-controlled modes. Tidal volume is not set on the ventilator. It is the result of the pressure settings and the patient’s lung mechanics and the inspiratory time. On some ventilators, the speed with which the PIP is achieved (i.e., the pressure rise time) is adjustable. That adjustment affects the shape of the pressure waveform and the mean airway pressure. Mean airway pressure is important because, within reasonable limits, as the mean airway pressure increases, arterial O₂ tension increases. Mean airway pressure is higher for pressure-controlled modes than for volume-controlled modes (at the same tidal volume) owing to the differences in the shapes of the airway pressure waveforms.

Phase Variables

A complete ventilatory cycle or breath consists of four phases: (1) the initiation of inspiration, (2) inspiration itself, (3) the end of inspiration, and (4) expiration. To understand a breath cycle, the clinician must know how the ventilator starts, sustains, and stops inspiration and must know what occurs between breaths.

The phase variable is a variable that is measured and used by the ventilator to initiate some phase of the breath cycle. The variable causing a breath to begin is the trigger variable. The variable limiting the magnitude of any parameter during inspiration is the target variable. The variable causing a breath to end is the cycle variable. To describe what happens during expiration, the baseline variable that is in effect must be known. Figure 42-7 shows the criteria for determining phase variables.

Trigger Variable

On most modern ventilators, either the machine or the patient can initiate a breath. If the machine initiates the breath, the trigger variable is time. If the patient initiates the breath, pressure, flow, or volume may serve as the trigger variable. Manual or operator-initiated triggering is also available on most ventilators.

Other Trigger Variables

Other variables can be used to trigger inspiration, such as a decrease in total minute ventilation below a preset threshold (e.g., mandatory minute ventilation mode on the Draeger Evita XL ventilator) or the electrical signal generated by diaphragmatic contraction (e.g., neurally adjusted ventilatory assist mode on the Maquet SERVO-i ventilator).

Mini Clini

Calculate the Expiratory Time Setting from the Frequency and Inspiratory Time

 Problem

The RT is assigned to the neonatal ICU and has successfully intubated a newborn term infant diagnosed with respiratory distress syndrome. The RT is manually ventilating the infant with a flow-inflating bag at a frequency of 25 breaths/min. An inline manometer indicates PIP is approximately 24 cm H₂O, and PEEP is 4 cm H₂O. The infant’s vital signs (heart rate, respiratory rate, and blood pressure) are within normal limits, and SpO₂ is 95%. The pressure and frequency the RT is using manually will serve as the initial ventilator settings on a Sechrist IV100B infant ventilator. With this particular ventilator, however, setting the inspiratory and expiratory times determines the frequency. The attending physician has requested an inspiratory time (T_I) of 0.7 second.

Calculate the expiratory time (T_E) necessary to give the desired frequency.

Discussion

1. Compute the total cycle time (TCT) using the frequency (f):

TCT=1f=TI+TE

f=25breathsminute×1minute60seconds=25breaths60seconds

TCT=125breaths/60seconds=60seconds25breaths=2.4secondsbreath

2. Compute the expiratory time:

TE=TCT−TI=2.4−0.7=1.7seconds

3. Check the digital display to verify the set frequency remains at 25 breaths/min.

Target Variable

A target variable is one that can reach and maintain a preset level before inspiration ends but does not terminate inspiration. Pressure, flow, or volume can serve as a target variable.

Clinicians often confuse target variables with cycle variables. A cycle variable always ends inspiration. A target variable does not terminate inspiration—it sets an upper bound only for pressure, volume, or flow. Figure 42-8 illustrates the importance of distinguishing between target and cycle variables.

Cycle Variable

The inspiratory phase always ends when some variable reaches a preset value. The variable that is measured and used to end inspiration is called the cycle variable. The cycle variable can be pressure, volume, flow, or time. Manual cycling is also available on some modern ventilators.

Time Cycling

Time cycling means that expiratory flow starts because a preset time interval has elapsed. There are several time intervals of interest during inspiration. One is the inspiratory flow time. As the name implies, this is the time during which inspiratory flow is delivered to the patient. Another interval is the inspiratory hold time, during which inspiratory flow has ceased but expiratory flow is not yet allowed. The sum of these two intervals is the inspiratory time. Time cycling occurs when the inspiratory time has elapsed. An inspiratory hold time may not be used. If it is used, it may be set directly, or it may occur indirectly if the set inspiratory time is longer than the inspiratory flow time (determined by the set tidal volume and flow; time = volume/flow).

Mini Clini

Calculate Inspiratory Hold Time Given Set Inspiratory Time, Tidal Volume, and Flow When Using the Siemens Servo

 Problem

The RT is performing a ventilator check on an ICU patient with a blunt chest trauma. The ventilator is set to volume-controlled IMV on a Draeger Evita 4. The physician wants the minimum mean airway pressure for the given level of ventilation to preserve the patient’s already low cardiac output. The physician asks the RT to ensure the night shift therapist removed the inspiratory hold the patient had been on.

Determine from the ventilator settings alone whether there is an inspiratory hold, and if so, make the appropriate changes to eliminate it. Ventilator settings are:

Tidal volume: 500 ml = 0.5 L

Inspiratory flow: 60 L/min = 1 L/sec

Inspiratory time: 0.8 second

Frequency: 10 breaths/min

Discussion

1. Calculate the inspiratory flow time (TIF) using appropriate unit conversions:

TIF=tidalvolumeinspiratoryflow=500ml60L/minute×1L1000ml×60seconds1minute=0.5L1L/second=0.5second

2. Compare the set inspiratory time (0.8 second) with the flow time resulting from the tidal volume and flow settings (0.5 second). Inspiratory time lasts longer than inspiratory flow. Because inspiration is time cycled, this means that there is an inspiratory hold of duration equal to 0.8 − 0.5 = 0.3 second.

3. You could eliminate the inspiratory hold either by decreasing the inspiratory flow or by decreasing the inspiratory time. The goal is to minimize the mean inspiratory pressure. You choose to decrease inspiratory time for two reasons: (1) It decreases the I : E ratio and may allow more time for spontaneous breaths to occur, lowering mean intrathoracic pressure; (2) decreasing inspiratory flow may make tidal volume delivery slower than the patient demands, decreasing patient-ventilator synchrony.

Patient versus Machine Triggering and Cycling

Trigger and cycle signals can be grouped into two categories: patient-generated and machine-generated. The determination of whether the patient or the machine generated the signal is based on the equation of motion. However, this time the equation is expressed in another way:

Pinsp=PE+PR Equation 42-5

Equation 42-5

where P_insp reflects the pressure required for inspiration, P_E is the pressure resisting inspiration by the elastance of the respiratory system, and P_R is the pressure resisting the inspiration by the resistance of the respiratory system. This equation can be expanded as follows:

Pvent+Pmus=(PE,vent+PR,vent)+(PR,mus+PR,mus) Equation 42-6

Equation 42-6

where P_vent is the pressure generated by the ventilator, independent of the patient, P_mus is the pressure generated by the patient’s ventilatory muscles independent of the ventilator, P_E,vent is the elastic load supported by the ventilator, P_E,mus is the elastic load supported by the patient’s muscles, P_R,vent is the resistive load supported by the ventilator, and P_R,mus is the resistive load supported by the patient’s muscles. In this form of the equation, we can easily differentiate sources of patient versus machine trigger and cycle signals.

For the definition of patient trigger and cycle signals, the patient’s ventilatory efforts are represented by P_mus, P_E,mus, and P_R,mus. Because P_E = EV and P_R = RV, it follows that patient trigger and cycle signals are based on three variables (P_mus, V_mus, and ) and two parameters (E and R). The ventilator can be triggered or cycled by a signal representing P_mus (e.g., the electrical signal of the diaphragm as with neurally adjusted ventilatory assist or a calculated estimate of P_mus) or the volume or flow generated by the patient’s ventilatory muscles (V_mus and [e.g., volume triggering in the Draeger Babylog or flow triggering on many other ventilators]). The ventilator can be triggered or cycled by the patient’s passive respiratory system mechanics (E and R). Examples are flow triggering with the AutoRelease feature on the Draeger Evita Infinity V500 ventilator and flow cycling during pressure support on any ventilator. Manufacturers may have specific marketing terms for flow cycling. For example, flow cycling on the Puritan Bennett 840 ventilator (Pleasanton, California) is termed E-sensitivity. Given the definition of patient trigger and cycle signals, we can define machine trigger and cycle signals as being anything else that starts or ends inspiration (excluding operator-generated manual signals).

Baseline Variable

The baseline variable is the parameter controlled during expiration. Although pressure, volume, or flow could serve as the baseline variable, pressure control is the most practical and is implemented by all modern ventilators.

Baseline or expiratory pressure is always measured and set relative to atmospheric pressure. For baseline pressure to equal atmospheric pressure, it is at zero. For baseline pressure to exceed atmospheric pressure, it is set at a positive value, called positive end expiratory pressure (PEEP). Although seldom used, the baseline pressure could be set below atmospheric pressure, a technique called negative end expiratory pressure (NEEP).

Zero end expiratory pressure (ZEEP) is the default baseline value during positive pressure ventilation, meaning that it is normally in effect unless purposely changed. Regardless of the mechanism by which gas is delivered to the lungs, it must leave before the next inspiration. Exhalation normally occurs by virtue of the stored pressure in the expanded lungs and thorax. With ZEEP in effect, when exhalation begins, the ventilator’s expiratory valve simply opens to the atmosphere, exposing the patient’s airway to a relative pressure of zero. At this point, alveolar pressure exceeds airway pressure, gas moves from alveoli out to the atmosphere, and the lungs and thorax passively recoil down to their resting volume, or functional residual capacity (FRC).

PEEP is the application of pressure above atmospheric pressure at the airway throughout expiration. PEEP elevates a patient’s FRC and can help improve oxygenation by preventing collapse of alveolar units that are made unstable by lack of surfactant or disease.

NEEP is the application of subatmospheric pressure to the airway during expiration. NEEP was originally developed to overcome the harmful cardiovascular effects of positive pressure ventilation. The assumption was that NEEP could offset the impedance to venous return created by the positive pressure during inspiration. NEEP has occasionally been promoted as a way to help patients overcome expiratory airway resistance. In this approach, negative pressure is applied only to help return airway pressure to baseline. Because NEEP can cause airway collapse and decrease the FRC if misapplied, great care must be taken in applying this technique.

Control Variable

In the equation of motion, the control variable is the variable that is predetermined for a given inspiration (i.e., pressure or volume). Predetermined means that the operator presets the parameters of the variable’s waveform independent of the patient’s mechanics. In volume control, the operator may preset the inspiratory flow and tidal volume. In pressure control, the operator may preset the inspiratory pressure and inspiratory time. In a more exotic form of pressure control, such as proportional assist ventilation, the operator presets the parameters of elastance and resistance to be supported, and the ventilator delivers pressure according to the equation of motion.

There are clinical advantages and disadvantages to volume versus pressure control, which are discussed in Chapter 43. Briefly, volume control results in a more stable minute ventilation (and more stable blood gases) than pressure control if lung mechanics are unstable. Pressure control allows better patient-ventilatory synchrony with the patient because inspiratory flow is not constrained to a preset value. Although it is possible to control only one variable at a time, a ventilator can automatically switch between pressure control and volume control in an attempt to guarantee minute ventilation while maximizing patient synchrony. This feature is called dual control¹³ and is discussed in the section on Targeting Schemes.

Breath Sequence

Specifying only the control variable for a mode, the clinician can distinguish only among pressure and volume control modes; this is often all that is needed to communicate. At the bedside, the clinician might simply have to indicate that the patient has become asynchronous with the ventilator, and the mode has been changed from “volume control” to “pressure control.”

The second component of the mode classification scheme is the breath sequence. A breath is defined as a positive change in airway flow (inspiration) paired with a negative change in airway flow (expiration), both relative to baseline flow and associated with ventilation of the lungs. This definition excludes flow changes caused by hiccups or cardiogenic oscillations, but it allows the superimposition of a spontaneous breath on a mandatory breath or vice versa (these breath types are defined subsequently). Traditionally, the flow baseline is taken as flow = 0 L/min (i.e., end expiration). However, because “breaths” can be superimposed on existing flow in various circumstances (e.g., high-frequency ventilation), the inspiratory and expiratory movements of gas must be judged relative to the level of flow existing when these movements occur.

Typically, expiration immediately follows inspiration. However, with some modes, such as airway pressure release ventilation, there can be a large inspiration, followed by several small inspirations and expirations, followed by a large expiration.

The classification of modes requires the definition of two basic categories of breaths: spontaneous and mandatory. A spontaneous breath is a breath for which the start and end of inspiration may be determined by the patient, independent of any machine settings for inspiratory time and expiratory time. In other words, the patient both triggers and cycles the breath. As a consequence, the patient retains substantial, if not complete, control over the timing (frequency and inspiratory time) and size (tidal volume) of the breath. A spontaneous breath may occur during a mandatory breath (Figure 42-9). A spontaneous breath may be assisted or unassisted. An assisted breath is a breath during which all or part of inspiratory or expiratory flow is generated by a change in transrespiratory pressure (i.e., airway pressure minus body surface pressure) owing to an external agent (e.g., manual or automatic resuscitator, mechanical ventilator).

A mandatory breath is a breath for which the start or end of inspiration (or both) is determined by the ventilator, independent of the patient; the machine triggers or cycles the breath. As a consequence, the patient substantially, if not completely, loses control over the timing (frequency and inspiratory time) and size (tidal volume) of the breath. Mandatory breaths are generally assisted. Box 42-1 shows an algorithm defining spontaneous and mandatory breaths.

Box 42-1   Algorithm Defining Spontaneous and Mandatory Breaths

Having defined spontaneous and mandatory breaths, there are three possible sequences of breaths, designated as follows:

The abbreviation CMV has been used to mean a variety of things by ventilator manufacturers. The most logical use in this classification system is to represent continuous mandatory ventilation as part of a continuum from full ventilatory support to unassisted breathing. The abbreviation IMV has a long history of consistent use to mean intermittent mandatory ventilation (i.e., a combination of mandatory and spontaneous breaths). However, the development of the active exhalation valve and other innovations has made it possible for the patient to breathe spontaneously during a mandatory breath. This is primarily a feature used to help ensure synchrony between the ventilator and patient in the event that the mandatory breath parameters (e.g., preset inspiratory time, pressure, volume, or flow) do not match the patient’s inspiratory demands. This feature blurs the historical distinction between CMV and IMV.

The key difference now between CMV and IMV is that with CMV, the clinical intent is to make every inspiration a mandatory breath, whereas with IMV, the clinical intent is to partition ventilatory support between mandatory and spontaneous breaths. This means that during CMV, if the operator decreases the ventilatory rate (often considered to be a safety “backup” rate in the event of apnea), the level of ventilatory support is unaffected, provided that the patient continues triggering mandatory breaths at the same rate (i.e., each breath is assisted to the same degree). With IMV, the rate setting directly affects the number of mandatory breaths and the level of ventilatory support (assuming that spontaneous breaths are not assisted to the same degree as mandatory breaths). CMV is normally considered a method of “full” ventilatory support, whereas IMV is usually viewed as a method of partial ventilatory support. For classification purposes, if spontaneous breaths are not allowed between mandatory breaths, the breath sequence is CMV; otherwise, the sequence is IMV (Figure 42-10). Given that almost every ventilator has a mechanism to synchronize breath delivery with patient effort, it is no longer necessary to add an S to designate synchronized IMV (SIMV). The term SIMV was important in the early days of mechanical ventilation but is an anachronism now. Patient triggering can be specified in the description of mode phase variables.

There has been no consistent abbreviation to signify a breathing pattern composed of all spontaneous breaths. However, the logical progression would be from CMV to IMV to CSV.

When a breath sequence is added to the control variable in classifying a mode, the result is a breathing pattern. A breathing pattern provides a greater ability to discriminate between similar modes. For example, it is possible to distinguish between pressure-control IMV (PC-IMV) and pressure-control CSV (PC-CSV). By classifying modes based solely on the breathing pattern, there are only five possibilities in two groups (Table 42-3). The utility of this system is immediately obvious. A new mode, such as airway pressure release ventilation (APRV), can be explained as simply a form of PC-IMV. Assuming the clinician already understands the concept of PC-IMV, it takes little effort to understand the additional nuances of APRV (e.g., different labels for control settings, alarms). This level of description avoids the cumbersome verbal ad hoc definition for APRV, such as “a mode that allows spontaneously breathing patients to breathe at a positive-pressure level but drops briefly to a reduced pressure level for carbon dioxide (CO₂) elimination during each breathing cycle,” or the misleading description of some authors who explain APRV as two levels of continuous positive airway pressure.

PC-IMV and PC-CSV can also be used to clarify what bilevel positive airway pressure ventilation means. For example, on the Respironics BiPAP S/T or BiPAP AVAPS (Phillips Healthcare, Andover, Massachusetts) noninvasive home ventilators, the “timed” mode is PC-IMV, whereas the “spontaneous” mode is PC-CSV. BiPAP ventilation and bilevel ventilation are particularly ambiguous terms. To make matters even more confusing, the Puritan Bennett 840 ventilator has a “bilevel” mode that allows for additional pressure support during a pressure-limited, time-cycled, mandatory breath. With PEEP, the mandatory pressure limit, and the pressure support limit, the mode actually provides “trilevel” ventilation.

The specification of breathing pattern is also useful for grouping together modes that function the same way but are given different names. Both “CMV plus pressure limited ventilation” (Draeger Evita 4) and “volume-assured pressure support” (Bird 8400ST) are PC-IMV. It is also possible to group ventilators in terms of the number of breathing patterns they offer; some offer only one or two, whereas others offer all five. This grouping might be useful as an initial screening tool when planning ventilator purchases. Although the Bird 8400ST and 8400 STi may still be in use, this particular ventilator is not actively supported by the manufacturer (CareFusion, San Diego, California).

Targeting Schemes

Control variables and the differences between pressure and volume control have been discussed, but what is meant by control has not been explained. There are two general ways to control a variable: open loop control and closed loop control.¹⁴

Exponential

Exponential pressure waveforms are common outputs of neonatal ventilators and can be produced by adjusting the pressure rise control on some newer adult ventilators. The resulting pressure and volume waveforms can take on various shapes ranging from an exponential rise (same shape as the volume waveform in Figure 42-11, A) to a linear rise (same shape as the volume waveform in Figure 42-11, B). Generally, the flow waveform is similar to that seen in Figure 42-11, A except that peak inspiratory flow is reached gradually rather than instantaneously (resulting in a rounded rather than peaked waveform), and peak flow is lower than with a rectangular pressure waveform.

Sinusoidal

As previously described, a sinusoidal pressure waveform can be created by attaching a piston to a rotating crank. In addition, a linear drive motor driven by a microprocessor can produce a sine wave pressure pattern. In response, the volume and flow waveforms are also sinusoidal, but they attain their peak values at different times (see Figure 42-11, E).

Oscillating

Oscillating pressure waveforms can take on various shapes, from sinusoidal to ramp (SensorMedics 3100 Oscillator, Care Fusion Inc, Yorba Linda, CA) to roughly triangular (some jet ventilators). The distinguishing feature of a ventilator classified as an oscillator is that it can generate negative transrespiratory pressure. If the mean airway pressure is set equal to atmospheric pressure, the airway pressure waveform oscillates above and below zero. If the pressure waveform is sinusoidal, volume and flow are also sinusoidal but out of phase with each other (i.e., their peak values occur at different times).

Sinusoidal

This volume waveform is most often produced by ventilators whose drive mechanism is a piston attached to a rotating crank (e.g., Emerson). The output waveform of this type of ventilator can be approximated by the first half of a cosine curve, whose shape in this case is referred to as a sigmoidal curve (see Figure 42-11, E). Because volume is sinusoidal during inspiration, pressure and flow are also sinusoidal.

Ramp

Many respiratory care practitioners (and ventilator manufacturers) refer to ramp waveforms as either accelerating or decelerating flow patterns. The use of either of these terms is usually inappropriate. If a car slows, we do not say that its velocity decelerates; we say that the car decelerates. We do not say that a cyclotron is a velocity accelerator but that it is a particle accelerator. The rate of change of position of an object is the velocity of the object; analogously, the rate of change of volume is flow. The rate of change of velocity of an object is the acceleration of the object; likewise, the rate of change of flow is the acceleration of volume, not the acceleration of flow. So if we want to say that flow changes, we should simply talk about an increasing flow or a decreasing flow (or an accelerating volume or a decelerating volume), not an accelerating flow or a decelerating flow.

Sinusoidal

Some ventilators offer a mode in which the inspiratory flow waveform approximates the shape of the first half of a sine wave (see Figure 42-11, E). As with the ramp waveform, ventilators often truncate the sine waveform by starting and ending flow at some percentage of the set peak flow rather than start and end at zero flow. In response to a sinusoidal flow waveform, the pressure and volume waveforms are also sinusoidal but out of phase with each other.

Mode Settings

There is no standardization among ventilator manufacturers regarding the names of modes, their classification, or how the operator identifies and sets them on a given device. This requires the operator to learn the specific terminology and layout of each individual brand of ventilator for every function it provides. For clinical settings where many different ventilator brands are used, this can be a daunting and perhaps unachievable task. For an example of how different and confusing ventilator displays can be regarding mode selection, compare Figures 42-16 and 42-17.

Trigger, Target, and Cycle Variable Settings

There was a time when all the input settings and output displays of a modern ICU ventilator could fit on the face of the ventilator, even in digital format (Figure 42-18). However, with the proliferation of computer screens for displays and the increasing complexity of modes, the user is now often faced with the need to switch between multiple screens to access all of the ventilator’s capabilities (Figure 42-19). This is a relatively new challenge for the ventilator industry, and more work needs to be done to perfect the operator interface. More recent research on the topic suggests that the design of the user interface is relevant to the occurrence of operational failures and that ventilator designers could optimize the user-interface design to reduce operational failures.²³

Trigger variables include selection of either pressure or flow triggering and the trigger thresholds (in cm H₂O or L/min). Sometimes these variables are explicitly described, and sometimes they are just referred to as a sensitivity setting. Frequency is also a trigger variable, setting the time delay between time-triggered inspirations. A preset minute ventilation can also be a trigger variable in modes such as Draeger’s mandatory minute ventilation mode and Hamilton’s adaptive support mode; if the total minute ventilation from mandatory and spontaneous breaths falls below the preset value, the ventilator triggers mandatory breaths. Draeger has introduced a novel trigger mechanism (called AutoRelease on its Evita Infinity V500 ventilator) based on the percentage of peak expiratory flow. Mandatory pressure-controlled breaths are flow triggered when expiratory flow decays to the preset threshold, usually expressed as a percentage of the peak expiratory flow for the breath. Setting the cycle threshold at a lower value (e.g., 25%) increases the average inspiratory time, whereas setting the threshold higher (e.g., 75%) decreases the average inspiratory time. The actual inspiratory time for any given breath is determined by the patient’s mechanics (i.e., resistance, compliance, and inspiratory effort). The trigger mechanism is designed for use during the airway pressure release mode, and target variables for volume control typically include inspiratory flow and inspiratory flow waveform (e.g., square, descending ramp, or sinusoidal).

Target variables for pressure control include inspiratory pressure and sometimes pressure rise time. The latter variable sets the speed with which inspiratory pressure increases to the target value and controls peak inspiratory flow. Inspiratory pressure is another term that causes confusion because of nonstandard usage. On some ventilators, it is measured relative to atmospheric pressure (and should be called peak inspiratory pressure), and on others it is measured relative to PEEP and should be called simply inspiratory pressure.²⁴ The problem is that when a patient is changed from one ventilator (or mode) using one convention to another ventilator (or mode) with the other convention, the risk of inadvertently setting the wrong inspiratory pressure increases and could lead to adverse events.

For example, imagine a ventilated patient with respiratory system resistance of 10 cm H₂O/L/sec and compliance of 0.035 L/cm H₂O. This patient is transported on a ventilator with set inspiratory pressure of 25 cm H₂O (relative to atmospheric pressure), PEEP of 10 cm H₂O, and inspiratory time of 1.0 second. Under the assumptions of passive ventilation and no intrinsic PEEP, the measured peak airway pressured with this particular ventilator is 25 cm H₂O, the ΔP is 25 − 10 = 15 cm H₂O, and the tidal volume would be 495 ml. On arrival to the ICU, pressure-control ventilation is initiated with a ventilator in which inspiratory pressure is set relative to PEEP. If the same settings were used (inspiratory pressure 25 cm H₂O and PEEP 10 cm H₂O) the new peak airway pressure would be 25 + 10 = 35 cm H₂O, ΔP would be 25 cm H₂O, and the tidal volume would 825 ml. In this example, the patient is put at risk of ventilator-induced lung injury and cardiac compromise because of the inadvertent increase in ΔP and mean airway pressure. If the transport had been in the opposite direction, the patient would have been at risk of hypoventilation because of a lower ΔP and loss of oxygenation owing to decreased mean airway pressure. One could argue that a knowledgeable clinician would not make such a mistake because of familiarity with the two ventilators. What if the patient was being manually ventilated during intrahospital transport to the ICU and the only data available were the inspiratory pressure and PEEP? The risk of confusion, if not actual harm, is inherent in the way ventilator settings are documented, owing to lack of standardization.

Cycle variables for volume control are usually tidal volume or inspiratory time. Some ventilators allow the operator to preset minute ventilation instead of tidal volume. This arrangement sets tidal volume indirectly as a function of frequency and minute ventilation (i.e., tidal volume = minute ventilation/frequency). Inspiratory pause time is a time-based cycle variable that may be an option. Some ventilators allow the operator to set an inspiratory pause time directly, and others allow setting both inspiratory flow and inspiratory time. In the latter case, inspiratory flow time is equal to the tidal volume divided by inspiratory flow, and inspiratory pause time is inspiratory time minus inspiratory flow time.

For example, if the tidal volume is 500 ml (0.5 L) and the inspiratory flow is 60 L/min (1 L/sec), inspiratory flow time is 0.5 L/(1 L/sec), or 0.5 second. If the inspiratory time is set to 1 second, an inspiratory pause is created lasting 1.0 − 0.5 = 0.5 second. Cycle variables for pressure control generally include inspiratory time (or I : E ratio) for mandatory breaths and possibly flow cycle threshold for spontaneous (pressure support) breaths. Pressure support breaths are flow cycled when inspiratory flow decays to the preset threshold, usually expressed as a percentage of the peak inspiratory flow for the breath. Setting the cycle threshold at a lower value (e.g., 25%) increases the average inspiratory time, whereas setting the threshold higher (e.g., 75%) decreases the average inspiratory time. The actual inspiratory time for any given breath is determined by the patient’s mechanics (i.e., resistance, compliance, and inspiratory effort). All ventilators except for some home care devices, allow adjustment of the baseline pressure, or PEEP.

Alarm Settings

The purpose of ventilator alarms is to bring events to the attention of the clinician. Events are conditions or occurrences that require clinician awareness or intervention. Events can be classified according to four levels of priority.²⁵ Level 1 events are immediately life-threatening. These include insufficient or excessive gas delivery to the patient, exhalation valve failure, control circuit failure, or loss of power. Alarm indicators in this category should be mandatory (cannot be turned off by the operator), redundant, and noncanceling. Level 2 events range from mild irregularities in machine function to dangerous situations that could threaten patient safety if left unattended. These include failure of the air-O₂ blending system, inadequate or excessive PEEP, autotriggering, circuit leak, circuit occlusion, inappropriate I : E ratio, and failure of the humidification system. Alarms in this category are not redundant and may be self-canceling (i.e., automatically turned off if the event ceases). Level 3 events reflect changes in the level of ventilatory support. Examples include changes in the patient’s ventilatory drive or respiratory system mechanics and the presence of auto-PEEP. Level 3 events often trigger the same alarms as levels 1 and 2. Level 4 events are focused entirely on the patient. These include changes in gas exchange, dead space, oxygenation, and cardiovascular functions. Many ventilators do not warn of these events, and external monitors are required for surveillance.

Ventilators do not display alarm settings in terms of levels of priority. Instead, they tend to lump them all together on the screen (Figures 42-20 and 42-21; see Figure 42-15). The setting of alarm thresholds is a complicated topic that has been studied but for which little information is available regarding mechanical ventilation. The basic problem is to maximize true alarms and minimize false alarms. A high false alarm rate leads to habituation and clinicians ignoring warnings. False alarms can also lead to inappropriate responses. In one 200-hour study of an ICU, 1214 alarms occurred, and 2344 tasks were performed. On average, alarms occurred six times per hour; 23% were effective, 36% were ineffective, and 41% were ignored.²⁶ In another ICU study, during 982 hours of observation, 5934 alarms occurred, corresponding to six per hour. About 40% of the alarms did not correctly describe the patient condition and were classified as technically false; 68% of those were caused by manipulation. Only 885 (15%) of all alarms were considered clinically relevant.²⁷

Although these studies did not address mechanical ventilator alarms specifically, it is not hard to imagine similar results for such a study. Ventilator alarms are usually set by the operator (or as default values by the ventilator) as either a set value or a set percentage of the current value. Examples would be low and peak airway pressure alarms set at the current value ±5 cm H₂O or low and peak tidal volume and minute ventilation set at ±25% of the current value.²⁵ The problem is that the parameters we want to set alarms for, in particular, airway pressure, tidal volume, and minute ventilation, are highly variable, with significant portions at extreme values (Figure 42-22).²⁸ Limits set as absolute values or percentages may reduce safety for some extreme values, while increasing nuisance events for other values. An alternative approach might be to reference the alarm limits to the current value of the parameter such that extreme values have tighter limits. Further research is needed to identify optimization algorithms (i.e., minimize both harmful and nuisance events) for intelligent targeting schemes to set alarms automatically during mechanical ventilation.

Numeric Values

The most common data represented as numeric values are FiO₂; peak, plateau, mean, and baseline airway pressures; inhaled and exhaled tidal volume; minute ventilation; and frequency. Depending on the ventilator, a wide range of calculated parameters may also be displayed, including resistance, compliance, time constant, P_0.1, % leak, I : E ratio, and peak inspiratory/expiratory flow.

Waveforms and Loops

Most ICU ventilators display waveforms (sometimes called scalars) of airway pressure, volume, and flow as functions of time. Such displays are useful for identifying the effects of changes in settings or mechanics on the level of ventilation. They are also very useful for identifying sources of patient-ventilator asynchrony, such as missed triggers, flow asynchrony, and delayed or premature cycling.²⁹ They can also display one variable against another as an x-y or “loop” display. The most common loop displays show pressure on the horizontal axis and volume on the vertical axis or volume on the horizontal axis and flow on the vertical axis. Pressure-volume loop displays are useful for identifying optimal PEEP levels (quasistatic loops only) and overdistention. Flow-volume loops are useful for identifying the response to bronchodilators. An example of a composite display showing numeric values, waveforms, and loops is shown in Figure 42-23.

Trends

Many ventilators provide trend graphs of just about any parameter they measure or calculate. These graphs show how the monitored parameters change over long periods so that significant events or gradual changes in patient condition can be identified (Figure 42-24).

Picture Graphics

A new development in ventilator displays involves the use of picture graphics to represent useful information about the patient-ventilator system. A study³⁰ showed that subjects with graphic rather than conventional displays of obstructed endotracheal tubes and auto-PEEP problems were detected and treated faster. The investigators also reported significantly lower subjective workloads using the graphic display.

Hamilton Medical was the first manufacturer to use innovative picture graphics on their G5 ventilator. In particular, they created a graphic representation of the lungs, called a dynamic lung panel, which visually displayed information about resistance and compliance by the shape and color of the lungs and airways (Figure 42-25). In addition, Hamilton Medial created a unique graphic representation called the vent status panel that displays key parameters (e.g., oxygenation, ventilation, and spontaneous breathing activity) and shows when each item is in or out of an acceptable zone and for how long. This graphic display makes weaning status easy to identify. Draeger Medical followed with a similar graphic display called Smart Pulmonary View, which is a graphic display of respiratory system compliance and resistance and of the spontaneous and mandatory minute volume (Figure 42-26).

High-Frequency Ventilators

The availability of sophisticated devices, such as jet ventilators and high-frequency oscillators, facilitates ventilatory management of infants, children, and adults in the PC-IMV mode who fail to maintain adequate oxygenation and acid-base balance with conventional ventilatory support. High-frequency jet ventilators, such as the Bunnell Life Pulse (Bunnell Inc, Salt Lake City, Utah), deliver short bursts, or jet pulses, of mixed gas through a special adapter for endotracheal tubes or a specially designed endotracheal tube. The LifePort Endotracheal Tube Adapter (Bunnell Inc, Salt Lake City, Utah) is designed to replace the 15-mm outer diameter adapter on the proximal portion of a traditional endotracheal tube. This adapter has a high-frequency ventilator circuit connection port. The use of this adapter minimizes the need to reintubate the patient with a specially designed endotracheal tube. The connection port may be capped off or occluded when there is no longer a need for high-frequency ventilation. An endotracheal tube specially designed for use with high-frequency ventilation is similar in appearance to a conventional tube, with two additional small chambers molded into the wall. This triple-lumen design enables tracheal pressures to be monitored during the delivery of jet pulses.

High-frequency jet ventilators require a high pressure source (20 to 50 psig) to function. This type of ventilator consists of a system for regulating inlet pressure (psig) and a cycling mechanism and a device such as an air-O₂ blender for controlling FiO₂. The small volumes of gas are delivered at rapid rates (4 to 250 times the normal respiratory rate).³¹ The benefits of rescue and elective use of high-frequency jet ventilation as a treatment for acute lung disease and acute respiratory distress syndrome (ARDS) are unclear.³¹ However, the literature supports its effectiveness in maintaining alveolar ventilation and reducing morbidity during surgical repair of tracheal and airway anomalies.^32,33

High-frequency oscillation also allows very small tidal volumes to be delivered at rapid frequencies (180 to 1200 cycles/minute). High-frequency oscillators use a piston-driven diaphragm to produce the airflow oscillations. Tidal volume depends on the force and distance the piston moves from baseline. A special endotracheal tube is not required to implement this form of PC-IMV. The SensorMedics 3100A and 3100B high-frequency oscillators (CareFusion) are approved and commercially available for use in neonatal/pediatric (<35 kg) and pediatric/adult (>35 kg) populations. There are independent reports of improved morbidity and mortality with the use of high-frequency oscillation in infants and children with diffuse alveolar and small airways disease.³⁴ Combined evidence from observational and clinical trials appears to be favorable with respect to improved mortality rates in patients with heterogeneous lung disease, such as ARDS.^35,36 In contrast to conventional ventilators, convention plays a minor role in gas transport with high-frequency jet ventilation and high-frequency oscillation. Rather, the mixing and diffusion of gases in the airways enhances fresh gas delivery to and elimination of CO₂ from the alveoli.