Prehospital Setting

Mechanical ventilation in the prehospital setting is typically limited to continuous positive airway pressure (CPAP) or NPPV and critical care transport ventilators. The presence of CPAP/NPPV machines is becoming more commonplace in emergency medical system (EMS) vehicles. Patients who require intubation typically need bag-valve ventilation during transport. Critical care transport ambulances are fewer in number but carry transport ventilators, which have become increasingly smaller and more complex. An important step in transporting a patient on a transport ventilator is to test the patient on the transport ventilator before leaving the facility, preferably before moving the patient to the stretcher. This allows paramedics and EMS nurses to coordinate ventilator settings with respiratory therapists at the transferring facility. Critically, it allows early determination of intolerance to the transport ventilator.

Hospital Setting

The typical setting for initiation of mechanical ventilation will be the prehospital or hospital setting. Occasionally, patients with chronic respiratory failure managed with home ventilators will arrive at the ED in acute respiratory failure; such patients should be placed on critical care ventilators while the work-up is in progress, and their home ventilator should accompany them to the ED. This functions in two ways: first, it addresses any possible complications caused by the home ventilator, and second, it allows ease of familiarity with the ventilator.

Consultation

Patients suffering from respiratory failure requiring mechanical ventilation will need to be admitted to a critical care unit or be transferred to a facility able to take care of mechanically ventilated patients. Consultation with a critical care medicine provider is preferable, though not always available.

Control Mode

Control mode ventilation (CMV) is used almost exclusively in anesthesia, but knowledge of this mode’s limitations aids in comprehension of other modes’ features (Fig. 3.1). In CMV, all breaths are triggered, limited, and cycled by the ventilator. In volume-targeted mode, the physician selects a tidal volume (VT), RR, inspiratory flow rate (IFR), fraction of inspired oxygen (FIO₂), and positive end-expiratory pressure (PEEP). The machine then delivers positive pressure and applies as much pressure as required to deliver the set VT at the set IFR. (In pressure-targeted mode the physician sets the pressure high, RR, FIO₂ and pressure low or PEEP.) Note that patients can set their own flow rate in pressure-targeted modes. The machine then delivers positive pressure and applies as much pressure as required to reach the set pressure high. The VT values generated are a function of respiratory system compliance. The patient is not able to initiate or terminate a breath. If inspiratory effort is initiated before the machine is triggered to deliver a breath, airflow would not occur regardless of the patient’s inspiratory effort. If exhalation is incomplete and the time for the machine to deliver a breath has occurred, the ventilator would provide as much pressure as necessary to cause inhalation. Imagine forcibly exhaling, or coughing, when the ventilator begins to deliver a breath. This lack of synchrony would cause distress and risk structural lung or airway injury. For these reasons, CMV is never used except for apneic, paralyzed, or anesthetized patients.

Fig. 3.1 Control mode.

Tidal volume, respiratory rate, inspiratory flow rate, FIO₂, and positive end-expiratory pressure are controlled. In this mode there is no synchronization with the patient’s respiratory effort.

Assist/Control Mode

AC mode usually provides the greatest level of ventilatory assistance (Fig. 3.2). In volume-targeted ventilation, the physician sets VT, RR, IFR, FIO₂, and PEEP. (In pressure-targeted mode, the physician sets the pressure high, RR, FIO₂ and pressure low or PEEP.) In contrast to all other modes, the trigger that initiates inspiration can be either an elapsed time interval (determined by the set RR) or the patient’s spontaneous inspiratory effort. When either occurs, the machine delivers the set VT (in volume-targeted mode) or pressure high (in pressure-targeted mode). The machine follows a time algorithm that synchronizes the machine with patient-initiated breaths. If the patient is breathing at or above the set RR, all breaths are initiated by the patient. If the patient breathes below the set RR, machine-initiated breaths are interspersed among the patient’s breaths. Work of breathing (WOB) is primarily the effort that the patient exerts to cause airway pressure to drop to the threshold that triggers onset of the ventilator. (Manipulating the sensitivity of the ventilator sets this threshold.) Furthermore, WOB may be performed to a variable degree during inspiration, depending on how much the respiratory muscles are activated. WOB with the volume-targeted AC mode may be extreme in two situations: when the VT drawn by the patient is greater than the set VT and when the patient inspires at a rate that exceeds the set IFR (see later).

Fig. 3.2 Volume-targeted, assist/control mode.

Tidal volume is controlled. A minimum mandatory respiratory rate is set and synchronized with patient effort. If the patient breathes at a rate higher than the set rate, all breaths are assisted. The dashed line represents esophageal/intrathoracic pressure dynamics. The first two breaths represent machine-initiated (M) breaths; the second two breaths represent patient-initiated (P) breaths.

In the majority of situations, AC mode is used as described earlier and is termed volume-targeted or volume-cycled ventilation. As an alternative, some ventilators allow pressure-targeted (cycled) ventilation (PCV, not to be confused with PSV, described later) (Fig. 3.3). Instead of IFR, the limit during PCV is a set airway pressure. Instead of VT, the cycle during PCV is a set inspiratory time (TI). On some ventilator models, RR and the inspiratory-to-expiratory (I : E) ratio are set, and TI is calculated from these settings. On other models, TI is available as a setting. Because VT is not set, the VT delivered varies slightly from breath to breath, depending on lung compliance, airway resistance, and patient effort. PCV may offer a slight advantage over VCV in clinical scenarios that require control of the I : E ratio, but a body of literature investigating this concept does not exist. Historically, PCV was commonly used in neonates and infants, although modern ventilators that precisely measure small VT are currently favored. PCV may be the only mode available on some portable and transport ventilators.

Fig. 3.3 Pressure-targeted, assist/control mode.

Inspiratory pressure support (PS) and inspiratory time (TI) are controlled. Tidal volume may vary from breath to breath. The minimum mandatory respiratory rate is set and synchronizes with patient effort. The dashed line represents esophageal/intrathoracic pressure dynamics. The first two breaths represent patient-initiated breaths; the third breath is a machine-initiated (mandatory) breath. PEEP, Positive end-expiratory pressure.

Synchronized Intermittent Mandatory Ventilation and Pressure Support

Synchronized intermittent mandatory ventilation (SIMV) is probably the most commonly misunderstood mode of mechanical ventilation (Fig. 3.4). The physician sets VT, RR, IFR, FIO₂, and PEEP, as in AC mode. In contrast to AC mode, however, the trigger that initiates inspiration depends on the patient’s RR relative to the set RR. When the patient breathes at or below the set RR, the trigger can be either elapsed time or the patient’s respiratory effort. In this case, WOB is equivalent to AC. When the patient breathes above the set RR, the ventilator is not triggered to assist in making spontaneous breaths in excess of the set RR. The work associated with such breaths may be quite high because the patient must generate enough negative force to pull air through the ventilator and overcome the resistance to airflow caused by the ventilator circuit tubing and the endotracheal tube (ETT), in addition to the WOB required as a result of the underlying disease process.

Fig. 3.4 Volume-targeted, synchronized intermittent mandatory ventilation (SIMV).

Tidal volume is controlled only during machine-assisted breaths. Tidal volume may vary during nonassisted (patient-initiated) breaths. The minimum mandatory respiratory rate is set. If the patient breathes more slowly than the set rate, the machine synchronizes with patient effort (third breath). If the patient breathes faster than the set rate, breaths in excess of the set rate are not assisted (second and fourth breaths). M, Machine-initiated breath; P, patient-initiated breath. The dashed line represents esophageal/intrathoracic pressure dynamics.

This limitation of SIMV can be diminished by the addition of PSV (Fig. 3.5). PSV causes inspiratory positive pressure to be applied during patient-initiated breaths that exceed the set RR. The patient initiates and terminates inspiration, thereby determining VT. Once the patient triggers pressure support, it is maintained until the machine detects cessation of patient effort, as indicated by a fall in inspiratory airflow. VT, IFR, and TI are not controlled but instead are determined by patient effort. The WOB performed during PSV involves triggering the ventilator to deliver the pressure and maintaining inspiratory effort throughout inhalation. Contrast this with machine-assisted ventilation in AC or SIMV, where WOB involves triggering the ventilator but lung inflation continues regardless of the patient’s inspiratory effort. WOB during PSV also depends on the set level of pressure support. Insufficient pressure support is associated with high WOB, which leads to a small VT and a high RR. Adequate pressure support reduces WOB and improves VT and RR. Many experts view RR as the best index of the adequacy of the level of pressure support. It should be adjusted to maintain an acceptable RR of less than 30 but preferably less than 24 breaths per minute.

Fig. 3.5 Volume-targeted, synchronized intermittent mandatory ventilation (SIMV) with pressure support ventilation (PSV).

Tidal volume is controlled during machine-initiated (M) breaths (first and third breaths) and synchronized patient-initiated (P) breaths (second breath). Pressure support is provided whenever the patient initiates a breath over the set rate (fourth breath). Tidal volume may vary during pressure-supported breaths. The dashed line represents esophageal/intrathoracic pressure dynamics.

SIMV can be used in pressure-targeted ventilation. Essentially, the ventilator is set to reach a target pressure for each of the ventilator-initiated breaths and potentially a different target for patient-initiated breaths. Another way to consider pressure-targeted SIMV is as PSV with a set rate.

Continuous Positive Airway Pressure

CPAP alone is not a true form of assisted mechanical ventilation because inspiration is not assisted by increasing airway pressure. Pressure greater than ambient atmospheric pressure is supplied, but it is held constant throughout the respiratory cycle. During inhalation, the gradient between the airway and intrathoracic pressure is higher than it would be if breathing ambient air. Conversely, the gradient is lower during exhalation. As a result, inhalation requires slightly less effort than normal breathing does, and the airways are held open during exhalation to allow better expiratory airflow. As in SIMV, PSV may be added to CPAP. CPAP with PSV is a form of assisted ventilation because inspiratory pressure is augmented, and this is more appropriately referred to as simply PSV. As discussed earlier, the patient initiates and terminates each breath; therefore, WOB is performed as the patient initiates each breath and maintains inspiratory effort throughout inhalation.

Other Modes

The most recent innovations in ventilator modes are those that combine volume and pressure targets, which are referred to as dual modes. Pressure-regulated volume control, autoflow, volume ventilation plus, adaptive support ventilation, variable pressure control, and variable pressure support are all dual modes that adjust pressure or volume targets from breath to breath to reach the goals desired. Volume-ensured PSV and pressure augmentation alter parameters within the breath to reach goals. Unfortunately, few studies have compared these latest modes with one another or with conventional modes.

Finally, modes that have rarely been used in the ED setting and are beyond the scope of this chapter include high-frequency ventilation, airway pressure release ventilation, bilevel ventilation, proportional assist ventilation plus, and proportional pressure support.

Peak Inspiratory Airway Pressure

Peak inspiratory airway pressure (P_peak) is the highest pressure that is generated during inflation of the lung. Because pressure decreases incrementally along the path at each point of resistance, the pressure delivered at the alveolar level may be significantly less than the measured P_peak, particularly when resistance to airflow is high. Therefore, P_peak is not an ideal surrogate measurement for alveolar pressure and does not correlate with VILI.

Plateau Pressure

Plateau pressure (P_plat) is the end-inspiratory airway pressure and is measured just after airflow has ceased. Because this is a static measurement (absence of airflow), resistance of the circuit and airways does not play a role. Therefore, P_plat is a logical surrogate measurement for mean alveolar pressure. Its primary limitation is that compliance is not equal in all regions of the lung. The degree of alveolar distention in healthy regions of the lung may be significantly greater than that in heavily diseased lung regions at the same P_plat. In a healthy adult undergoing mechanical ventilation with normal lung compliance, P_plat is low, usually in the range of 5 to 15 cm H₂O. Patients with alveolar disease (pneumonia, cardiogenic pulmonary edema, acute lung injury [ALI], and ARDS) have poor lung compliance, and P_plat is typically much higher in these states. Measures to maintain P_plat below the currently recommended limit of 30 cm H₂O are discussed later.

Intrinsic Positive End-Expiratory Pressure

PEEP indicates that the airway pressure measured at the end of exhalation is above ambient air pressure. When PEEP is set by the clinician and applied by the ventilator, it is termed extrinsic PEEP (PEEP_e). In contrast, intrinsic PEEP (PEEP_i) arises when exhalation is incomplete because of either intrathoracic airway obstruction, early airway closure during exhalation, or inadequate exhalation time. The common end point is trapping of air in the lung at the end of exhalation, which ultimately leads to increased intrathoracic pressure. PEEP_i can cause problems through several mechanisms. First, because exhalation is incomplete, air is progressively being trapped in the lungs, thereby leading to early airway closure and dynamic hyperinflation with an associated risk for VILI. Second, PEEP_i leads to difficulty triggering the ventilator and increased WOB, as discussed previously. Third, PEEP_i can cause patient-ventilator dyssynchrony when the patient continues active contraction of the respiratory muscles at end exhalation as the ventilator is triggered. Lung inflation may begin while the patient is attempting to complete exhalation. Finally, increased intrathoracic pressure can impede venous return to the heart and consequently lead to hemodynamic instability. Simultaneously, impaired venous return may compromise pulmonary blood flow, increase physiologic dead space, and result in worsening hypercapnia. Control of PEEP_i is discussed later.

Spontaneous and Spontaneous/Timed Modes

In spontaneous mode, airway pressure cycles between inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP). This mode is commonly referred to as biphasic (or bilevel) positive airway pressure, but other proprietary names refer to the same mode. The trigger to switch from EPAP to IPAP is the patient’s inspiratory effort. A variety of ventilator models use one or several of the following to indicate patient effort: a drop in airway pressure, measured inspired volume (usually 5 to 6 mL), or an increase in airflow rate. The limit during inspiration is the set level of IPAP. The inspiratory phase cycles off when the machine senses cessation of patient effort, as indicated by a decrease in inspiratory flow below a set threshold (typically 60% of the peak IFR) or attainment of maximum inspiratory time (usually 3 seconds). The latter is a safety mechanism to prevent lung hyperinflation as a result of ventilator “runaway,” but it was not available on early generations of noninvasive ventilators. VT may vary from breath to breath, dependent primarily on the magnitude and duration of patient effort, but also on lung compliance. WOB is predominantly related to initiating and maintaining airflow throughout the inspiratory phase. Additional WOB may occur if the patient actively contracts the expiratory muscles.

Spontaneous mode is dependent on the patient’s effort to trigger inhalation. Respiratory acidosis will develop in a patient breathing at a slow, inadequate rate. To prevent this adverse consequence, spontaneous/timed mode allows the machine to be triggered either by patient effort or after an elapsed time interval that is calculated from a set minimum RR. If the patient does not initiate inspiration during the set interval, IPAP is triggered. For machine-initiated breaths, the machine cycles back to EPAP based on a set inspiratory time. For patient-initiated breaths, the ventilator cycles as it would in spontaneous mode.

Pragmatically, NPPV (noninvasive) and PSV (invasive) are similar but have a few noteworthy differences. First, the trigger for PSV is a drop in airway pressure sensed by the ventilator. Some ventilators monitor airflow in the inspiratory and expiratory limbs of the ventilator circuit and will be triggered if airflow in the inspiratory limb is greater than airflow in the expiratory limb. The sensitivity of the trigger can be adjusted on a conventional ventilator by setting the magnitude of the change in pressure required for triggering. This is contrasted with NPPV, in which sensitivity is continuously and automatically adjusted by the noninvasive ventilator based on the amount of air leak and is not able to be adjusted by the physician. Second, because PSV is supplied by a critical care ventilator, leaks are not tolerated or compensated. Because airflow through a leak may be misinterpreted in this mode as patient inspiratory effort, a leak may lead to early triggering before exhalation is complete. Leaks may also cause failure to cycle off in synchrony with cessation of patient effort. These phenomena are less likely to occur when using a noninvasive ventilator. Finally, the nomenclature used for airway pressure is different. Pressure during the expiratory phase is termed PEEP, analogous to the EPAP of NPPV. Pressure during the inspiratory phase is termed peak inspiratory pressure, analogous to the IPAP of spontaneous mode. The distinction is that in PSV the numerical value for pressure support is the equivalent of the difference between IPAP and EPAP.

Initiation of Noninvasive Positive Pressure Ventilation

The process of initiating a trial of noninvasive ventilatory support consists of four basic steps. First, the patient must be willing to accept face mask ventilation. Because the patient should remain awake and cooperative during ventilation, the process should be explained before the mask is applied. Initially, an FIO₂ of 100% with 3 to 5 cm H₂O of CPAP is provided. Acceptance may improve if the patient holds the mask against the face. The mask is secured with straps once the patient demonstrates acceptance.

Next, after explaining that the pressure will change, ventilation is switched to NPPV with an EPAP of 3 to 5 cm H₂O and an IPAP of 8 to 10 cm H₂O. IPAP is titrated in 2– to 3–cm H₂O increments until exhaled VT (measured by the ventilator) is in the range of 6 to 9 mL/kg IBW. Further adjustment of IPAP should be directed toward obtaining an RR of less than 30. Another option is to start with high IPAP (20 to 25 cm H₂O) and titrate down based on patient comfort. Of note, no studies have compared a low-to-high IPAP versus a high-to-low IPAP approach.

EPAP is then adjusted to the lowest level that allows synchrony between the patient and ventilator. Understanding this process requires review of the components of WOB related to triggering the ventilator. The patient activates the inspiratory muscles to decrease intrathoracic pressure. As intrathoracic pressure falls below airway pressure, transpulmonary pressure becomes positive, airflow begins, and the ventilator is triggered. In a normal patient, the inspiratory muscle force required to lower intrathoracic pressure to a level that triggers the ventilator is not great. In a patient with high PEEP_i (also known as auto-PEEP), intrathoracic pressure is high at end exhalation. The inspiratory muscle force required to lower intrathoracic pressure below airway pressure is significantly greater. Thus the WOB that is performed to trigger the ventilator is proportional to the amount of PEEP_i that is present.

While delivering NPPV, it is impossible to measure PEEP_i without invasive means. Instead, to detect PEEP_i, signs of difficulty triggering the ventilator or signs of expiratory airflow obstruction should be sought. On physical examination, recruitment of the accessory muscles of inspiration suggests that PEEP_i is a problem. A useful technique is palpation of the sternocleidomastoid muscle while simultaneously watching the ventilator flow graphs or listening for the ventilator to trigger. When the muscle is felt to contract before the ventilator triggers, PEEP_i may be the culprit. Observation of active abdominal muscle recruitment during exhalation indicates airflow obstruction as a cause of elevated PEEP_i. When elevated PEEP_i is suspected, EPAP should be increased in increments of 2 to 3 cm H₂O until the problem is controlled. The maximum safe level of EPAP that should be used during NPPV has not been determined in an evidence-based manner. Typical initial settings range from 0 to 5 cm H₂O; maximum settings described in the methods sections of various trials range from 12.5 to 15 cm H₂O. It is prudent to measure the heart rate and blood pressure and perform pulse oximetry after each increase in EPAP because high levels may compromise cardiac output. As EPAP is increased, corresponding increasing increments in IPAP are required to maintain a differential between EPAP and IPAP that ensures adequate VT.

Finally, FIO₂ is adjusted to maintain adequate O₂ saturation. In many clinical situations, continuous pulse oximetry alone is adequate for this purpose. Arterial blood gas determinations are not routinely required but may be helpful in select patients to assess improvement in respiratory acidosis.

Controlling Airway Pressure—Lung-Protective Ventilator Strategies

Causes of difficulty with mechanical ventilation fall into four general categories:

Follow-Up, Next Steps In Care, and Patient Education

Patients placed on mechanical ventilation (either invasive or noninvasive) require admission to the hospital. Some centers are able to manage NPPV outside the intensive care unit, but this should be considered only in patients who have demonstrated marked improvement and are on minimal settings. Patients admitted to the floor with NPPV should still be monitored carefully.

Patients who required NPPV during their ED course and subsequently improve to the point where they no longer needed mechanical ventilation should be considered for admission or observation. Asthmatic patients who have improved may be considered for discharge, but only after a period of observation.

Occasionally, patients with chronic respiratory failure will be seen in the ED. Evaluation of such patients should be based on their chief complaint. Determination for admission is essentially the same as for other patients. If admission is required for simple issues, these patients will need to be admitted to the intensive care unit because it is the only location with personnel trained to manage the ventilator.

Invasive Mechanical Ventilation

In the ED, complications of mechanical ventilation (Box 3.1) can begin during the preintubation period. Induction agents may cause or worsen hypotension. Overly aggressive bag-valve-mask ventilation may lead to decreased venous return and hypotension. Airway trauma and mechanical complications may be caused by the act of intubation. Initiating mechanical ventilation and transitioning from negative pressure ventilation to positive pressure ventilation may lead to hypotension. Positive pressure ventilation may worsen an existing pneumothorax or give rise to pneumothorax. Auto-PEEP (also known as PEEP_i, dynamic hyperinflation, breath stacking) can lead to hypotension and circulatory collapse. Ventilator-associated lung injury can be caused by barotrauma, volutrauma, or trauma related to atelectasis. Long-term complications can include inability to be liberated from the ventilator, ventilator-associated pneumonia, tracheal stenosis, and vocal cord injury.

Some of the commonly underrecognized problems that arise in the support of critically ill patients fall into the category of patient-ventilator dyssynchrony. These situations can markedly increase WOB and lead to increased CO₂ and lactic acid production with both respiratory and metabolic acidosis.

Intrinsic Positive End-Expiratory Pressure

Maneuvers directed at elimination of PEEP_i have in common the effect of decreasing inspiratory time and therefore providing more expiratory time. Decreasing RR and VT and increasing IFR effectively accomplish this goal. Frequently, this cannot be achieved without sedation, sometimes requiring the addition of pharmacologic paralysis.

Equipment Failure

Whenever a patient decompensates while receiving mechanical ventilation, consideration should be given to equipment failure as the cause. Interruption of the oxygen supply, accidentally rotated knobs, disconnected ventilator circuitry, and obstructed tubes are all potential culprits. Immediate action should include disconnection from the ventilator and bag ventilation with 100% O₂. The mnemonic made popular by the American Heart Association’s Pediatric Advanced Life Support Course is useful to recall the causes of unexpected decompensation: DOPE (dislodgment of the ETT, obstruction of the tube, pneumothorax, and equipment failure). Confirmation of ETT placement, suctioning via an endotracheal catheter, auscultation, chest radiography, and equipment troubleshooting are necessary actions.

Noninvasive Mechanical Ventilation

The main complication of NPPV is an inability to tolerate the mask or the pressure. Long-term complications can include an inability to eat or drink, nasal and oral dryness, and pressure necrosis on the bridge of the nose, the cheeks, or the chin or above the ears.