a. Physiologic effects

Continuous positive airway pressure (CPAP) and positive end-expiratory pressure (PEEP) increase the patient’s functional residual capacity (FRC). In neonates, the most common cause of a decreased FRC is infant respiratory distress syndrome (RDS). This condition is caused by the lack of surfactant in the lungs of the premature neonate. The neonate with RDS has relatively airless lungs that are prone to atelectasis. This results in hypoxemia. In addition, each tidal volume breath requires a greater than normal inspiratory effort (Figure 16-1). The restoration of FRC in the neonate increases its Pao₂, decreases the percentage of shunt, narrows the alveolar to arterial difference in oxygen, and reduces its work of tidal volume breathing. CPAP must be used with caution in neonates with persistent pulmonary hypertension (PPHN) of the newborn. An excessive amount of pressure in the alveoli compresses the capillary bed. This decreases pulmonary blood flow, which in turn increases blood flow through the patent ductus arteriosus and worsens the problem.

Figure 16-1 Pressure-volume curves for normal neonate and for one with infant respiratory distress syndrome (RDS). Normal neonate’s lungs inhale a relatively large tidal volume at low pressure. Note how when the same pressure is placed against the lungs of an infant with RDS, the tidal volume is much smaller. To get a normal tidal volume, the RDS infant must generate a much greater negative pressure.

(From Carlo WA, Martin RJ: Pediatr Clin North Am 33:221, 1986.)

b. Indications, contraindications, and hazards

CPAP is indicated for any condition that results in an unacceptably low Pao₂ secondary to a decreased FRC. Some neonates respond so well to CPAP that mechanical ventilation is not needed. In addition, CPAP has been used to keep open the airways of infants with tracheal malacia or other conditions in which the airways collapse abnormally. In general, contraindications include any CPAP-related condition that results in a worsening of the patient’s original status. Some neonates cannot tolerate CPAP and progressively hypoventilate as the pressure level is increased. Clinical judgment is needed to decide how high the PaCO₂ should be allowed to rise before discontinuing the CPAP and beginning mechanical ventilation. In general, the PaCO₂ should not be greater than 50 torr as long as the pH is at least 7.25. An absolute contraindication is apnea resulting in hypoxemia and hypotension. These infants should be mechanically ventilated. Box 16-1 gives a complete listing of indications, contraindications, and hazards.

c. Initiation

Before starting CPAP, a set of baseline arterial blood gases should be taken. Transcutaneous oxygen monitoring or pulse oximetry may be substituted in some clinical situations if oxygenation is the only parameter that must be measured. The neonate’s vital signs should also be recorded. Assemble the CPAP circuit and pressure device. The decision must be made whether to apply the CPAP above the epiglottis (Figures 16-2 and 16-3) or to intubate the infant and apply the CPAP within the trachea. Nasal CPAP (NCPAP) or nasopharyngeal tube CPAP (NP-CPAP) are both widely used to apply pressure from above the epiglottis. The neonate or infant must have an endotracheal tube placed to apply CPAP within the trachea. Among the factors to be considered are the neonate’s gestational age and weight, the amount of secretions that need to be suctioned, the pulmonary problem, and the likelihood of mechanical ventilation eventually becoming necessary. More mature and larger infants with few secretions and relatively stable pulmonary conditions will most likely have CPAP applied above the epiglottis by nasal prongs or nasopharyngeal tube. In contrast, less mature and smaller infants (less than 1000 to 1200 g) who need suctioning and have relatively unstable pulmonary conditions will probably be intubated. Mechanical ventilation can then be easily started if needed.

Figure 16-2 A, Nasal CPAP is delivered through a set of prongs and placement system. Shown is a Kendall Health Argyle CPAP prong and headgear set. B, Nasopharyngeal CPAP is delivered through a shortened endotracheal tube. Shown are 2.5, 3.0, and 3.5 mm ID endotracheal tubes that have been cut off at the proximal end and attached to the ventilator adapter. Always select the prongs or tube that are the proper size to fit the neonate’s airway.

(From Cario J, Pilbeam S: Mosby’s Respiratory Therapy Equipment, ed 8, St Louis, 2010, Mosby.)

Figure 16-3 An assembly for supporting nasal CPAP prongs in an infant.

(Modified from advertisement of Stayflex tubing system from Ackrad.) (From Sills JR: Respiratory care registry guide, ed 1, St Louis, 1994, Mosby.)

CPAP is usually started at about 4 to 5 cm water pressure whether the pressure is applied above or below the epiglottis. The inspired oxygen percentage is usually kept at the previously set level. It is important to make only one change at a time so that each adjustment in care can be evaluated for its own effect. For example, if you simultaneously increased the oxygen percentage by 10% and started 5 cm water of CPAP, it would not be known whether the increase in Pao₂ was from the additional oxygen, the CPAP, or both. Usually the long-term inspired oxygen is limited to 40% to 50% because of concern of the possibility of pulmonary oxygen toxicity.

d. Monitor and adjust alarm settings (ELE Code: IIIG3g) [Difficulty: ELE: R, Ap, An]

Depending on the clinical situation, CPAP can be administered through a freestanding, improvised system or through a mechanical ventilator set on the CPAP mode. A variety of alarms and monitoring systems should be used to ensure the patient’s safety. Patient monitors should have visual and audible alarms and include the following:

e. CPAP adjustment

Blood gases and vital signs must be evaluated at the starting CPAP level. The heart rate, blood pressure, and respiratory rate should be stable or improved. Wait at least 10 minutes after a change in CPAP before getting an arterial blood gases sample. See Table 16-1 for the recommended blood gas limits. In general, the Pao₂ should be kept between 60 and 70 torr, PaCO₂ less than 50 to 55 torr, and pH at least 7.25. If the Pao₂ is too low and the patient’s vital signs are acceptable, the CPAP may be increased in a step of 1 to 2 cm water. The vital signs and blood gases should then be reevaluated. In addition, the neonate’s work of breathing can be indirectly assessed. Improved lung function will be demonstrated by seeing decreased respiratory rate, retractions, expiratory grunting, and nasal flaring. If necessary, the process of adding CPAP and reassessing the patient can be continued. The maximum CPAP level in a neonate is generally held to be 10 cm water; the maximum CPAP level in an infant is generally held to be 15 cm water.

TABLE 16-1 Commonly Recommended Blood Gas Goals for CPAP and Mechanical Ventilator Therapy

NOTE: Keep the Pao₂ no greater than 80 torr in the premature neonate to reduce the risk of retinopathy of prematurity.

* Ptco₂ values may be used after they have been shown to correlate within 15% of the Pao₂ from an arterial blood gas.

† With CPAP, this value may be increased to 50 to 55 torr as long as the pH is at least 7.25.

Mechanical ventilation is usually indicated if more than these maximum CPAP pressures are needed to correct hypoxemia. Depending on the patient, even levels less than these maximum CPAP pressures may not be well tolerated. The infant may become exhausted from exhaling against the back pressure of the CPAP system. That is seen clinically as decreased chest movement from the smaller tidal volume. The PaCO₂ will probably increase. It may be necessary to place the infant on mechanical ventilation to decrease the work of breathing and then add PEEP to maintain the FRC. When nasal prongs or a nasopharyngeal tube are used, CPAP pressures of greater than 8 cm water may cause the infant’s mouth to open. This results in the loss of CPAP. A crying infant also opens its mouth and loses the CPAP. In either case, the CPAP pressure gauge drops to zero or fluctuates below the set pressure.

f. Independently initiate weaning from CPAP (Code: IIIF2i12) [Difficulty: ELE: R, Ap; WRE: An]

As the patient improves, it is necessary to reduce the CPAP level so as not to cause pulmonary barotrauma. The pressure level can be reduced in steps of about 2 cm water. The vital signs and blood gases should be reassessed after each step. The apparatus is usually removed when the CPAP level is down to 2 to 4 cm water. The infant is then placed into an oxyhood at the same oxygen percentage as before or 5% to 15% higher. If the infant has an endotracheal tube that is needed for suctioning or a secure airway, the pressure is usually left at 2 to 4 cm water. After extubation, the infant is placed into an oxyhood as before. Alternatively, the neonate may be weaned to a high flow nasal cannula and then to a traditional, low-flow nasal cannula. See Chapter 6 for more discussion on the high-flow nasal cannula.

If the infant was breathing more than 50% oxygen while on the CPAP, it may be more important to lower the oxygen before decreasing the CPAP level. The following guidelines may prove helpful when deciding whether to first lower the oxygen percentage or the CPAP level:

a. Recommend changing the type of ventilator to be used on the patient (Code: IIIG3i) [Difficulty: ELE: R, Ap; WRE: An]

b. Independently change the type of ventilator to be used on the patient (Code: IIIF2i9) [Difficulty: ELE: R, Ap; WRE: An]

Historically, most neonatal patients requiring life sup-port receive time-triggered, pressure-limited, time-cycled mechanical ventilation (TPTV). These ventilators are pneumatically powered with electrical controls and alarm systems. They are used in the IMV mode and feature a continuous flow of gas from which the neonate can breathe spontaneously. TPTV units are pressure limited to prevent an excessive peak airway pressure and can have therapeutic PEEP added. Furthermore, they are capable of reaching the Food and Drug Administration (FDA) limited rate of 150 breaths/min. The majority of neonatal and small pediatric patients can be effectively ventilated on these types of units.

High-frequency ventilation (HFV) with a small tidal volume has been approved by the FDA for use in the rescue of neonates with RDS and a bronchopulmonary fistula or pulmonary interstitial emphysema (PIE) who fail under conventional TPTV ventilation. HFV has also been used in the short-term support of neonates with a congenital diaphragmatic hernia until corrective surgery can be performed.

Volume-cycled ventilators are commonly used on infants who weigh more than 10 kg (22 lb). The most recent generation of conventional volume-cycled ventilators can be used to deliver a tidal volume as small as 2 to 3 mL. They often have chest wall sensor systems that note movement and match that breathing effort with a machine delivered breath. Volume-oriented ventilation can also be performed with a neonatal TPTV-type ventilator if the patient is apneic. This technique is discussed later.

When selecting a ventilator, it is important to choose one that offers the features needed to ventilate the patient. For example, if real-time graphics are needed to evaluate the patient’s response to a ventilator adjustment, the proper unit will be needed. Any selected ventilator must be able to meet the typical tidal volume goal for a neonatal of 4 to 6 mL/Kg or small pediatric patient of 6 to 8 mL/Kg. A patient receiving HFV for an FDA-approved pulmonary problem has a smaller tidal volume goal.

a. Initiate and adjust continuous mechanical ventilator settings (Code: IIID2b) [Difficulty: ELE: R, Ap; WRE: An]

b. Recommend changes in mechanical ventilation to modify ventilator techniques (Code: IIIG3e) [Difficulty: ELE: R, Ap; WRE: An]

c. Independently modify ventilator techniques (Code: IIIF2i5) [Difficulty: ELE: R, Ap; WRE: An]

With traditional neonatal TPTV, a set tidal volume is not ordered and cannot be measured. With volume-cycled ventilation of a pediatric patient, the tidal volume is ordered just as it is with an adult patient. Typically the therapist is expected to set the following ventilator controls based on a protocol or the patient’s condition and response to the ventilator: sensitivity, flow, I : E ratio, alarms, and humidified gas temperature. Specific examples of ventilator settings, based on pathological problem, will be presented next.

d. Sensitivity

e. Flow

Flow is adjusted to set the inspiratory time, the I : E ratio, or to meet the patient’s needs to better synchronize breathing efforts with the ventilator. The flow should be increased if the patient appears to be attempting to inhale faster than the ventilator is delivering the tidal volume.

f. I : E ratio

g. Alarms

Alarm systems are different for each type of ventilator. Generally speaking, they are set with a safety margin of ±10% from the patient’s normal ventilator settings. A variation of greater than 10% results in an audible or visual alarm condition.

h. Humidified gas temperature

The goal for most patients is to minimize their humidity deficit by giving gas that is humidified and warmed to near body temperature. It is common to have the gas warmed to 90° to 95° F (about 35° C). It should be measured in the inspiratory limb of the circuit as close to the patient as possible.

A review of the literature dealing with neonatal critical care shows that there are few universally accepted strategies for the use and modification of CPAP/PEEP and mechanical ventilation. The discussion presented here is an attempt to describe what are widely accepted approaches to neonatal TPTV and related therapy. The approach used in this discussion focuses on adjusting the ventilator and treating a neonate based on its pathologic problem.

a. Patients with normal cardiopulmonary function

Patients with normal cardiopulmonary function may need mechanical ventilation because of apnea from anesthesia, paralysis, or a neurologic condition. The initial TPTV ventilator parameters for this type of patient are listed in Box 16-3. Once mechanical ventilation is established, it is important to evaluate the patient’s blood gases, vital signs, breath sounds, and any other pertinent clinical information before changing any ventilator parameters.

As the patient recovers and begins to breathe spontaneously, it will probably be necessary to reduce the ventilator-delivered minute volume. This encourages the child to breathe more because the final goal is to completely wean and extubate the patient. The most accepted way to reduce the ventilator-delivered minute volume is to reduce the ventilator rate. A reduction of about 10% is a good starting place but must be tailored to meet the patient’s needs. The tidal volume is maintained as originally set. Obtain a set of blood gases in 10 to 20 minutes (or follow the transcutaneous or pulse oximetry values), and check the patient’s vital signs to see how well the adjustment is tolerated.

If the blood gases show an elevated PaCO₂, the ventilator-delivered minute volume must be raised. Do this by increasing either the alveolar ventilation or respiratory rate. Alveolar ventilation can be increased by increasing either the inspiratory flow or the pressure limit (if it has been reached) to increase the tidal volume. The ventilator rate may be increased if the flow and pressure limit cannot be increased. An increase of about 10% is a good starting place but must be tailored to meet the patient’s needs. As before, blood gases and vital signs should be monitored after every change to see if the increase is well tolerated and accomplishing what was intended.

If the blood gases show that the PaCO₂ is lower than desired, the ventilator-delivered minute volume must be decreased. The first parameter to adjust is usually the rate. Try decreasing the rate by about 10% and check another set of blood gas values. If other parameters need to be reduced, try decreasing the peak inspiratory pressure, inspiratory flow, or inspiratory time about 10% to decrease the tidal volume. Again, check the blood gas values after every adjustment.

If the blood gases show that the Pao₂ is higher or lower than necessary, the oxygen percentage must be adjusted. An increase or decrease of about 5% is a good starting place but must be adjusted as needed. If the patient does not respond to the increased oxygen as expected, the patient should be reevaluated. It may be necessary to reclassify him or her into one of the following categories.

Math Review

CALCULATION OF ESTIMATED TIDAL VOLUME DURING TPTV MECHANICAL VENTILATION

The NBRC examination content outline does not specifically list estimated tidal volume calculations. However, the information may be useful in understanding concepts presented later in the text.

If the neonatal patient receiving TPTV is apneic and neither assisting nor fighting against the ventilator-delivered breath, it is possible to calculate an approximate tidal volume. This is referred to as volume-oriented ventilation. The following formula is used:

In which

T_I = inspiratory time

(It is important that either the pressure limit is not reached or the pressure limit is reached at the same time inspiratory time is completed. If the pressure limit is reached before the inspiratory time limit is reached, part of the inspiratory time is spent as an inflation hold, and no additional tidal volume is delivered.)

= inspiratory flow rate on the ventilator in mL/sec

Vc = volume compressed in the circuit and ventilator

(This is found by multiplying the peak inspiratory pressure by the manufacturer’s stated compliance factors for the circuit and ventilator.)

For example, estimate the delivered tidal volume for an apneic 5 kg infant. The ventilator parameters are inspiratory flow of 5.5 l/min, frequency of 20/min, I : E ratio of 1 : 3, inspiratory time of 0.75 seconds, and expiratory time of 2.25 seconds. Peak inspiratory pressure (PIP) is 15 cm water. The internal compliance of the ventilator is 0.4 mL/cm water, and the circuit compliance factor is 1.6 mL/cm water.

In which:

T_I = .75 seconds

= 5.5 L/min. This is converted to mL/sec by dividing the flow in L/min by 60 seconds. So 5.5 L/min = .092 L/sec or 92 mL/sec:

Vc = 0.4 + 1.6 mL/cm water = 2 mL/cm water = 2 mL/cm water × 15 cm water PIP = 30 mL

Therefore,

It must be emphasized that this is only a calculated tidal volume. Leaks in the system, a decrease in the patient’s compliance, or an increase in the patient’s resistance decreases the true tidal volume. Conversely, an increase in the patient’s compliance or a decrease in the patient’s resistance increases the true tidal volume. Also, if the pressure limit is reached before the inspiratory time is completed, less volume than expected will be delivered. This is because part of the inspiratory time is spent as an inflation hold and no additional tidal volume is delivered (Figure 16-5). Finally, the infant must be completely passive during the delivery of the breath.

Figure 16-5 Pressure-time curve seen with pressure limit set at 25 cm water and inspiratory time being increased from 0.5 to 1 second. Pressure limit is reached when inspiratory time is about 0.5 second. As inspiratory time is increased to 1 second (or greater), the pressure limit is held at 25 cm water resulting in a “square wave” pressure curve.

(From Betis P, Thompson JF, Benjamin PK: Mechanical ventilation. In: Koff PB, Eitzman DV, Neu J: Neonatal and pediatric respiratory care, ed 2, St Louis, 1993, Mosby.)

b. Patients with decreased lung compliance and normal airway resistance such as infant respiratory distress syndrome (RDS)

Although RDS in infants weighing less than 1000 g is the most common cause of decreased lung compliance with normal airway resistance, stiff lungs are also found in patients with other lung conditions such as pneumonia and pulmonary edema (Figure 16-6). The greatest challenge presented in the care of these infants is to oxygenate them without causing oxygen toxicity or pulmonary barotrauma. Common recommendations for the initial ventilator settings are listed in Box 16-4. As discussed earlier, blood gases, vital signs, and so forth must be monitored after the infant is placed on the ventilator. Any further adjustments can then be determined and evaluated by another set of blood gases and vital signs.

Figure 16-6 Comparison of the lung volumes and capacities of a 6-year-old child, normal infant, and an infant with IRDS. Note relatively high compliance and low resistance of a normal child compared with an infant, and infant with RDS compared with normal infant.

(From Chatburn RL, Lough MD. Mechanical ventilation. In: Lough MD, Doershuk CF, Stern RC, editors: Pediatric respiratory therapy, ed 3, St Louis, 1985, Mosby.)

The issue of time constants of ventilation helps to better explain the various options available for adjusting the ventilator. As presented earlier, the time constant of ventilation (T_c or time constant of the respiratory system T_RS) is calculated as the product of compliance and resistance. For example, using the following values for a mechanically ventilated neonate with RDS, its time constant is calculated as

Neonates with RDS have a relatively short time constant; therefore the tidal volume and ventilating pressure are delivered rather quickly to the lungs. However, because the lungs are so stiff, there is usually no problem with the tidal volume being fully exhaled as long as five time constants are allowed. An expiratory time of at least 0.5 seconds is usually set initially. The various options available for increasing oxygenation are discussed on the following pages.