Role of Positive Pressure Ventilation in Neonatal Resuscitation

Normal Transition and Initial Spontaneous Breathing

In healthy, newborn term infants, normal transition is characterized by repeated, short, deep inspirations, each followed by a prolonged expiratory phase through a partially closed glottis (crying). The inspiratory efforts generate large negative trans-pulmonary pressures, typically −50 cm H ₂ O to −100 cm H ₂ O, such that the high-resistance liquid filling the infant's airways is driven distally, moving the air-liquid interface toward the distal airways.

Detailed imaging of rabbit lungs demonstrates that initial lung aeration depends on the generation of this transpulmonary pressure, with consequent stepwise increases in FRC over three to five repeated inspiratory efforts. More volume is inspired than expired with each breath, which overcomes small losses in FRC between inspiratory efforts as lung liquid moves back into airways. The lung liquid then moves into the interstitial tissue and is cleared over the following hours.

Prolonged expiration through a partially closed glottis with simultaneous abdominal muscle contraction pressurizes the chest, further pushing back the air-liquid interface; this is called expiratory braking ( Fig. 32.1 ). Recordings of the first breaths in preterm infants have also shown that they use crying and expiratory braking to facilitate lung recruitment and development of FRC ( Fig. 32.2 ).

Delayed Transition and Need for Respiratory Support

Some infants do not breathe or have ineffective breathing at birth. Guidelines recommend initial steps of umbilical cord clamping, warming, drying and stimulating the baby, and opening, and in some cases clearing, the airway. They stipulate that positive pressure support should be commenced by 60 seconds of age if the baby remains apneic, has irregular or gasping respirations, or has a heart rate less than 100 beats/min (bpm). However, following these recommendations may not always be straightforward.

• Healthy babies may not take their first breath immediately after birth; there may be a delay of 30 seconds or longer.

• In practice, initial evaluation often takes longer than 60 seconds.

• In healthy term and preterm babies, the median heart rate at one minute is <100 bpm, rising to ~140 bpm by 2 minutes of age.

• Clinical assessment of heart rate at birth, either by palpation or auscultation, is intermittent and inaccurate.

• Assessment of a newborn's color is subjective and unreliable. Although assessment of color is not used in the most recent resuscitation guidelines, it remains part of the Apgar assessment.

These factors make assessment at birth, and the decision whether to intervene, more complex. If decisions are made too early, unnecessary interventions may be applied. Whereas, if decisions are delayed, there may be further cardiorespiratory compromise. It may be helpful to be aware that:

• Accurate measurements of oxygen saturation (SpO ₂ ) and heart rate can be obtained by about 90 seconds using pulse oximetry, and heart rate values may be seen within 1 minute using electrocardiography.

• The key sign of an infant's condition in the minutes after birth, and response during stabilization, is the heart rate.

• An infant who has good tone is unlikely to be severely hypoxic.

Basics of Positive Pressure Support

When an infant fails to establish spontaneous breathing after birth, the caregiver must commence positive pressure support. Typically, this is initially applied using a face mask and a pressure-generating device. If the infant is breathing but has not completely established regular, effective breathing, continuous positive airway pressure (CPAP) may aid inflation of the lungs and establishment of FRC. CPAP helps establish and maintain end expiratory lung volume, reduce alveolar collapse, and decrease work of breathing. It aids lung expansion and helps to conserve surfactant. CPAP also improves oxygenation, lung compliance, and ventilation-perfusion mismatch. However, an infant who has no respiratory effort, or who is bradycardic, will require positive pressure ventilation (PPV). Ideally, PPV should be given along with positive end expiratory pressure (PEEP), although not all resuscitation devices are capable of delivering PEEP ( Table 32.1 ). Studies in intubated preterm animals have demonstrated that the addition of PEEP to PPV results in more rapid acquisition of FRC, improved oxygenation and lung compliance, and decreased lung injury. PEEP is recommended for the resuscitation of preterm infants, although clinical trials have failed to show a significant difference in the proportion of infants requiring intubation in the delivery room, or in SpO ₂ at 5 minutes of age when PEEP was added. Use of PEEP during stabilization of preterm infants in the delivery room in major units in the UK and Canada is now reported to be higher than 75%.

Endotracheal intubation should be considered for infants who have an ongoing need for PPV and those who remain bradycardic and/or hypoxic in spite of adequate mask PPV. The management of preterm infants who require ongoing respiratory support has evolved over the last decade, moving away from immediate intubation and surfactant administration toward continuing CPAP support with either rescue intubation or surfactant treatment if certain criteria are met, or surfactant administration without endotracheal intubation plus ongoing CPAP. Meta-analysis of trials that compared immediate intubation with CPAP found a reduction in the combined outcome of death or bronchopulmonary dysplasia in the CPAP group, without any difference in pneumothorax risk. Both options may be appropriate, however, international guidelines now recommend initial CPAP for spontaneously breathing preterm infants with respiratory distress rather than routine intubation. As a gentler approach to early respiratory support has become more popular, more preterm infants are likely to be supported by CPAP in the delivery room.

Application of Noninvasive Positive Pressure Support

How to Generate Positive Pressure

Various devices are available for generating positive pressure in the delivery room. The choice of device may be made based on availability of a gas supply, the skills of the resuscitator, the desire to deliver sustained inflations, PEEP and CPAP, and on local preferences. A European survey reported that centers typically use more than one device, with self-inflating bags (SIB) being most commonly used (85%), whereas a recent Japanese survey reported flow-inflating bags (FIB) to be most commonly used (63%).

Self-Inflating Bags (SIB)

Self-inflating bags ( Fig. 32.3 A1 and A2 ) re-expand after compression. These are the only devices that can be used without a gas supply; they have been shown to be an effective method for reducing mortality from birth asphyxia in resource-limited areas. Their use is an integral part of the international Helping Babies Breathe program.

Several types and sizes of SIB exist. The smallest sizes, ~240 mL, are most appropriate for newborns. The peak pressure delivered by an SIB depends on how hard and fast the bag is squeezed. SIBs usually incorporate a valve, which limits the maximum deliverable pressure. The valve can be manually overridden to deliver higher pressures but can be inadvertently overridden if the SIB is squeezed very hard or very fast. Pressures greater than 100 cm H ₂ O have been reported, resulting in very high delivered volumes, >20 mL/kg. The more fingers used to squeeze the bag, the more pressure is generated ; for most resuscitations, a gentle squeeze with a finger and thumb is all that is required. Many studies, including Bassani et al., have shown that it is difficult to give consistent peak pressures when using an SIB, particularly if an operator is inexperienced. Manometers attached to the SIB help improve the consistency of peak pressure delivery, and newer “upright” designs of SIB have been shown to provide more consistent volume and pressure delivery, even with novice operators. If a PEEP valve is attached to an SIB, some PEEP can be delivered. However, it is difficult to achieve the desired PEEP, and delivered PEEP is rate dependent. PEEP levels decay quickly between inflations, although newer models are better. Different brands of SIB have different capacities to deliver a sustained inflation ; closing the pressure relief valve improves the ability to perform sustained inflation. It is not possible to deliver CPAP using an SIB, therefore, it may not be the optimal device for stabilizing preterm infants.

Flow-Inflating Bags (FIB)

A flow-inflating bag (see Fig. 32.3 B ) needs a continuous gas supply, something which may not always be readily available. The delivered pressure and tidal volume depend on how hard the bag is squeezed. A pressure-limiting valve can be attached to prevent high pressure being inadvertently delivered. PEEP can be delivered with an FIB by controlling the rate of gas escaping from the back of the bag during expiration. This technique requires experience as it can inadvertently lead to dangerously high PEEP. It is difficult to consistently deliver both the desired PEEP and peak pressures with a flow-inflating bag, and many operators find the flow-inflating bag more difficult to use than the SIB. A sustained inflation can be delivered by a skilled operator, but the pressure achieved is more variable than that delivered using a T-piece. It is very difficult to deliver CPAP with an FIB.

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