Respiratory and cardiovascular support in the delivery room

Understanding the transition to newborn life

What is the primary trigger that facilitates cardiopulmonary transition in the newly born infant?

The transition from fetal to newborn life represents one of the greatest physiologic challenges that humans encounter. During fetal life, the lungs are liquid-filled and at birth this liquid must be rapidly cleared from the airways to allow the entry of air and the onset of pulmonary gas exchange. Pulmonary blood flow (PBF) must markedly increase, and several specialized vascular shunts must also close to separate the pulmonary and systemic circulations. While it is often considered that these events are independent, we now know that they are intimately linked. Lung aeration is the primary trigger that not only facilitates the onset of pulmonary gas exchange but also stimulates an increase in PBF, which in turn initiates the cardiovascular changes. The fact that lung aeration triggers the physiologic transition at birth underpins the well-established tenet that establishing effective pulmonary ventilation is the key step in neonatal resuscitation.

How is airway liquid removed from the newborn infant’s lung?

Radiographic imaging studies have demonstrated that lung aeration can occur rapidly (in three to five breaths) and mostly occurs during inspiration in spontaneously breathing newborns or during positive-pressure inflations in ventilated newborns ( Fig. 2.1 ). ^, The hydrostatic pressure gradients generated by inspiration, or positive-pressure inflations, drive liquid from the airways into the surrounding lung tissue. ^, However, as the interstitial tissue compartment of the lung has a fixed volume, the clearance of airway liquid into this compartment during lung aeration increases lung interstitial tissue pressures. Thus, immediately following lung aeration, the neonatal lung is essentially edematous, which affects lung tissue mechanics and increases the likelihood of liquid reentering the airways during expiration. Use of positive end-expiratory pressure (PEEP) during assisted ventilation opposes liquid reentry and ensures that distal airways and gas exchange units remain aerated at end-expiration ( Fig. 2.2 ). As a result, PEEP allows gas exchange to continue throughout the respiratory cycle.

How does the timing of umbilical cord clamping affect the cardiovascular transition at birth?

Before birth, most of the right ventricular output bypasses the lung and flows through the ductus arteriosus into the systemic circulation mainly because pulmonary vascular resistance (PVR) is high. In the fetus, PBF is low and contributes little to venous return and the supply of preload for the left ventricle. Instead, this mainly comes from umbilical venous return via the ductus venosus and foramen ovale. Thus, blood flow through the placenta is vital not only for oxygen and nutrients supply during fetal life but also for delivering left ventricle preload. Clamping the umbilical cord at birth immediately decreases preload, thereby causing a large reduction in cardiac output. Furthermore, cardiac output remains low until the lungs aerate and PBF increases to restore preload for the left ventricle. Recognition of this shift in dependence from umbilical venous return to pulmonary venous return for sustaining ventricular preload at birth has led to the concept of physiology-based cord clamping. That is, if cord clamping is delayed until after the lungs have aerated and PBF has increased, then PBF can immediately replace umbilical venous return as the major source of left ventricular preload as soon as the cord is clamped, with no diminution in supply. This procedure greatly mitigates the large changes in cardiac output associated with umbilical cord clamping before the onset of ventilation. It also greatly reduces the instantaneous increase in arterial blood pressure associated with cord clamping caused by the removal of the low-resistance placental vascular bed. As a redistribution of cardiac output and increased blood flow to the brain is critical for protecting the fetal brain from hypoxia, any constraint on cardiac output caused by a lack of venous return at birth is potentially catastrophic. However, because uterotonic medications given to the mother at birth to contract the uterus also reduce umbilical blood flows ( Fig. 2.3 ), uterotonic administration prior to cord clamping can mitigate some of the benefits of physiological-based cord clamping.

Is pulmonary blood flow increased only in the aerated portions of the newborn’s lung?

At birth, lung aeration triggers the increase in PBF required for facilitating pulmonary gas exchange. As noted earlier, lung aeration is important for replacing left ventricular preload and maintaining cardiac output potentially lost following cord clamping. Imaging studies have shown that partial lung aeration after birth is sufficient to stimulate a global increase in PBF ( Fig. 2.4 ). ^, While oxygen can enhance this response, it is not oxygen dependent. ^, Although partial aeration has the potential to cause large ventilation-perfusion mismatches in the lung at birth, as the high PBF is vital for maintaining cardiac output, this response has major benefits during transition. Indeed, by not limiting the increase in PBF to the degree of lung aeration, venous return and cardiac output are not limited by the degree of lung aeration.

Anticipating and preparing for neonatal resuscitation

How often are resuscitative interventions needed after birth?

The incidence of stabilization and resuscitative interventions after birth varies based on setting and gestational age. Within 30 seconds of birth, most newly born term and late preterm infants initiate respirations spontaneously or in response to drying and tactile stimulation. In a population-based study of all births in three Norwegian hospitals (n = 1507 live-born infants), 4% received positive-pressure ventilation (PPV), 0.4% were intubated, and a very small number received either chest compressions (0.2%) or intravenous epinephrine (0.1%). The probability of requiring resuscitative interventions increases with decreasing gestational age. In the study, 21% of newborns < 34 weeks’ gestation received PPV and 54% of those < 28 weeks’ gestation were intubated. Others have demonstrated that even late preterm newborns (34–36 weeks) have a twofold risk of receiving PPV compared with term newborns.

Can the need for resuscitation be anticipated before birth?

Although the likelihood of requiring resuscitation is higher in the presence of identified risk factors ( Table 2.1 ), individual risk factors and scoring systems that combine risk factors have limited discriminatory power. They identify many births as high risk where subsequently no interventions are needed. Moreover, some newborns require intervention in the absence of any risk factors. In two Canadian cohort studies, approximately 14% of newborns considered low risk and 7% of normal term deliveries without identified risk factors received PPV in the delivery room. ^, Even among very low-birth-weight newborns, antenatal risk factors have low discriminatory power to identify those that will require extensive resuscitation. In a multicenter case-control study of term and late preterm newborns, Berazategui incorporated 10 antenatal and intrapartum risk factors in a multivariate regression model to predict the need for tracheal intubation, chest compressions, or emergency medications in the delivery room. The final model correctly predicted the need for these advanced intervention with 72% sensitivity and 93% specificity. Because the model used a case control design, the results could not be used to directly predict the need for advanced resuscitation in an unselected population. Subsequently, the authors performed a series of bootstrap simulations with the original dataset and have published an online calculator to predict the need for advanced neonatal resuscitation among term and late preterm newborns ( https://medicine.ouhsc.edu/Academic-Departments/Pediatrics/Sections/Neonatal-Perinatal-Medicine/Research/Risk-Calculator ). Because the need for resuscitative interventions cannot always be predicted, trained personnel must be available at every birth to assess the newborn and initiate resuscitation without delay. If the need for advanced resuscitation can be anticipated, a trained team proficient in all steps of resuscitation should be present at the time of birth.

What ambient temperature should be maintained in the delivery room?

The ideal delivery room or operating room ambient temperature that prevents neonatal and maternal hypothermia without increasing the risk of hyperthermia or interfering with medical staff performance has not been established. The World Health Organization (WHO) recommends maintaining the newborn’s temperature between 36.5°C (97.7°F) and 37.5°C (99.5°F). Among term and late preterm newborns delivered by cesarean birth, an operating room temperature of 23°C (74°F) compared to 20°C (68°F) modestly increased admission temperature (mean difference 0.3°C) and decreased the probability of moderate hypothermia (relative risk 0.26). Among extremely low-birth-weight newborns, where hypothermia significantly increases mortality, an environmental delivery room/operating room temperature of 23°C (74°F), in addition to other adjuncts, prevents admission hypothermia. For anticipated births of newborns < 32 weeks’ gestation, the International Liaison Committee on Resuscitation (ILCOR) suggests an ambient temperature between 23°C (74°F) and 25°C (77°F) in combination with adjuncts such as warm blankets, polyethylene plastic bags or wraps, and head caps. In a recent Consensus on Science and Treatment Recommendation (COSTR) update, ILCOR suggested maintaining a room temperature of 23°C (74°F) for infants ≥ 34 weeks’ gestation. For those at low risk of needing resuscitation, skin-to-skin care is suggested immediately after birth. Although WHO recommends a higher (25°C–28°C) environmental temperature, surgeons performing cesarean deliveries report discomfort at this temperature and concern that it would negatively impact their performance.