Divergent effects of initial ventilation with delayed cord clamping on systemic and pulmonary arterial flows in the birth transition of preterm lambs

Extended period of ventilation before delayed cord clamping augments left-to-right shunting and decreases systemic perfusion at birth in preterm lambs

Previous studies have suggested that an extended period of ventilation before delayed cord clamping (DCC) augments birth-related rises in pulmonary arterial (PA) blood flow. However, it is unknown whether this greater rise in PA flow is accompanied by increases in left ventricular (LV) output and systemic arterial perfusion or whether it reflects enhanced left-to-right shunting across the ductus arteriosus and/or foramen ovale (FO), with decreased systemic arterial perfusion. Using an established preterm lamb birth transition model, this study compared the effect of a short (∼40 s, n = 11), moderate (∼2 min, n = 11) or extended (∼5 min, n = 12) period of initial mechanical lung ventilation before DCC on flow probe-derived perinatal changes in PA flow, LV output, total systemic arterial blood flow, ductal shunting and FO shunting. The LV output was relatively stable during initial ventilation but increased after DCC, with similar responses in all groups. Systemic arterial flow patterns displayed only minor differences during brief and moderate periods of initial ventilation and were similar after DCC. However, an increase in PA flow was augmented with an extended initial ventilation (P < 0.001), owing to an earlier onset of left-to-right ductal and FO shunting (P < 0.001), and was accompanied by a pronounced reduction in total systemic arterial flow (P = 0.005) that persisted for 4 min after DCC (P ≤ 0.039). These findings suggest that, owing to increased left-to-right shunting and a greater reduction in systemic arterial perfusion, an extended period of ventilation before DCC does not result in greater perinatal circulatory benefits than shorter periods of initial ventilation in the birth transition. KEY POINTS: Previous studies suggest that an extended period of initial ventilation before delayed cord clamping (DCC) augments birth-related rises in pulmonary arterial (PA) blood flow. It is unknown whether this greater rise in PA flow is accompanied by an increased left ventricular output and systemic arterial perfusion or whether it reflects enhanced left-to-right shunting across the ductus arteriosus and/or foramen ovale, with decreased systemic arterial perfusion. Anaesthetized preterm fetal lambs instrumented with central arterial flow probes underwent a brief (∼40 s), moderate (∼2 min) or extended (∼5 min) period of ventilation before DCC. Perinatal changes in left ventricular output were similar in all groups, but extended initial ventilation augmented both perinatal increases in PA flow, owing to earlier onset and greater left-to-right ductal and foramen ovale shunting, and perinatal reductions in total systemic arterial perfusion. Extended ventilation before DCC does not confer a greater perinatal circulatory benefit than shorter periods of initial ventilation.

Assisted ventilation prior to umbilical cord clamping: Potential benefits, challenges, and research studies

Delayed cord clamping (DCC) is beneficial for many infants, and the American College of Obstetrics and Gynecology recommends at least 30-60 seconds of DCC for both term and preterm vigorous infants. For newly born infants that are not vigorous, some evidence in animal models suggests that providing assisted ventilation prior to cord clamping (V-DCC) leads to a more stable transition of cerebral, pulmonary and systemic circulation and oxygenation and may confer not only short-term physiologic benefits but perhaps also improvement in clinically important outcomes. This review is based around 7 questions to help the reader understand the physiologic underpinnings and challenges of V-DCC as well as the published and ongoing research studies aimed at determining whether V-DCC is beneficial for preterm or term infants.

Aeration strategy at birth does not impact carotid haemodynamics in preterm lambs

BACKGROUND

The impact of different respiratory strategies at birth on the preterm lung is well understood; however, concerns have been raised that lung recruitment may impede cerebral haemodynamics. This study aims to examine the effect of three different ventilation strategies on carotid blood flow, carotid artery oxygen content and carotid oxygen delivery.

METHODS

124-127-day gestation apnoeic intubated preterm lambs studied as part of a larger programme primarily assessing lung injury were randomised to positive pressure ventilation with positive end-expiratory pressure (PEEP) 8 cmH2O (No-RM; n = 12), sustained inflation (SI; n = 15) or dynamic PEEP strategy (DynPEEP; maximum PEEP 14 or 20 cmH2O, n = 41) at birth, followed by 90 min of standardised ventilation. Haemodynamic data were continuously recorded, with intermittent arterial blood gas analysis.

RESULTS

Overall carotid blood flow measures were comparable between strategies. Except for mean carotid blood flow that was significantly lower for the SI group compared to the No-RM and DynPEEP groups over the first 3 min (p < 0.0001, mixed effects model). Carotid oxygen content and oxygen delivery were similar between strategies. Maximum PEEP level did not alter cerebral haemodynamic measures.

CONCLUSIONS

Although there were some short-term variations in cerebral haemodynamics between different PEEP strategies and SI, these were not sustained.

IMPACT

Different pressure strategies to facilitate lung aeration at birth in preterm infants have been proposed. There is minimal information on the effect of lung recruitment on cerebral haemodynamics. This is the first study that compares the effect of sustained lung inflation and dynamic and static positive end-expiratory pressure on cerebral haemodynamics. We found that the different ventilation strategies did not alter carotid blood flow, carotid oxygen content or carotid oxygen delivery. This preclinical study provides some reassurance that respiratory strategies designed to focus on lung aeration at birth may not impact cerebral haemodynamics in preterm neonates.

Pulmonary hypertension in the newborn- etiology and pathogenesis

A disruption in the well-orchestrated fetal-to-neonatal cardiopulmonary transition at birth results in the clinical conundrum of severe hypoxemic respiratory failure associated with elevated pulmonary vascular resistance (PVR), referred to as persistent pulmonary hypertension of the newborn (PPHN). In the past three decades, the advent of surfactant, newer modalities of ventilation, inhaled nitric oxide, other pulmonary vasodilators, and finally extracorporeal membrane oxygenation (ECMO) have made giant strides in improving the outcomes of infants with PPHN. However, death or the need for ECMO occurs in 10-20% of term infants with PPHN. Better understanding of the etiopathogenesis of PPHN can lead to physiology-driven management strategies. This manuscript reviews the fetal circulation, cardiopulmonary transition at birth, etiology, and pathophysiology of PPHN.