Cardiovascular effects of carbon dioxide in ventilated preterm infants

Future perspectives on the use of deformation analysis to identify the underlying pathophysiological basis for cardiovascular compromise in neonates

The assessment of the wellbeing of the cardiovascular status in premature infants has come to the forefront in recent years. There is an increasing realisation that myocardial performance, systemic blood flow and end-organ perfusion (particularly during the transitional period) play an important role in determining short and long-term outcomes in this population. The recent open access series on Neonatologist Performed Echocardiography (NPE) published in this journal outline the necessary techniques for image acquisition and analysis and provide a framework for the potential clinical applications of NPE in neonatal, and specifically preterm care. In this "Future Perspectives" review, we describe the important determinants of adequate cellular metabolism and myocardial performance (e.g. loading conditions, intrinsic contractility and morphological change), we discuss the maladaptive state of the preterm cardiovascular system, and highlight the emerging role that non-invasive echocardiography techniques, such as deformation analysis, serve in identifying the underlying physiological basis for cardiovascular instability.

Left ventricular output and aortic blood flow in response to changes in preload and afterload in the preterm piglet heart

Low systemic blood flow occurs in up to 30% of infants born at less than 30 wk gestation. It is associated with increased morbidity and mortality, and current treatments are ineffective in 40% of cases. The aim of this study was to assess the ability of the preterm heart to respond to the acute shifts in preload and afterload that occur at the time of birth. Myocardial and coronary vascular function was assessed using an isolated working heart model in term (115 days) and preterm (92 days) piglets. Cardiac output/kg body wt in preterm hearts was ∼50% lower than that of term hearts (P = 0.001). Pressure development was similar in term and preterm hearts. Elevations in preload increased cardiac output and aortic flow similarly in term and preterm hearts, demonstrating significant preload "reserve". By contrast, elevations in afterload markedly depressed aortic flow, with a greater proportion of cardiac output being distributed to coronary flow in preterm hearts at high afterloads. The demands of increased workload were associated with greater increases in coronary flow in preterm hearts compared with term hearts. In preterm hearts, exposure to maternal glucocorticoids resulted in increased aortic flow when afterload was below 25 mmHg. These data suggest the preterm heart lacks the functional capacity to acutely adapt to postnatal afterload. To maximize systemic blood flow in preterm infants, treatments limiting afterload, while harnessing significant preload reserve, should be further explored.

Acute systemic complications in the preterm fetus after asphyxia: role of cardiovascular and blood flow responses

1. Poor perfusion of the kidneys and gut, and associated functional impairment, are major problems in the first days of life in very preterm infants. These complications can be associated with a substantial mortality and further problems such as reduced kidney growth and chronic renal problems in later childhood. 2. There is very little information, and consequently considerable debate, about how or even whether to improve perfusion of the vital organs of this most vulnerable group of babies. Current treatments simply do not consistently improve babies' perfusion generally or kidney and gut perfusion and function in particular. 3. In this review we critically examine clinical and experimental evidence that suggests that exposure to low oxygen levels before and during birth may be a significant contributor to impaired systemic perfusion, and highlight areas requiring further research. 4. This knowledge is essential to develop and refine ways of improving perfusion of the kidneys and other vital organs in premature babies.

Low systemic blood flow and pathophysiology of the preterm transitional circulation

The transitional circulation of the preterm infant differs significantly from the term infant. The preterm infant is uniquely at risk of hypotension and low systemic blood flow states due to failure or delay in the normal transitional circulation processes. The maintenance of normal tissue oxygenation requires maintenance of systemic blood flow and normal blood oxygen levels. Reduction in either of these physiological parameters may result in organ damage, leading to complications such as intraventricular haemorrhage and longer term neurodevelopmental disability. The identification and ongoing monitoring of low systemic blood flow in the preterm infant is more challenging than in older children and some of the assessment techniques used in paediatric and adult intensive care settings do not apply to the neonatal setting. The presence of ductal and atrial shunts makes the measurement of cardiac output problematic in the preterm infant so other newer ways of measuring systemic blood flow need to be considered. The proper treatment of hypotensive and low cardiac output states in the preterm infant requires primarily an understanding of the underlying pathophysiology.

Low systemic blood flow in the preterm infant

Low systemic blood flow in the first hours after birth of a preterm infant is an often unrecognized complication. Traditional measures of cardiovascular adequacy used in the neonatal intensive care unit such as capillary refill time and blood pressure may not identify this problem. Longitudinal measurement of systemic blood flow demonstrates a falling off of blood flow in the first 6-12 h after birth, often to less than half of normal, before a gradual return to normal values by 24-48 h of age. Identification and appropriate treatment of this reduction in flow may assist in preventing some of the complications of prematurity.

Accumulation of CO(2) in reservoir devices during simulated neonatal mechanical ventilation

Aerosolized albuterol is frequently administered to mechanically ventilated neonates by metered dose inhaler (MDI) and a reservoir device. These reservoirs are often placed between the Y-piece and endotracheal tube, thereby creating mechanical dead space and increasing the risk of rebreathing carbon dioxide (CO(2)). The objectives of this study were: 1) to quantify CO(2) accumulation in two commonly used reservoirs (ACE(R), Aerochamber(R)-MV) and a bidirectional nonreservoir actuator (Airlife(R) Minispacer) during mechanical ventilation of a neonatal lung model; and 2) to determine the effect of tidal volume (V(T)) on CO(2) accumulation. We hypothesized that the accumulation of CO(2) in these devices is clinically insignificant at the small tidal volumes used in mechanically ventilated premature neonates. The model was constructed to simulate CO(2) exhalation by a ventilated neonate and consisted of a neonatal ventilator circuit (rate = 40/min; peak inspiratory pressure (PIP) = 20 cm H(2)0) attached to a reservoir/actuator and neonatal test lung. The ventilator delivered inspiratory gas (room air) to the test lung, which was vented into the atmosphere by a small adjustable leak. Expiration was simulated by manually ventilating 7.1% CO(2) (partial pressure of CO(2) (PCO(2)) = 48 mm Hg) back through the model. Accumulation of CO(2) within the reservoir/actuator was measured using an end-tidal CO(2) monitor. Each 4-min experiment was conducted at three V(T) (7.5 mL, 15 mL, and 25 mL), and the median PCO(2) was calculated in 0.5-min increments. For V(T) = 7.5 mL, CO(2) accumulated slowly in the ACE(R) and Minispacer(R) and reached a maximum at 4.0 min (PCO(2) = 2.3 mm Hg and 7.3 mm Hg, respectively). In contrast, the Aerochamber(R)-MV rapidly reached a PCO(2) of 9.5-10.0 mm Hg by 1-1. 5 min. A similar trend occurred with V(T) = 15 mL; however, higher partial pressures (approximately 10-12 mm Hg) were achieved with all devices. At V(T) = 25 mL, PCO(2) rose rapidly with the ACE(R), Aerochamber(R)-MV, and Minispacer(R), reaching peaks of 17.2, 12.3, and 20.3 mm Hg, respectively (P < 0.05). In conclusion, accumulation of CO(2) in reservoir/actuator depends on V(T) as well as the chamber design and internal volume. Due to the short duration of use when administering drugs via MDI, accumulation of CO(2) in these devices is not likely to be clinically relevant for the majority of ventilated newborns.

Relationship between blood pressure and cardiac output in preterm infants requiring mechanical ventilation

OBJECTIVE

To assess the contribution of cardiac output in determining the blood pressure of preterm infants and to identify other factors that may be important.

METHODS

Sixty-seven preterm infants requiring mechanical ventilation (median birth weight, 1015 gm: median gestational age, 28 weeks) underwent on echocardiographic study at on average age of 19 hours (range, 7 to 31 hours). Measurements taken included left ventricular ejection fraction, left and right ventricular outputs by means of pulsed Doppler and the diameter of both the ductal and atrial shunt jets with the use of color Doppler as a measure of the size of shunt. Simultaneous measurements of intraarterial blood pressures, mean airway pressure, and inspired fraction of oxygen were recorded.

RESULTS

After we allowed for the influence of ductal shunting, the correlation between the left ventricular output and mean arterial blood pressure was significant but weak (r = 0.38). There were infants with low blood pressures and normal cardiac outputs, and conversely there were infants with low cardiac outputs and normal blood pressure. The infants with a mean arterial blood pressure of less than 30 mm Hg had a significantly lower gestational age (27 vs 28 weeks), higher mean airway pressure (9.0 vs 7.0 cm H2O), larger ductal diameter (1.6 mm vs 0.7 mm) and a lower systemic vascular resistance (163 vs 184 mm Hg/L per minute per kilogram of body weight). Multilinear regression identified higher mean airway pressure and larger ductal diameter as significant negative influences on mean arterial blood pressure, with higher gestational age and higher left ventricular output as significant positive influences.

CONCLUSIONS

Normal blood pressure cannot necessarily be equated with normal systemic now. These data emphasize the importance of other influences, and in particular that of varying systemic vascular resistance, in the determination of blood pressure in preterm infants.

Haemodynamic changes during high frequency oscillation for respiratory distress syndrome

In a crossover trial left ventricular output (LVO), cerebral blood flow velocity (CBFV), and resistance index (RI) of the anterior cerebral artery were compared using Doppler ultrasonography, in eight preterm infants with respiratory distress syndrome (RDS) during conventional mechanical ventilation and high frequency oscillation. LVO was 14% to 18% lower with high frequency oscillation. There were no significant changes in CBFV. On the first day of life there was a trend towards lower RI on high frequency oscillation; the fall in LVO on high frequency oscillation was not related to lung hyperinflation. Changes in ventilation type (from conventional mechanical ventilation to high frequency oscillation, or vice versa) can induce significant LVO changes in preterm infants with RDS.