Journal of Perinatology | 2021
Towards optimization of cardiovascular stability in neonates with hypertrophic cardiomyopathy: uniqueness of the neonatal cardiovascular system
Abstract
We thank Loomba et al. [1] for their comments and thoughtful appraisal of this small case series of neonates with hypertrophic obstructive cardiomyopathy (HOCM). We share their sentiments that systemic arterial pressure (SAP) alone is an insufficient metric of circulatory health, in particular because of its lack of predictable association with cardiac output in the neonatal period. Although high SAP is often not desirable in neonatal critical care due to the poor tolerance of the immature myocardium to afterload, newborns with HOCM have increased ventricular mass [2] which, not only tolerates high afterload, but may benefit from it. Poor LV compliance may result in complete emptying if left ventricular end-systolic pressure is greater than aortic root pressure, thus exacerbating obstruction. Augmentation of afterload to increase end-systolic volume with systemic vasoconstrictors such as phenylephrine, are established principles used to care for adults with HOCM [3]. The bio-physiological impact of HOCM in our small cohort was exacerbated by concurrent pulmonary hypertension, secondary to high pulmonary vascular resistance [PVR], which leads to poor left atrial preload and potential exaggeration of LV outflow tract obstruction. It is therefore plausible that augmentation of SAP and systemic vascular resistance [SVR] may positively contribute to the observed improvements in LV obstruction and clinical features including reduction in lactate. We agree that mixed venous saturation is a useful adjunct; however, in the neonate there are technical limitations that restrict practicality. Importantly, half our patients had concomitant hypoxic ischemic encephalopathy (HIE) which, due to end-organ-injury, impacts the reliability of lactate trends as a surrogate of systemic perfusion. Nevertheless, plasma lactate fell in all patients (Fig. 1A), which aligns with the improvement in other end-organ markers of systemic perfusion (arterial pH, base deficit, and urinary output). We agree that nearinfrared spectroscopy offers value as a longitudinal tool to refine the use of blood pressure interventional thresholds; however, in patients with HIE the reliability of regional oxygen saturation is questionable due to end-organ injury. The rapid improvement in efficacy of oxygenation, with no additional volume or pulmonary vasodilator support, endorses the likelihood of enhanced oxygen delivery after vasopressin administration. This may have been due to reduced PVR or related to increased transductal pressure gradient due to increased SVR:PVR ratio. Regardless of the mechanism, this is an important change due to the positive effect of increased pulmonary blood flow (PBF) on left heart filling. The increase in right ventricular output (Fig. 1B), used as a surrogate for systemic blood flow (SBF) among patients with left-to-right ductal shunt, argues against the author’s suggestion that PBF solely increased at the expense of SBF. We acknowledge our oversight in leaving out the timing of the echocardiography re-evaluation, which was conducted following a median of 10.5 [6, 18] hours. Although HOCM related to maternal diabetes is typically asymptomatic by 1 month of age [4], natural evolution of the disease is unlikely to have occurred over the short time-course in which clinical improvement was noted. The author’s comments related to rate pressure product (RPP) are very interesting. HR is proportionally related to myocardial oxygen consumption (MVO2). Gobel et al. suggest RPP to be superior in settings whereby HR and * P. J. McNamara [email protected]