Echocardiographic and Ultrasound Evaluation of Haemodynamic Parameters in Hypoxic Neonates Treated With Hypothermia.

Impact of hypoxia on the cardiovascular system Transitory hypoxia of the myocardium (and its resultant dysfunction), which may, but not necessarily, present clinically, occurs in two thirds of neonates born with perinatal asphyxia. There is no doubt that that this is one of the more frequent cause of circulatory insufficiency. Both an episode of ischaemic hypoxia in the perinatal period and the changes in the distribution of blood may lead to diminished perfusion of the cardiac muscle. An additional but no less important impact on cardiac function is the immaturity of the neonatal myocardium and its reduced contractility secondary to acidosis and hypoxia. Ischaemia and acidosis lead to imbalance in favour of production of endothelin 1, which leads to reduced production of nitric oxide and vasoconstriction of pulmonary vessels and therefore greater pulmonary vascular resistance (PVR) which has a detrimental effect on the already impaired right ventricular (RV) function. The weak RV function and increase PVR impair filling and function of the left ventricle (LV) and thus, they can affect systemic and cerebral blood flow.

The impact of therapeutic hypothermia on the cardiovascular system

TH improves the distant results in cases of moderate and severe HIE and is currently the standard of care for neonates born at or near term (> 35 weeks of gestation). The direct effect of TH on the cardiovascular system includes the following:

• Moderate bradycardia resulting from the decreased effect of the parasympathetic system on cardiac function. Indeed, sinus bradycardia leads to reduced stroke volume and decreased requirement for energy by the myocardium. In turn, administration of inotropes increase metabolic requirements.
• Additionally, TH leads to increased PVR, potentially resulting in a clinical picture of persistent pulmonary hypertension in the neonate (PPHN) or its exacerbation in cases of pre-existing raised PVR. In animal studies, TH was associated with increased PVR, while an increased risk of PPHN with TH was not found in RCTs.
• The resulting RV dysfunction and reduced stroke capacity of the RV leads to reduced pulmonary venous return and therefore inferior filling and stroke volume of the LV. A consequence of the effects of TH mentioned above and of an episode of HI in the perinatal period is therefore exacerbation of respiratory and circulatory failure.

Impact of the warming process on the cardiovascular system following administration of hypothermia