Cardiovascular system The newborn has the highest cardiac output per weight than any other age group (200 ml/kg/min). The major differences in myocardial function between the neonate and the adult heart translate into important clinical applications. The high content of collagen and high ratio of Type I to type III collagen may account for the relative non compliance of the neonatal heart and consequently, its limited capacity to handle a volume load. The Frank Starling response is limited and the heart rate is the critical factor in maintaining cardiac output in the newborn. The normal range of heart rate in a neonate is 120 – 180 beats/min. Neonate
Adult
Cardiac output
Rate dependent
Stroke volume and rate
Contractility
Reduced
Normal
Starling response
Limited
Normal
Catecholamine response
Reduced
Normal
Compliance
Reduced
Normal
Table 2: Cardiovascular differences between a newborn and an adult Parasympathetic control of heart rate matures earlier in gestation and to a greater extent than beta adrenergic control. For this reason, neonates may not respond to hypovolemia or an inadequate depth of anesthesia with tachycardia. Additionally, the vagotonic response caused by laryngoscopy and succinylcholine and synthetic opioids may lead to bradycardia. These reflexes can be offset by the vagolytic effects of atropine. The newborn also requires a higher extracellular calcium concentration to achieve maximal contractility. Extracellular, rather than intracellular calcium has a primary role in control of contractility of the neonatal myocardium. Neonates are more sensitive to the negative inotropic effects of anesthetic agents than older children. Any factor which increases the pulmonary vascular resistance in a neonate (hypoxia, hypercarbia, acidosis) may result in a return to the fetal-type circulatory pattern with shunting of deoxygneated blood from the right to left side of the heart through the PFO and the PDA.