Pediatric emergency medicine trisk 0259 0259

CLINICAL AND PATHOPHYSIOLOGIC CONSIDERATIONS
Delivery of oxygen and nutrients is predominantly regulated by cardiac output
and microvascular blood flow. Cardiac output is calculated by multiplying the
stroke volume (volume of blood ejected by the left ventricle) by the heart rate
(ejection cycles per minute). The stroke volume depends on the filling volume of
the ventricle (preload), myocardial contractility and rhythm, and the systemic
vascular resistance (SVR) against which the ventricle is pumping blood to the
body (afterload). Heart rate, preload, contractility, rhythm, and afterload are
regulated through a variety of neurohumoral and metabolic factors, including the
sympathetic nervous system, endogenous catecholamine release, tissue pH and
oxygen tension, nitric oxide, reactive oxygen species, and calcium homeostasis,
that themselves respond to changes in blood volume, circulating inflammatory
cytokines, and endothelial cell activation ( Fig. 10.2 ). Infants require special
consideration, as they have relatively little reserve myocardial contractility to
increase stroke volume and thus depend almost exclusively on heart rate to
increase cardiac output.