POSTNATAL CIRCULATION
Postnatal circulation is mainly dependent on rhythmic contractility of the heart (cardiac cycle) and various pressure/resistance gradients between cardiac chambers and peripheral circulation.
Cardiac cycle: Each rhythmic contraction of the heart completes a set of changes in cardiac physiology, termed as cardiac cycle. In adults, each cycle lasts for ~0.8 seconds, including a smaller phase of ventricular systole (~0.3 second) and longer phase of ventricular diastole (~0.5 second). Atrial systole is limited to a smaller fraction of ventricular diastolic period (~0.1 second). During the remaining phase of cardiac cycle, atria remain in diastolic state to accept the venous return from systemic/ pulmonary circulation.
A temporal sequence of various events in cardiac cycle is as follows (Fig. 17.4):
• Onset of atrial systole by generation of electrical impulse from sinoatrial (SA) node, with rapid filling of ventricles,
• Closure of atrioventricular mitral and tricuspid valves, producing first heart sound,
• Isovolemic contraction of ventricles,
• Opening of semilunar aortic and pulmonary valves,
• Rapid ejection of blood from ventricles due to forceful ventricular systole (early systole),
• Slower ejection of blood from ventricles as the pressure gradient between ventricles and great vessels nears equalization and active ventricular contraction ends (late systole),
• Closure of semilunar valves, producing second heart sound,
• Isovolemic relaxation of ventricles,
• Opening of atrioventricular valves,
• Rapid filling of ventricles due to atrioventricular pressure gradient (early diastole),
• Slower filling of ventricles due to reducing atrioventricular pressure gradient (late diastole),
• Onset of atrial systole to restart the cycle.
Pressure and resistance relationships: Pressures in various cardiac chambers and circulation circuits depend on the amount of blood flow and resistance offered by forward flow.
Note that:• Amount of blood flow in systemic and pulmonary circulation is equal in normal children,
• As the systemic circulation provides more resistance to blood flow than the pulmonary circulation, LV pressure is much higher (120/0-10 mm Hg) than in the RV (25/0-6 mm Hg). LV pressure is also higher due to more muscle mass than the RV. Hence, whenever there is an artificial communication between two sides, e.g. VSD or PDA, shunt is from left to right side, unless other abnormalities are present.
• As both atria have negligible muscle mass, atrial pressures are much lower than ventricular pressures and nearly comparable on both sides (0-6 mm Hg in RA and 6-10 mm Hg in LA). Hence, there is no significant shunting in ASD.
Relative pressures/resistance gradients between various points in systemic and pulmonary circulation have important implications in hemodynamic changes during health and disease. Figure 17.5 illustrates normal pressure/resistance relationship between various heart chambers and major vessels, which should be remembered while interpreting origin of clinical signs in heart diseases.
17.2