Page 63 - Cardiac Nursing

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CHAPTER 1 / Cardiac Anatomy and Physiology 39

slower rate than during rapid ejection. Blood continues to flow Right Ventricular Cardiac Cycle
into the aorta. This period of reduced ventricular ejection com-
prises approximately the latter two thirds of the total ejection The sequence of events in the right ventricle during the cardiac
period (see Table 1-6). Atrial pressure and volume continue to cycle is exactly the same as in the left ventricle, but the timing of
increase. Ventricular repolarization is usually complete by this events in the two ventricles is slightly different. Right ventricular
time, as indicated by the end of the T wave. and pulmonary artery pressures are much lower than left ventric-
ular and aortic pressures and right atrial pressures are usually
slightly less than left atrial pressures.
Ventricular Diastole Several factors lead to differences in the timing of events be-
Protodiastole (Initial Part of Period d, Fig. 1-38). As tween the right and left heart. Contraction of the left ventricle be-
ventricular muscle relaxation begins, there is a brief period before gins before contraction of the right ventricle. Left ventricular iso-
ventricular pressure becomes lower than aortic pressure when no volumetric contraction and relaxation last longer than right
blood is being ejected from the ventricle. Blood flow momentar- ventricular isovolumetric contraction and relaxation, presumably
ily reverses. This backflow at a time when ventricular pressure is because the left ventricle must develop more contractile force to
becoming less than aortic pressure facilitates the closure of the overcome higher systemic pressures. Right ventricular ejection be-
aortic valve. The second heart sound occurs. During this time, a gins before, lasts longer than, and ends after left ventricular ejec-
slight transient decrease in atrial pressure may occur, reflecting the tion. Thus, right ventricular filling and ejection periods are longer
effect of ventricular relaxation. than left ventricular periods, but the durations of left and right
ventricular electromechanical systole are almost equal.
Isovolumic Ventricular Relaxation (Latter Part of Period
d, Fig. 1-38). Ventricular pressure decreases rapidly as the ven- Cardiac Valvular Events and
tricle relaxes. There is no change in ventricular volume during this
period when all the cardiac valves are closed. After closure of the Normal Heart Sounds
aortic valve, aortic pressure increases by a few millimeters of mer- Valvular Events
cury, and the incisura or dicrotic notch is noted on the aortic pres- The differences in timing of right and left ventricular events lead
sure tracing. Atrial pressure continues to increase as the atrium to differences in timing of right and left valvular events. The AV
continues to receive pulmonary venous blood.
valves close at the onset of ventricular systole. The mitral valve
Rapid Ventricular Filling (Period e, Fig. 1-38). The AV normally closes before the tricuspid valve because left ventricular
(mitral) valve opens when atrial pressure exceeds ventricular pres- contraction begins before right ventricular contraction.
sure. The ventricle fills rapidly with blood that has been accumu- The aortic and pulmonic valves open when ventricular pres-
lating in the atrium, but ventricular pressure continues to decrease sures exceed arterial pressures. The pulmonic valve opens before
during this period because ventricular relaxation continues. Most of the aortic valve. Right ventricular isovolumetric contraction is
the blood that was sequestered in the atrium during systole is emp- shorter than left ventricular isovolumetric contraction.
tied into the ventricle by the time the ventricle reaches maximal di- The aortic and pulmonic valves close when ventricular pres-
astolic size. Atrial pressure decreases as the atria empty but remains sures fall below arterial pressures. The aortic valve closes before the
slightly greater than ventricular pressure throughout this period. pulmonic valve. The right ventricular ejection period is longer
than the left.
Late Diastole (Diastasis; Period f, Fig. 1-38). The mitral The AV valves open during diastole when ventricular pressures
valve remains open, and pressures in the atrium and ventricle are lower than atrial pressures. The tricuspid valve opens before
equilibrate in the time after rapid ventricular filling and before the the mitral valve because of the more rapid isovolumetric right ven-
beginning of atrial contraction. Blood from the lungs continues to tricular relaxation.
enter the left ventricle passively, so ventricular volume and pres-
sure slowly increase. Coronary artery blood flow usually is maxi- Normal Heart Sounds
mal during late diastole. The beginning of atrial depolarization is The specific mechanisms responsible for heart sounds are disputed.
indicated by the upstroke of the ECG P wave.
Sudden accelerations and decelerations of blood, turbulent blood
Atrial Contraction (Period g, Fig. 1-38). Atrial muscle flow, and the movements of valves, heart walls, and blood vessels
contraction follows atrial depolarization and results in an increase may all produce vibrations and sounds audible at the body surface.
in left atrial pressure. Ventricular volume and pressure are in-
creased slightly as the atrium forces much of its remaining blood First Heart Sound. Mitral valve closure and oscillations in
into the ventricles. Between 15% and 25% of the end-diastolic the movement of blood in the ventricles are associated with vi-
ventricular volume consists of blood that has been ejected from brations of the entire valvular apparatus and of atrial and ventric-
the atrium during atrial contraction. The contribution of atrial ular walls. This creates the early components of the first heart
contraction to total ventricular volume depends on venous return sound. Later components of the first heart sound may be due to
and heart rate; it is greater at faster heart rates. This atrial contri- the acceleration of blood ejected into the aorta.
bution to ventricular volume may be lost when the atria and ven- Second Heart Sound. The second heart sound actually be-
tricles are electrically and mechanically dissociated, such as during gins before semilunar valve closure. The mechanisms responsible
atrial fibrillation or complete heart block. Aortic pressure contin- for the second heart sound include arterial blood flow decelera-
ues to decrease as blood in the aorta flows into the periphery. tions caused by ventricular relaxation, blood vessel wall vibrations,
Toward the end of this period, the ventricles begin to depolarize. and semilunar valvular vibrations.
Diastole ends with the onset of ventricular contraction. The car- Pulmonic valve closure follows aortic valve closure and leads to
diac cycle is repeated. a two-component sound, which is accentuated during inspiration.