Cardiac cycle

The cardiac cycle is the period of time from beginning of one heart beat to beginning of the next. As such, it consists of two alternating phases  

Atria and ventricles undergo phases of systole and diastole however, the duration of each phase in the chambers differs. In the atria, whose primary function is to receive blood returning to the heart from the veins, diastole is the predominant phase, lasting for almost 90% of each cardiac cycle at rest. In the ventricles, whose primary function is to develop enough force to eject blood into the pulmonary or systemic circulations, systole is much longer lasting and accounts for almost 40% of each cycle at rest. 

A discussion of the cardiac cycle requires the correlation of pressure changes ventricular volume changes valve activity and heart sounds. In this section, the focus will be on the left side of the heart (see Table 13.3). Identical events occur simultaneously on the right side of the heart however, the pressures are lower. 

Ventricular contraction. This process occurs during ventricular systole. When the ventricular myocardium begins to contract and squeeze down on the blood within the chamber, the pressure increases rapidly. In fact, ven- 

Pressures Pa Pv Paorta Pa, Py 0-10 mmHg Paorta = 0 mmHg Pa Pv Paorta Pv increases toward 80 mmHg Pa Pv P aorta PVr Paorta = 120 mmHg Pa Pv Paorta Pv decreases toward 0 mmHg 

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The Cardiac Cycle. The heart (Eig. lb) performs its function as a pump as a result of a rhythmical spread of a wave of excitation (depolarization) that excites the atrial and ventricular muscle masses to contract sequentially. Maximum pump efficiency occurs when the atrial or ventricular muscle masses contract synchronously (see Eig. 1). The wave of excitation begins with the generation of electrical impulses within the SA node and spreads through the atria. The SA node is referred to as the pacemaker of the heart and exhibits automaticity, ie, it depolarizes and repolarizes spontaneously. The wave then excites sequentially the AV node the bundle of His, ie, the penetrating portion of the AV node the bundle branches, ie, the branching portions of the AV node the terminal Purkinje fibers and finally the ventricular myocardium. After the wave of excitation depolarizes these various stmetures of the heart, repolarization occurs so that each of the stmetures is ready for the next wave of excitation. Until repolarization occurs the stmetures are said to be refractory to excitation. During repolarization of the atria and ventricles, the muscles relax, allowing the chambers of the heart to fill with blood that is to be expelled with the next wave of excitation and resultant contraction. This process repeats itself 60—100 times or beats per minute... 

Describe the mechanical events, status of the valves, and pressure changes that take place during each phase of the cardiac cycle... 

Table 13.3 Summary of Events Occurring during Cardiac Cycle...

Diastole is the period in the cardiac cycle in which relaxation of the myocardium and ventricular filling take place. In an individual with a resting... 

Another noteworthy anatomical feature of the arteries is the presence of elastic connective tissue. When the heart contracts and ejects the blood, a portion of the stroke volume flows toward the capillaries. However, much of the stroke volume ejected during systole is retained in the distensible arteries. When the heart relaxes, the arteries recoil and exert pressure on the blood within them, forcing this "stored" blood to flow forward. In this way, a steady flow of blood toward the capillaries is maintained throughout the entire cardiac cycle. 

At rest, the MAP is closer to the diastolic pressure because the diastolic phase of the cardiac cycle lasts almost twice as long as the systolic phase. During exercise when heart rate increases and the length of diastole decreases, systolic pressure contributes more to the MAP. 

The QT interval is a dynamic physiological variable depending on multiple factors such as cardiac cycle length (heart rate), autonomic nervous system activity, age, gender, plasma electrolyte concentrations, genetic variations in ion channels involved in cardiac repolarization. In addition, circadian and seasonal variations of the QT interval have been described [93]. 

The key point of the cardiac cycle diagram is to be able to use it to explain the flow of blood through the left side of the heart and into the aorta. An appreciation of the timing of the various components is, therefore, essential if you are to draw an accurate diagram with which you hope to explain the principle. 

The cardiac cycle diagram is sometimes plotted with the addition of a curve to show ventricular volume throughout the cycle. Although it is a simple curve, it can reveal a lot of information. 

As with all members of its class, propafenone has its major effect on the fast inward sodium current. The IC agents depress over a wide range of heart rates and shift the resting membrane potential in the direction of hyperpolarization. The 1C agents bind slowly to the sodium channel and dissociate slowly. Therefore, they exhibit rate-dependent block. Inhibition of the sodium channel throughout the cardiac cycle will result in a decrease in the rate of ectopy and trigger ventricular tachycardia. 

Fig. 11. Changes In gated NMR spectra during the cardiac cycle. Top panel isovolumic left ventricular pressure In a ferret heart paced at 0.99 Hz in 8 mM [Ca +]. NMR spectra were acquired at the two times indicated on the pressure record (a) 10 ms prior to stimulation (b) 75 ms after stimulation. Middle panel shows gated F NMR spectra (each from 800 acquisitions) recorded at (a) and (b), as indicated. The bound (B) and free (F) peaks of 5F-BAPTA exhibit distinct chemical shifts at approximately 8 and 2 ppm, respectively, downfield from a standard of 1 mM 6-Ftryptophan at 0 ppm. It appears that the free [Ca +] varied during the cardiac cycle. Bottom panel shows gated P spectra (400 scans) acquired at times a and b in the same heart. The major peaks correspond to phosphocreatine (0 ppm), ATP (the three peaks upfield from phosphocreatine), and inorganic phosphate (the small peak at 4-5 ppm) (Reproduced from Marban et al. Circ. Res. 1988 63 673-678 [311] with permission of Lippincott, Williams Wilkins).

The transmembrane potential of cardiac cells is determined by the concentrations of several ions—chiefly sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-)—on either side of the membrane and the permeability of the membrane to each ion. These water-soluble ions are unable to freely diffuse across the lipid cell membrane in response to their electrical and concentration gradients they require aqueous channels (specific pore-forming proteins) for such diffusion. Thus, ions move across cell membranes in response to their gradients only at specific times during the cardiac cycle when these ion channels are open. The movements of the ions produce currents that form the basis of the cardiac action potential. Individual channels are relatively ion-specific, and the flux of ions through them is... 

The ECG consists of the P-wave, the QRS complex, and the T-wave. These components, represented in Figure 4.2, are associated with different aspects of the cardiac cycle atrial activity, excitation of the ventricles, and repolarization of the ventricles, respectively. 

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