Cardiac Cycle
The cardiac cycle consists of two interrelated phases: systole, the contraction phase, and diastole, the filling phase. During diastole, the ventricles fill with blood from the atria via open atrioventricular valves. he 526 SECTION II Intervention Strategies for Rehabilitation Coronary circulation. (A) left main (LM); (B) left anterior descending (LAD); (C) left circumlex (CX); (D) right coronary (RCA); (E) posterior descending (PDA).
The branches of the LAD are known as diagonals; the branches of the CX are known as marginals. atrioventricular valves lie between the atria and the ventricles and include the tricuspid valve on the right and mitral valve on the left. he first two-thirds of ventricular filling is passive; during the last one-third the atria contract and push the blood into the ventricles. his contraction is known as the atrial kick. After the atrial kick, diastole ends and the atrioventricular valves close. Systole begins with both the atrioventricular and semilunar valves closed. An initial isovolumetric contraction, similar to an isometric contraction of striated muscle, increases the pressure within the ventricles, and the semilunar valve opens. he LV then undergoes a concentric contraction, causing a volume to be ejected, termed the stroke volume (SV). After the SV is ejected, the aortic valve closes and systole is complete. he cardiac cycle is defined by the presence of normal heart sounds, S1 and S2. Heart sounds are associated with valvular closings; S1 is associated with atrioventricular valve closure, and S2 is associated with semilunar valve closure. Systole occurs between S1 and S2, and diastole occurs between S2 and S1
Stroke is the fourth leading cause of death and the leading cause of long-term disability among adults in the United States. An estimated 7,000,000 Americans older than 20 years of age have experienced a stroke. Each year approximately 795,000 individuals experience a stroke; approximately 610,000 are first attacks and 185,000 are recurrent strokes.
Stroke (cerebrovascular accident [CVA]) is the sudden loss of neurological function caused by an interruption of the blood flow to the brain. Ischemic stroke is the most common type, affecting about 80% of individuals with stroke, and results when a clot blocks or impairs blood flow, depriving the brain of essential oxygen and nutrients.
Four heart valves ensure one-way blood flow through the heart. Two atrioventricular valves are located between the atria and ventricle. The atrioventricular valve, positioned between the RA and RV, is termed the tricuspid valve; the left atrioventricular valve is the mitral valve (also known as the bicuspid valve), located between the left atrium and ventricle. he semilunar valves lie between the ventricles and arteries and are named based on their corresponding vessels (i.e., pulmonic valve on the right in association with the pulmonary artery, and aortic valve on the left relating to the aorta).
Flaps of tissue called leaflets or cusps guard the heart valve openings. he right atrioventricular valve has three cusps and is therefore termed tricuspid, whereas the left atrioventricular valve has only two cusps and hence is termed bicuspid. These leaflets are attached to the papillary muscles of the myocardium by chordae tendineae. the primary function of the atrioventricular valves is to prevent backflow of blood into the atria during ventricular contraction or systole, while the semilunar valves prevent backflow of blood from the aorta and pulmonary artery into the ventricles during diastole. Opening and closing of each valve depends on pressure gradient changes within the heart created during each cardiac cycle.
MORE DETAILS VISTS PHYSIOTHERAPY CLINIC (ROYAL-PHYSIO)
The coronary arteries originate in the sinus of Valsalva located in the wall of the aorta near the aortic valve.The right coronary originates from the area near the right aortic leaflet, the left coronary from the area near the left aortic leaflet. When the aortic valve is open during systole, the origins of the coronary arteries are located behind the aortic leaflets within the wall; when the aortic valve is closed during diastole, the openings of the coronaries are clearly exposed, allowing them to be easily perfused.5 he coronary arteries therefore receive the majority of their blood flow during diastole, unlike the other arteries of the body that are perfused during systole.
The left coronary artery begins as the left main (LM) and then branches into the left anterior descending (LAD) and the circumflex (CX) (Fig. 13.3). he LAD may have further divisions, known as diagonal branches, that come off of the primary LAD. he LAD and its diagonal branches primarily supply the anterior and apical surfaces of the LV, as well as portions of the interventricular septum. he circumflex may also have branches, known as marginal branches. he circumflex and its marginal branches supply the lateral and part of the inferior surfaces of the LV and portions of the left atrium (LA). he right coronary artery (RCA) supplies the RA, most of the RV, part of the inferior wall of the LV, portions of the interventricular septum, and the conduction system. he posterior descending artery (PDA) is most commonly a branch of the RCA and perfuses the posterior heart. If the RCA does not perfuse the posterior heart, the CX will supply this area. When the PDA comes from the RCA, the anatomy is referred to as being right dominant; if the PDA comes from the circumflex, the anatomy is referred to as being left dominant. For physical therapists, there is no clinical importance to whether the anatomy of the myocardium is either left or right dominant.
The inner diameter (i.e., the opening) of the arteries through which the blood flows is the lumen. he size of the lumen is critical for adequate blood flow. A significant narrowing of the lumen, such as that which occurs with a fixed atherosclerotic lesion of CAD, will decrease the available blood supply to the myocardium. Lumen size may also be altered by the smooth muscle within the walls of the arteries, because smooth muscle regulates vasomotor tone of the coronary arteries. Vasodilation will increase lumen diameter as a result of relaxation of smooth muscle, and vasoconstriction will decrease lumenal diameter as a result of smooth muscle contraction. he responsiveness of arterial smooth muscle is also influenced by the integrity of the endothelium, the lining of the coronary artery that is in direct contact with the lumen. The endothelium has a number of normal functions and “plays the central role in controlling the biology of the vessel wall.”10, p. 1,265 Some of these important functions are anti-inflammatory actions, antithrombotic activity, and its influence on vasodilation. Endothelial cells release endothelial-derived relaxing factor (EDRF), which facilitates vascular smooth muscle relaxation. Nitric oxide (NO) is the most prevalent EDRF. An injury to endothelium can result in impaired NO release and a decrease in vasodilation.11 NO release is influenced by many factors, including acetylcholine, norepinephrine, serotonin, adenosine diphosphate, bradykinin, and histamine.The etiology of the clinical condition known as coronary spasm, in which smooth muscle contraction within the walls of the artery results in narrowing of the coronary artery, is not clearly understood. Coronary spasm occurs in arteries that have endothelial injury (e.g., atherosclerosis), as well as in those arteries that appear to be normal but exhibit hyperreactivity to a variety of vasoconstrictor stimuli, such as serotonin and ergonovine, and loss of EDRF.
