CARDIAC PHYSIOLOGY
Alexandra Haraldsson PA-C
Myocardium
Myocardium is the muscular tissue of the heart composed of specialized involuntary muscle cells known as cardiomyocytes. The myocardium is a thicker layer of cells lying between the thinner epicardium (outer layer) and the endocardium (inner layer).
Cardiomyocytes are the main type of cell of the heart and are responsible for the contractile function that enables it to pump blood. The overlapping spiral arrangement of these cells allow for contraction in a “wringing” fashion to permit more efficient emptying of the cardiac chambers. The myocardium is thinner in the atria and thicker in the ventricles, with the left ventricle being the thickest. The right ventricle is similar in size to the left but the muscle wall is thinner due to the shorter distance the blood must travel.
Oxygen is supplied to the myocardium via epicardial blood vessels. Whereas most arterial blood is supplied to an organ during systole, the coronary arteries are filled during diastole. There are right and left main coronary arteries. The left main further branches into the left anterior descending artery (LAD), which provides blood supply to the anterior and superior septal regions, and the circumflex (Cx) which supplies the lateral wall via obtuse marginal branches. The posterior wall of the left ventricle and the inferior septum is supplied by the posterior descending artery (PDA). In most hearts (80-85%) the PDA is a branch of the RCA or is right dominant. In the remaining 15-20% it branches off the circumflex artery.
Conduction System
In addition to the cardiomyocytes, the heart also has another type of cells called pacemaker cells. These cells are unique in their ability to spontaneously generate an electrical impulse without input from the nervous system, leading to the organized depolarization of the myocytes that results in coordinated contraction of the atria and the ventricles.
Sinoatrial (SA) Node
The cells of the SA node depolarize more rapidly than other areas of the heart and thus set the heart rate. Normally fires about 60-100 beats per minute (bpm). This creates an electrical impulse that causes adjacent cardiomyocytes to depolarize across the atrium leading to contraction.
Atrioventricular (AV) Node
These cells have no intrinsic pacemaker ability but serve to slow down the impulse to allow atrial contraction to complete the filling of the ventricles.
Bundle of His
The impulse then travels down the Bundle of His, located in the interventricular septum, to the right and left bundle branches. The bundle of His possesses pacemaker cells that can fire at a rate of 40-60 bpm, providing back-up in the event the SA node fails to generate an impulse.
Right and Left Bundle Branches
The right bundle branch innervates the right ventricle and the left bundle branch the left. The left bundle branch divides into the posterior and anterior fascicles.
Purkinje Fibers
Theses fibers become con1nuous with the myocardium and conduct the electrical impulse into the muscle fibers causing ventricular contraction. These fibers are also able to generate an intrinsic rate of 20-40 bpm.
Action Potential
Depolarization and repolarisa1on of the cardiomyocytes refer to the electrical event of the cardiac muscle cell that (usually) results in contrac1on. To generate this electrical impulse, there has to be a difference in electrical charges. This is done by exchange of electrolytes (sodium, calcium, and potassium) across the cell membrane of myocardial cells creating electrical activity. This happens in a 4-phase cycle known as the action potential which reflects the difference in the concentration of these ions across the cell membrane at any time.
This electrical activity of the heart is viewable on a monitor using external electrodes.
Cardiac Cycle & Blood Flow
The heart acts as a pump to circulate deoxygenated blood to the lungs and oxygenated blood to the rest of the body. The right atrium receives deoxygenated blood from superior vena cava (SVC) and inferior vena cava (IVC). The blood passes through the tricuspid valve into the right ventricle. The blood then traverses the pulmonic valve into the pulmonary artery and is delivered into the lungs for gas exchange. The newly oxygenated blood is returned to the left atrium via right and left pulmonary veins where it then crosses the mitral valve into the left ventricle, across the aortic valve and into the systemic circulation.
The rhythmic sequential contraction of the myocardium to circulate blood has two phases: Systole and Diastole.
Systole is the phase during which the ventricles contract with the aortic & pulmonic valves open to propel blood into the pulmonary and systemic circulation. The mitral & tricuspid valves are closed to prevent backflow into the atria.
Diastole is when the ventricles relax and fill passively, the mitral & tricuspid valves are open, and the pulmonic & aor1c valves are closed. During diastole about two thirds of the blood leaving the atria fills the ventricles passively. The remaining third is propelled into the ventricle due to atrial contraction.
This “atrial kick” accounts for about 10-30% of cardiac output!!
Post operatively when we are ventricularly pacing and the patient has “no atrial kick” you can often see significant hypotension.
Cardiac Output
Cardiac output (CO) is the volume of blood circulated by the heart by unit of time. It is described by the equation CO=HR x SV where HR is heart rate and SV is stroke volume, or the volume of blood ejected by the left ventricle with every contraction.
There are three main determinants of stroke volume:
1.) Preload: the filling pressure of the ventricle as determined by the amount of venous re-turn. The volume of fluid available to fill the ventricle (how full is the tank?).
2.) Afterload: The force on the ventricular wall, the column of fluid the ventricle must eject against. This is mainly determined by systemic vascular resistance (SVR).
3.) Contractility
Normal CO in adults is 4-8 L/ min.
A more useful parameter for evaluating tissue perfusion in cardiac surgery patients is the Cardiac Index (CI) which considers the patient’s size. This is calculated by dividing the CO by the patient’s body surface area CI=CO/BSA.
A CI of >2 L/ min/ m² is accepted as the minimal requirement for effective perfusion.
Rate Control
The heart is innervated by both the sympathetic and parasympathetic nervous systems. Stimulation of the sympathetic nerve fibers results in the release of norepinephrine which causes an acceleration of heart rate and increased force of ventricular contraction. Stimulation of the parasympathetic fibers has an opposing effect by releasing acetylcholine which slows heart rate by inhibiting the rate of firing at the SA node and slowing transmission of the impulse through the AV node.
References:
1.Soltoski P, Karamanoukian H, Salerno T: Cardiac Surgery Secrets. Philadelphia: Hanley & Belfus, 2000
2.Bojar R: Manual of Periopera1ve Care in Adult Cardiac Surgery, 4th ed. Massachusejs: Blackwell, 2003
3.Aehlert B: ECGs Made Easy. Mosby, 1995.
4.Jakoi E: CV 1. HEART ELECTRICAL ACTIVITY
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