Cardiac Physiology Flashcards
What is the mean arterial pressure?
average pressure for circulation (about 2/3 of systolic pressure). Indicator of adequate tissue perfusion
What two factors determine blood pressure?
cardiac output and systemic vascular resistance
What can you use to assess perfusion at the bedside?
temperature of extremities and urine output
What is the stroke volume?
volume of blood expelled from each ventricle during contraction (EDV-ESV)
What is the ejection fraction?
fraction of blood volume that is expelled during contraction (stroke volume/EDV)
What factors influence EDV?
total blood volume, atrial filling (afib), ventricular compliance, venous tone (gravity/respirations)
What parameters can a PA catheter measure?
CVP, PA Occlusion Pressure (Wedge), PA pressure, systemic and pulmonary vascular resistance, CO, mixed venous saturation
What factors affect stroke volume?
preload, inotropic state (degree of contractility of the myocardium), afterload
What is afterload?
the tension the myocardium is required to develop during contraction OR the resistance the heart muscle must overcome to eject the blood into the arteries (SVR)
What factors affect SVR?
radius of arterioles, blood volume, BP
What increases contractility?
cardiac glycosides (positive inotropes), adrenergic agonists (epi), Ca+, increased HR
What is the Frank Starling Law of the heart?
The more cardiac muscle is stretched within physiological limits, the more forcibly it will contract. Increasing volumes of blood in ventricles increase the stretch & thus the force of contraction.
Greater stretch means more blood volume is pumped out
Describe the depolarization phase of cardiac cycle in relation to membrane potential
Cardiac cell resting membrane potential is -90mv
excitation spreads through gap junctions and fast Na+ channels open for rapid depolarization
Describe the plateau phase of the cardiac cycle in relation to membrane potential
slow Ca2+ channels open, let Ca2+ enter from outside cell and from storage in sarcoplasmic reticulum, while K+ channels close, Ca2+ binds to troponin to allow for actin-myosin cross-bridge formation & tension development
Describe the repolarization phase of the cardiac cycle in relation to membrane potential
Ca2+ channels close and K+ channels open & -90mv is restored as potassium leaves the cell
How does calcium affect contractility?
Increased Ca2+ promotes more cross bridge binding (actin and myosin) and enhances ventricular contractility
How does hypocalcemia affect the heart?
reduced Ca2+ blood levels depress the heart
How does hypercalcemia affect the heart?
Prolong the plateau phase of action potential. Dramatically increase heart irritability. Lead to spastic heart contractions that permit little rest
How does hypernatremia affect the heart?
Inhibits transport of Ca2+ into the cardiac cells, thus blocking heart contraction
How does hyperkalemia affect the heart?
Interferes with depolarization by lowering the resting potential, and may lead to heart block and cardiac arrest
How does hypokalemia affect the heart?
life threatening, heart beats feebly and arrhythmically
What is the most common way to change cardiac output?
increasing or decreasing the heart rate. the stroke volume usually remains relatively constant due to the Frank Starling Law
Describe the sympathetic cardioaccelatory center effects on the heart
projects to motor neurons in the T1-T5 level of the spinal cord. Innervate with the SA and AV nodes, heart muscle, and the coronary arteries. Stimulates heart rate (enhances calcium entry into contractile cells)
Describe the parasympathetic cardioinhibitory center effects on the heart
sends impulses to the dorsal vagus nucleus in the medulla, which in turn sends inhibitory impulses to the heart via branches of the vagus nerves. Project most heavily to the SA and AV nodes. Slows heart rate
What is the Bainbridge reflex?
Acceleration of heart rate by increased right atrial pressure due to stimulation of atrial receptors (ex- when you take a deep breath you get splitting of second heart sound).
What can cause systolic dysfunction?
Coronary artery disease is a common cause of systolic dysfunction. It can impair large areas of heart muscle because it can reduce blood flow to large areas of heart muscle.
What is the most common cause of diastolic dysfunction?
Inadequately treated high blood pressure. HTN stresses the heart because the heart must pump blood more forcefully than normal to force blood into the arteries against the higher pressure. Eventually, the heart’s walls thicken (hypertrophy), then stiffen.
Describe systolic dysfunction
the heart contracts less forcefully and cannot pump out as much of the blood that is returned to it. As a result, more blood remains in the ventricles
Describe diastolic dysfunction
the heart is stiff and does not relax normally. it may be able to pump a normal amount of blood out of the ventricles, but does not allow as much blood to enter its chambers. the blood returning to the heart then accumulates in the veins.
