Cardiac Physiology Overview

Correlation between electrical and mechanical events

• End of PR interval correlates with atrioventricular valves closure.
• End of T wave correlates with aortic valve closure.
• Third heart sound correlates with rapid ventricular filling.

Ventricular filling phases

• Rapid ventricular filling contributes to 75% of the ventricular volume.
• Atrial kick contributes to 25% of the ventricular volume.

Left ventricular structure and function

• The left ventricle has an ellipsoid shape due to its longitudinal-circumferential-longitudinal muscle orientation.
• Passive ventricular filling is the major component of LV filling.
• Diastasis is an energy-dependent phase that doesn't contribute to LV filling.
• Peak rate of left ventricular wall thinning measured with echocardiography reflects LV diastolic function.

Right ventricular dysfunction

• Left ventricle dilation or left ventricle hypertrophy can lead to right ventricle diastolic dysfunction in an otherwise healthy individual.

Hemodynamic concepts

• Preload is the ventricular load at diastole start.
• Afterload is the LV systolic load after contraction has begun.
• According to Frank-Starling law, a higher end-diastolic volume of 200 ml will result in a higher stroke volume.
• According to Laplace’s law, reducing the radius 1.5 fold will have the most significant reduction in cardiac O2 consumption.

Cardiac hypertrophy in aortic stenosis

• In a patient with aortic stenosis, a 3-fold increase in room pressure and maintained wall stress would result in a 3-fold increase in hypertrophy.

Digitalis in heart failure

• Digitalis is beneficial in heart failure due to its ability to improve the contractility and Frank-Starling relationship.

Cardiac work

• External cardiac work is the total external energy needed for a complete cardiac cycle.
• Draining the heart during cardiopulmonary bypass reduces cardiac external work, minimizing O2 consumption.

Cardiomyocyte structure and function

• Desmosomes connect structural elements between adjacent cardiomyocytes.
• Myofilaments occupy 60% of a cardiomyocyte.

Action potential phases

• IK1 is the principal determinant of diastolic repolarization in the SA node.
• Phase 2 of the action potential is characterized by K outflux less than Ca influx.

Calcium handling and inotropy

• A drug that phosphorylates phospholamban would increase inotropy.
• Actin connects directly to the Z line.
• Myosin is found in the sarcomere’s H band.

Familial cardiomyopathy

• Familial hypertrophic cardiomyopathy is a disease of the cytoskeleton.
• Familial dilated cardiomyopathy is an autosomal dominant disease.

Cardiac hypertrophy

• β1, α1 and β2 receptors mediate cardiac hypertrophy.
• A drug that increases the activity of cardiac adenylyl cyclase increases inotropic function.

Chronic heart failure

• Vasopressin is not involved in the pathogenesis or worse prognosis of chronic heart failure.

Cardiac reflexes

• The carotid sinus reflex has a systolic blood pressure activation point of 180 mmHg and a lowest effective point of 40 mmHg.
• Chemoreceptor sensors are located in the carotid body.
• The vagus nerve carries the afferent transmission of most cardiac reflexes.
• The Bezold-Jarisch reflex is induced by forced expiration against a closed glottis.
• The Oculocardic reflex is the only one of the listed reflexes that is not transmitted by the vagus nerve.

This quiz explores the correlation between electrical and mechanical events in cardiac function, focusing on key concepts such as ventricular filling phases, left ventricular structure, and right ventricular dysfunction. Test your knowledge on the critical role these processes play in heart function and how they interact.