USMLE Step 1 - Cardiovascular Physiology & Pathology

Cardiac Output (CO) is the volume of blood pumped by the heart per minute. The equation is: $\text{CO}=\text{HR}\times \text{SV}$, where HR is Heart Rate and SV is Stroke Volume.

The three main determinants of Stroke Volume (SV) are: Preload, Afterload, and Contractility.

Cardiac preload refers to the ventricular end-diastolic volume or pressure, reflecting the stretch of myocardial fibers before contraction. It is primarily determined by venous return.

Cardiac afterload is the resistance the ventricle must overcome to eject blood during systole. It is primarily determined by systemic vascular resistance (SVR) and aortic pressure.

The Frank-Starling mechanism states that increased venous return (preload) stretches cardiac myocytes, leading to a more forceful contraction and increased stroke volume, thereby optimizing cardiac output within physiological limits.

Mean Arterial Pressure (MAP) is the average arterial pressure during a single cardiac cycle. The equation is: $\text{MAP}=\frac{1}{3}\text{SBP}+\frac{2}{3}\text{DBP}$, where SBP is Systolic Blood Pressure and DBP is Diastolic Blood Pressure.

The four phases are:
1. Ventricular Filling: Mitral valve opens, ventricle fills with blood (volume increases, pressure low).
2. Isovolumetric Contraction: Mitral valve closes, aortic valve remains closed, ventricular pressure rises rapidly (volume constant).
3. Ejection: Aortic valve opens, blood is ejected (volume decreases, pressure rises then falls).
4. Isovolumetric Relaxation: Aortic valve closes, mitral valve remains closed, ventricular pressure falls rapidly (volume constant).

Aortic stenosis increases afterload, causing the left ventricle to generate higher peak systolic pressure to eject blood. This results in a taller, narrower pressure-volume loop (reduced stroke volume) and often leads to concentric hypertrophy.