Cardiac Physiology and Hemodynamic Parameters

The Ejection Fraction (EF) represents the percentage of the total end-diastolic volume that is pumped out with each beat:

$EF = \frac{SV}{EDV} \times 100$

Alternatively, it can be expressed as:

$EF = \frac{EDV - ESV}{EDV} \times 100$

To understand the total volume of blood circulated through the body over time, we look at Cardiac Output (CO). Cardiac output is the product of the volume pumped per beat and the frequency of those beats:

$CO = SV \times HR$

In this equation, Heart Rate (HR) is the number of beats per minute. For a healthy circulatory system, the Venous Return (VR)—the amount of blood returning to the left ventricle—should ideally equal the cardiac output to prevent blood from pooling in the venous system.

Exam Tip: In a physiological range, increasing heart rate increases cardiac output. However, at pathologic heart rates (such as certain arrhythmias), a very high heart rate can actually decrease cardiac output because the shortened diastole reduces filling time, thereby lowering stroke volume.

Hemodynamics: Resistance and Blood Pressure

Blood flow is not just about the pump; it is also about the "pipes" through which the blood travels. This is described by Total Peripheral Resistance (TPR), also known as Systemic Vascular Resistance (SVR).

TPR is the resistance to blood flow from peripheral structures. It is heavily influenced by the diameter of the arterioles:

• Arteriolar vasoconstriction increases TPR.
• Arteriolar vasodilation decreases TPR.

These mechanical factors—stroke volume and resistance—directly govern the components of blood pressure:

1. Systolic Blood Pressure (SBP): The peak pressure during contraction, largely determined by the stroke volume.
2. Diastolic Blood Pressure (DBP): The pressure during relaxation, largely determined by the TPR.
3. Pulse Pressure: The difference between systolic and diastolic pressure ($SBP - DBP$). This value is proportional to the stroke volume.
4. Mean Arterial Pressure (MAP): The average pressure in the arteries during one cardiac cycle. It is calculated as:

$MAP = DBP + \frac{1}{3}(SBP - DBP)$

Worked Example: Calculating MAP

If a patient has a blood pressure of $120/80 \text{ mmHg}$:

• $SBP = 120$
• $DBP = 80$
• $Pulse\ Pressure = 120 - 80 = 40$

Using the formula: $MAP = 80 + \frac{1}{3}(40)$ $MAP = 80 + 13.3 = 93.3 \text{ mmHg}$

Summary of Determinants

The "work" of the heart and its oxygen demand are influenced by four primary determinants:

• Preload: The stretch of the fibers (EDV).
• Afterload: The resistance the heart must overcome to eject blood (influenced by MAP and aortic valve health).
• Contractility: The innate strength of the muscle contraction (indexed by EF).
• Heart Rate: The frequency of contractions.

Increasing any of these factors generally increases the work of the heart and its myocardial oxygen requirement.