Overview
This lecture covers key concepts in cardiovascular physiology, focusing on pressure and flow, oxygen delivery, and determinants of cardiac output: preload, afterload, and contractility.
Circulatory Function and Oxygen Delivery
- The cardiovascular system's main goal is to maintain oxygen homeostasis by matching oxygen delivery to tissue demand.
- Perfusion pressure is the arterial pressure minus venous pressure; it ensures blood enters capillaries.
- Oxygen delivery (DO2) is calculated as cardiac output multiplied by blood oxygen content.
- Blood oxygen content is determined by hemoglobin concentration and arterial oxygen saturation, with normal arterial content ≈ 200 mL/L.
- Typical cardiac output is about 5 L/min, leading to a normal oxygen delivery of ~1,000 mL/min.
- Cardiac output = stroke volume × heart rate; stroke volume is the blood ejected per ventricular contraction.
Determinants of Stroke Volume: Preload, Afterload, Contractility
- Preload refers to ventricular filling and the stretch of myocardial fibers at end-diastole.
- Starling’s law: Greater preload increases stroke volume up to an optimal point (Frank-Starling curve).
- Mean systemic filling pressure is the driving pressure for venous return; affected by blood volume and venous tone.
- Central venous pressure (CVP) and pulmonary artery wedge pressure are traditional but flawed preload measures.
- Newer indicators like stroke volume variation and pulse pressure variation better assess fluid responsiveness.
- Contractility is the heart muscle’s intrinsic ability to contract independent of preload; increased by sympathetic activity and certain drugs, decreased by parasympathetic activity and depressants.
- Contractility can be measured by the slope of the end-systolic pressure-volume relationship (ESPVR) or by max dp/dt of arterial waveforms.
Afterload and Hemodynamic Optimization
- Afterload is the resistance the ventricle must overcome to eject blood, influenced by vascular resistance and aortic pressure.
- Systemic vascular resistance (SVR) is calculated from mean arterial pressure, right atrial pressure, and cardiac output.
- Reducing afterload increases stroke volume; increasing afterload opposes ventricular ejection and reduces stroke volume.
- Effective arterial elastance quantifies arterial load as end-systolic pressure divided by stroke volume.
- The intersection of venous return and cardiac function curves determines current cardiac output.
Key Terms & Definitions
- Cardiac Output (CO) — Volume of blood ejected by the left ventricle per minute.
- Stroke Volume (SV) — Amount of blood ejected by the left ventricle per beat.
- Preload — Stretch of ventricular fibers at end diastole; related to end-diastolic volume.
- Afterload — Resistance the ventricle faces to eject blood into the circulation.
- Contractility — Intrinsic ability of cardiac muscle to contract, independent of preload.
- Mean Systemic Filling Pressure (MSFP) — Pressure driving venous return to the heart.
- Central Venous Pressure (CVP) — Pressure in the vena cava near the right atrium.
- Systemic Vascular Resistance (SVR) — Overall resistance of the systemic circulation.
Action Items / Next Steps
- Review pressure-volume loops and Frank-Starling curves.
- Practice calculating oxygen delivery and cardiac output.
- Prepare for discussion on advanced cardiac monitoring techniques in the next session.