Review of Knowledge for CC Exam: Hemodynamics & Hemodynamic Monitoring

Determinants of Cardiac Output

Cardiac output (CO) is the volume of blood ejected by the left ventricle every minute. CO is determined by the heart rate (HR) and stroke volume (SV). Both HR and SV play critical roles in influencing CO:

• Cardiac Output (CO): The amount of blood ejected by the heart each minute. The normal value is 4 to 6 L/min, calculated as HR × SV.
  • Heart Rate (HR): The speed of the heartbeat, measured in beats per minute (bpm).
    • Too fast (Tachycardia): Shortens filling time, leading to decreased volume and reduced CO.
    • Too slow (Bradycardia): Decreases ejection, resulting in reduced CO.
  • Stroke Volume (SV): The amount of blood ejected with each heartbeat, with a normal range of 60–100 mL/heartbeat, typically the same amount is ejected by both the right and left ventricles.

Determinants of Stroke Volume

The primary factors that determine stroke volume are preload, afterload, and contractility:

• Preload: The stretch on the ventricular myocardium at end-diastole.
  • Determined by the volume left in the ventricles at the end of diastole.
  • Measured using pressures known as “filling pressures”.
• Afterload: The pressure (resistance) against which the right or left ventricle has to pump to eject the blood.
  • Right Ventricular Resistance: Pulmonary Vascular Resistance (PVR).
    • Elevations occur in conditions such as pulmonary hypertension, hypoxia, pulmonary embolism (PE), and pulmonary stenosis.
  • Left Ventricular Resistance: Systemic Vascular Resistance (SVR).
    • Elevations occur in hypertension (HTN), use of vasopressors, aortic stenosis, and hypothermia.
• Contractility: The ability of the myocardium to shorten the muscle fibers.
  • Influenced by electrolyte abnormalities (potassium and calcium), acid-base abnormalities, or myocardial oxygen supply/demand imbalances.
    • Decreased Contractility: Found in conditions such as hyperkalemia, hypocalcemia, myocardial ischemia, administration of negative inotropic medications, hypercapnia, hypoxia, and acidosis.
    • Increased Contractility: Found in conditions such as hypercalcemia, administration of positive inotropic medications, or sympathetic stimulation.

Determinants of O2 Supply to Tissues

The primary goal of the cardiopulmonary system is to maintain a balance between oxygen (O2) supply to the tissues and O2 demand of the tissues. An imbalance can lead to tissue hypoxia, organ failure, and cellular death. O2 supply to the tissues depends on:

• PaO2 (O2 Content): The amount of oxygen in the arterial bloodstream.
• O2 Delivery: How effectively the O2 content is being delivered to the tissues.