cardiac outpu_Physiology class for Mbbst.pptx

Cardiac output_Volume of blood

1. Physiology of Cardiac Output Understanding the Heart's Pumping Power

2. What is Cardiac Output? Defining the Flow Cardiac Output (CO) is the volume of blood pumped by the heart per minute. It's a fundamental measure of the heart's efficiency. The Core Formula CO is calculated as the product of Stroke Volume (SV) and Heart Rate (HR): CO = SV × HR . Resting Averages For a typical adult at rest, CO is approximately 5 liters per minute , though this can vary based on body size and activity levels.

3. Why Cardiac Output Matters Life-Sustaining Delivery CO ensures that oxygen and vital nutrients are efficiently delivered to every tissue and cell in the body. Meeting Metabolic Demands During exercise, the heart can dramatically increase CO, sometimes up to 20 liters/minute , to meet the body's elevated energy needs. Crucial for Organ Function Sustained CO is critical for the optimal functioning of major organs like the brain, kidneys, and muscles. Indicator of Health Impaired CO is a hallmark of heart disease, a leading cause of morbidity and mortality worldwide.

4. Components of Cardiac Output: Stroke Volume 1 Volume Per Beat Stroke Volume (SV) is the amount of blood ejected by the heart with each single heartbeat, typically around 70 mL at rest . 2 Calculating SV SV is the difference between End-Diastolic Volume (EDV - blood in ventricle after filling) and End-Systolic Volume (ESV - blood left after ejection). 3 Three Key Regulators Preload: The stretch on the ventricle before contraction. Afterload: The resistance the heart must overcome. Contractility: The intrinsic strength of the heart muscle.

5. Components of Cardiac Output: Heart Rate Beats Per Minute Heart Rate (HR) is simply the number of times the heart beats in one minute, with a normal resting range of 60–100 bpm . Impact on CO While increased HR generally boosts CO, an excessively high HR (e.g., >180 bpm) can reduce CO because the ventricles don't have enough time to fill adequately. Nervous System Control The autonomic nervous system regulates HR: Sympathetic stimulation (e.g., stress, exercise) increases HR. Parasympathetic stimulation (e.g., rest, relaxation) decreases HR.

6. Preload and Afterload Explained Preload: The Heart's Stretch Preload refers to the volume of blood and the resulting stretch on the ventricles at the end of diastole (filling phase). According to the Frank-Starling mechanism , increased preload leads to increased SV. Factors influencing preload: blood volume, venous return, and ventricular compliance. Afterload: The Resistance to Eject Afterload is the pressure the ventricles must overcome to eject blood into the arteries. An increase in afterload makes it harder for the heart to pump, thus decreasing SV . Factors influencing afterload: arterial pressure and systemic vascular resistance.

7. Contractility and Its Role Intrinsic Muscle Strength Contractility is the intrinsic strength of the heart muscle's contraction, independent of preload. It reflects the heart's ability to pump forcefully. Key Influences Sympathetic stimulation: Neurotransmitters like catecholamines (e.g., adrenaline) enhance contractility. Calcium availability: Critical for muscle contraction mechanisms. Myocardial health: The overall condition and integrity of the heart muscle. Direct Impact Increased contractility directly leads to a greater Stroke Volume and, consequently, a higher Cardiac Output.

8. Methods to Estimate Cardiac Output 1 Fick Principle This classical method calculates CO based on oxygen consumption (VO2) and the difference in oxygen content between arterial and venous blood. 2 Thermodilution A more invasive technique, it involves injecting a cold saline solution into the bloodstream and measuring temperature changes via a pulmonary artery catheter. 3 Non-Invasive Approaches Modern methods include Doppler ultrasound (measuring blood flow velocity) and bioimpedance (assessing electrical resistance changes in the chest with each heartbeat).

9. Clinical Relevance and Cardiac Index Why normalisation matters Cardiac Index (CI) normalizes Cardiac Output (CO) to the patient's Body Surface Area (BSA) . This provides a more accurate assessment of cardiac function relative to an individual's size. Normal CI ranges from 2.5 – 4.0 L/min/m² . "A low CO or CI is a critical indicator of compromised heart function, often seen in conditions like heart failure or various forms of shock, requiring urgent medical attention."