Cardiac Output

Introduction

• Cardiac output is the amount of blood pumped by each ventricle of the heart in a minute.
• It is vital for maintaining proper blood circulation throughout the body.
• The average resting cardiac output is about 5 L/min, although it varies depending on age, sex, body build, diurnal variations, emotional conditions, exercise, posture, and sleep.

Expression of Cardiac Output

• Heart Rate (HR): represents how fast the heart is beating, measured in beats per minute (bpm).
  • Normal heart rate: about 72bpm
• Stroke Volume (SV): the amount of blood pumped out by the heart with each beat, typically measured in millilitres (mL) per beat.
  • Normal stroke volume: 70ml per beat
• Minutes Volume: the amount of blood pumped out by each ventricle per 1 minute, calculated by the product of stroke volume and heart rate, measured in liters per minute (L/min).
  • Mathematically: Minutes Volume = Stroke Volume × Heart Rate
  • Normal minute volume: 5L/min

Distribution of Cardiac Output

• The whole volume pumped out of the right ventricle goes to the lungs to become oxygenated.
• The left ventricle pumps blood into different parts of the body based on the metabolic activity in that area or region.
• The heart itself receives the least amount of blood, while the liver receives the most blood supply.

Regulation of Cardiac Output

• Nervous Regulation:
  • Sympathetic Nervous System (SNS) stimulation: increases heart rate and enhances myocardial contractility, increasing cardiac output.
  • Parasympathetic Nervous System (PNS) stimulation: decreases heart rate, slowing down the heart's pumping action and reducing cardiac output.
• Hormonal Regulation:
  • Epinephrine (adrenaline) and norepinephrine: increase heart rate and contractility, directly increasing cardiac output.
  • Thyroid hormones (THs): regulate the cardiovascular system, exerting several effects on cardiac output.
• Baroreceptor Reflex:
  • Baroreceptors detect changes in blood pressure, triggering reflex adjustments to maintain blood pressure within the normal range.
• Chemical Factors:
  • Changes in blood pH, oxygen levels, and carbon dioxide levels can influence cardiac output.

Clinical Correlates

• Heart Failure:
  • Occurs when the heart cannot pump enough blood to meet the body's needs.
  • Caused by conditions like coronary artery disease, hypertension, or heart attacks.
  • Symptoms include fatigue, shortness of breath, and fluid retention due to reduced cardiac output.
• Arrhythmias:
  • Abnormal heart rhythms that disrupt the heart's pumping ability.
  • Conditions like atrial fibrillation or ventricular tachycardia can lead to irregular heartbeats.
  • Disruption can decrease cardiac output and may cause complications like stroke or heart failure.
• Shock:
  • Characterized by inadequate blood flow and reduced tissue oxygenation.
  • Causes include cardiogenic shock, mechanical/obstructive shock, hypovolemic shock, and distributive shock.
  • Decreased cardiac output contributes to insufficient tissue perfusion, potentially leading to organ damage or failure.
• Tachycardia:
  • A heart rate exceeding the normal resting rate (over 100 bpm).
  • Causes can include anxiety, infection, hypoglycemia, hypovolemia, or hyperthyroidism.
  • Tachycardia at rest may indicate underlying issues affecting cardiac output.