Cardiac Output and Heart Rate Control

Cardiac Output

• Cardiac output (COP) is the volume of blood ejected by each ventricle per minute.
• In a healthy young male at rest, COP equals 5 L/min.
• Cardiac index is COP divided by surface area.
• Normal cardiac index is 2.5 to 4 L/min/m².
• COP is affected by age, gender, and physical activity.

Control of Cardiac Output

• COP = Heart rate (HR) x Stroke volume (SV)
• Changes in HR or SV affect COP.

Heart Rate

• Average heart rate is 60-100 beats/minute.
• Above 100 bpm is tachycardia.
• Below 60 bpm is bradycardia.
• Factors affecting HR are described as chronotropic effects.

Normal Heart Rate Variations

• Age: HR decreases with age.
• Gender: Females generally have a slightly higher HR than males.
• Sleep: HR decreases during sleep.
• Training: Athletes have a lower resting HR.
• Neural Factors:
  • Sympathetic increases HR.
  • Parasympathetic decreases HR.
  • Chemoreceptors increase HR.
  • Baroreceptors decrease HR.
  • Bainbridge effect increases HR.

Hormones and Physical Factors Affecting Heart Rate

• Hormones (thyroid and catecholamines) increase HR.
• Fever or high temperature increase HR.

Heart Rate and Cardiac Output

• High heart rate = less time for cardiac filling = reduced stroke volume and COP.

Stroke Volume

• Stroke volume (SV) is the volume of blood ejected by each ventricle in each beat.
• SV = End diastolic volume (EDV) – End systolic volume (ESV).
• Normal SV is approximately 70 mL.

Stroke Volume Variations

• Age: SV increases with age.
• Gender: Males generally have a higher SV than females.
• Sleep: SV decreases during sleep.
• Exercise: SV is higher in athletes.
• Neural Factors:
  • Sympathetic increases SV.
  • Parasympathetic has no direct effect on SV.
  • Chemoreceptors increase SV.
  • Baroreceptors decrease SV.
• Hormones and Physical Factors:
  • Thyroid and catecholamines increase SV.
  • Fever or high temperature increase SV.

Ejection Fraction (EF)

• EF = SV/EDV x 100%
• Normal EF is approximately 65%.
• A sensitive indicator of myocardial contractility.
• EF decreases in heart failure.

Preload

• Preload is the tension in the ventricular muscle before contraction.

• Preload equals end-diastolic volume (EDV).

• Increased venous return results in increased preload.

• Increased preload results in increased stroke volume.

• Frank-Starling Law: As preload increases, the force of contraction increases.

• Chronic high preload can lead to cardiac muscle dilatation and heart failure.

Afterload

• Afterload is the resistance the ventricle must overcome to eject blood.
• High afterload is associated with hypertension and aortic valve stenosis.

Measurement of Cardiac Output

• Indirect methods: Indicator dilution technique, Fick principle, and thermodilution method.

• Direct methods: Electromagnetic or ultrasonic flow-meter devices.

• Hamilton dye dilution method: Dye injected into vein, arterial samples taken to measure dye concentration over time. COP calculated using dye dilution formulas.

• Fick principle: Blood flow to an organ can be calculated by measuring consumption or addition of a substance by that organ (e.g. oxygen)

• Cardiac output is measured using Fick's principle, where cardiac output is estimated by measuring the rate of oxygen consumption by the lungs and the differences in oxygen concentration between arterial and venous blood. Using the principle, the cardiac output (COP), can be calculated from the amount of oxygen consumed (or added) by the lungs in a given time. For example, if the lungs consume 250 ml of oxygen per minute, and the difference in oxygen concentration between arterial and venous blood is 50 ml/L, then the cardiac output is 250 ml/min / (50 ml/L) = 5 L/min, in a male adult at rest.

Venous Return

• Venous return is the volume of blood returning to the heart per minute.
• Normal venous return is 5 L/min.
• Factors affecting venous return include:
  • Pressure Gradient (arteries > capillaries > veins).
  • Blood Volume (reduced by bleeding).
  • Venomotor Tone (constriction increases return; dilation decreases return).
  • Muscle Pump and Respiratory Pump.
  • Gravity (standing decreases return).
  • Valvular Lesions/Pericardial Effusion (decrease return).