Physiology and Pathology of Heart Rate Regulation

Physiology Causes of Decreased Stroke Volume

• During sleep, athletes may experience decreased stroke volume
• Pathology causes include:
  • Heart failure
  • Congenital heart disease
  • Hypothyroidism
  • Hypothermia
  • Raised intracranial pressure
  • Certain drugs (antiarrhythmic, calcium channel blockers, beta blockers)

Stroke Volume

• Defined as the amount of blood pumped out of each ventricle per beat
• Average stroke volume: 70 mL
• Cardiac output: stroke volume x heart rate
• Cardiac output (approximate): 5040 mL/min (70 mL x 72 beats/min)
• Cardiac index: cardiac output per minute per square meter of body surface area
• Average cardiac index: 3.2 L/min/m²

Factors Affecting Stroke Volume

• End-diastolic volume (EDV) and end-systolic volume (ESV) affect stroke volume
• EDV: filled volume of ventricle prior to contraction (approx. 120 mL)
• ESV: residual volume of blood remaining in ventricle after ejection (approx. 50 mL)
• Stroke volume: EDV - ESV (approx. 70 mL)
• Increase in EDV increases stroke volume, while increase in ESV decreases stroke volume
• Strong heart contraction can decrease ESV to 10-20 mL, increasing stroke volume
• Large amounts of blood flow into ventricles during diastole can increase EDV to 150-180 mL, increasing stroke volume

Preload and Afterload

• Preload affects stroke volume through increased venous return to the heart, increasing EDV
• Increased EDV stretches muscle fibers, increasing force of ventricular contraction
• Afterload affects stroke volume through pressure generated by the ventricle to eject blood into the aorta
• Changes in afterload affect ESV and stroke volume
• Increase in afterload increases ESV, decreases stroke volume

Factors Affecting Cardiac Output

• Anxiety and excitement: increase cardiac output by 50-100%
• Eating: increase cardiac output by 30%
• Exercise: increase cardiac output by up to 700%
• High environmental temperature: increase cardiac output
• Pregnancy: increase cardiac output
• Epinephrine: increase cardiac output
• Sitting or standing from lying position: decrease cardiac output by 20-30%
• Rapid arrhythmias, heart disease: decrease cardiac output

Pressure-Volume Loop

• The body regulates stroke volume through three ways:
  • Filling pressure (preload)
  • Aortic pressure (afterload)
  • Contractility
• Changing filling pressure changes stroke volume only by changing LVEDV (example: transfusion)
• Lowering aortic pressure causes the ventricle to empty more completely, increasing stroke volume

Methods of Measuring Cardiac Output

• Direct Fick Method: accurate, but invasive (catheter insertion through patient's vein)
• Indicator dilution technique: accurate, but invasive (injection of marker substance)
• Thermodilution: accurate, non-invasive (saline injection through pulmonary artery)
• Doppler technique + echocardiography: non-invasive, accurate, but requires well-trained operator

Hyper- and Hypoeffective Heart

• Hyper-effective heart: pumps better than normal, with increased cardiac output
• Hypo-effective heart: pumps less than normal, with decreased cardiac output
• Factors that cause hypoeffectivity include:
  • Coronary artery blockage
  • Inhibition of nervous excitation
  • Pathological factors affecting heart rhythm or rate
  • Valvular heart disease
  • Increased arterial pressure
  • Congenital heart disease
  • Myocarditis
  • Cardiac hypoxia