Stroke Volume & Cardiac Cycle

Cardiac Output & Its Components

• The heart’s primary job ➜ pump blood through two circuits:
  • Right heart → pulmonary circuit.
  • Left heart → systemic circuit.
• Cardiac output (CO)
  • Formula: $CO = HR \times SV$
    • $HR$ = heart rate (beats \min^{-1})
    • $SV$ = stroke volume (mL \, beat^{-1})
  • Units: liters \min^{-1}.

Defining Stroke Volume (SV)

• SV = volume of blood ejected by one ventricle per single contraction.
• Requires closure of the atrioventricular (AV) valve ➜ opening of the semilunar valve.
• Applies equally to right and left ventricles (unless otherwise specified, discussions usually refer to the left).

Phases of the Cardiac Cycle

• One complete cycle = one systole + one diastole.
• Pronunciation & professional usage:
  • Systole (accent on the final “e” sound).
  • Diastole (accent on the “i”).
• Systole
  • Contraction phase.
  • High-pressure event → blood propelled into arteries.
  • Unless stated, assumed to reference the left ventricle.
• Diastole
  • Relaxation phase.
  • Low-pressure event → passive ventricular filling from atria & veins.
  • Again, typically refers to the left ventricle.
• Timing nuances
  • At low HR: relatively longer diastole (more filling time).
  • At high HR (exercise): shortened diastole ➜ less filling time, potential limitation on EDV.

Quantifying Stroke Volume

• $SV = EDV - ESV$
  • $EDV$ = End-Diastolic Volume (max blood in ventricle, end of filling).
  • $ESV$ = End-Systolic Volume (blood remaining after contraction).
• Typical resting adult values:
  • $EDV \approx 120\,\text{mL}$
  • $ESV \approx 50\,\text{mL}$
  ➜ SV \approx 70\,\text{mL\,beat^{-1}}
• Heart size matters: larger bodies → larger hearts → larger EDV/ESV/SV.

Ejection Fraction (EF)

• Functional index of contractile strength.
• Formula: $EF = \frac{SV}{EDV} \times 100\% = \frac{EDV-ESV}{EDV}\times 100\%$
• Interpretation:
  • 60–70 % (rest) = normal.
  • >70 % (during rhythmic endurance exercise) = heightened performance.
  • ~50 % = caution; heart weakening.
  • <40 % = heart failure (insufficient pump strength).

Three Determinants of Stroke Volume

1 | Preload

• Concept: “front-end loading” of the heart.
• Direct proxy = venous return.
• Measured via $EDV$.
  • ↑ Venous return → ↑ EDV (myocardial stretch) → ↑ SV.
  • ↓ Venous return → ↓ EDV → ↓ SV.
• Influencing factors
  • Total blood volume / hormones (e.g., ADH, aldosterone).
  • Body position: supine ↑ EDV; upright ↓ EDV.
  • Intrathoracic pressure changes (respiratory pump, Valsalva, etc.).

2 | Contractility

• Definition: intrinsic force of ventricular contraction at a given preload.
• Two avenues of modulation: a) Intrinsic (Frank-Starling Mechanism)
  • Stretching myocardium (↑EDV) increases overlap of actin-myosin → greater force.
  • Law discovered independently by Otto Frank (Germany) & Ernest Starling (UK).
    b) Extrinsic (Autonomic Nervous System)
  • Parasympathetic (vagus): modest negative inotropy by lowering HR; less calcium entry; ↓ contractility → ↑ ESV → ↓ SV.
  • Sympathetic (β1-adrenergic):
    • Chronotropic effect (rate): ↑HR → indirect ↑ contractility.
    • Inotropic effect (ion Ca²⁺): β1 → ↑cAMP → ↑Ca²⁺ influx & faster Ca²⁺ re-uptake.
    – More Ca²⁺ bound to troponin → ↑ cross-bridges → ↓ ESV → ↑ SV.

3 | Afterload

• “Back-end” factor: resistance the ventricle must overcome to eject blood.
• Practical surrogate measures:
  • Systemic diastolic BP.
  • Total Peripheral Resistance (TPR).
• Consequences
  • High afterload (↑TPR) → difficult ejection → ↑ESV → ↓SV.
  • Low afterload (↓TPR) → easier ejection → ↓ESV → ↑SV.
• Regulation of TPR primarily occurs in arterioles via vasoconstriction/dilation (covered in vascular chapters).

Integrated Summary

• Stroke Volume links preload (EDV), contractility (ESV), & afterload (ESV) into one performance metric.
• Equation set:
  • $CO = HR \times SV$
  • $SV = EDV - ESV$
  • $EF = \frac{SV}{EDV} \times 100\%$
• Practical rules of thumb:
  1. ↑ Venous return = ↑ EDV = ↑ SV (via Frank-Starling).
  2. ↑ Sympathetic tone = ↑ Ca²⁺ & ↑HR = ↓ ESV = ↑ SV.
  3. ↑ Arterial resistance (afterload) = ↑ ESV = ↓ SV.
• Clinical flags:
  • EF < 40 % = diagnostic benchmark for heart failure.
  • Exercise tips: endurance activities boost EF & SV; heavy resistance raises afterload, often reducing EF during the lift.

Connections & Implications

• Systolic vs Diastolic BP: measured pressures correspond to ventricular systole (SBP) and arterial recoil/TPR during diastole (DBP) → reflect afterload conditions.
• Wiggers Diagram: visualizes pressure-volume changes; pay attention to where EDV, ESV, systolic peak, & diastolic trough occur.
• Ethical/clinical relevance: early detection of reduced EF enables intervention (meds, lifestyle, devices) to prevent progression of heart failure.