Cardiac Output

Intracardiac

Extracardiac

HR

• ↓HR  → ↑ diastolic filling → ↑ preload → ↑ CO
• Arrhythmia (eg AF) → loss of atrial kick
  • less important in RV, can contribute as much as 30% of EDV

• SNS stimulation → ↑ HR → ↓ Diastolic filling time → ↓ Preload → ↓ RV output

Preload

PL = [(RVEDP – ITP) RVEDR]/2h

• h – thickness of the right ventricle
  • Less than LV → ↑ PL for equivlant end diastolic pressure (EDP) and end diastolic radius (EDR) compared with the left
• RVEDR – Right ventricular end diastolic radius
  • Ventricular wall compliance
    • Ventricular interdependence → coordinated, simultaneous contraction of ventricles limits the excursion of intraventricular septum → ↓ventricular compliance
• ITP – Intrathoracic pressure

Mostly RVEDP factors

• Central venous pressure
  • Venous compliance
    • ↑ venomotor tone → ↑ CVP → ↑ preload → ↑ RV output
• Thoracic venous blood volume
  • Total blood volume (TBV)
    • ↑ TBV → ↑MSFP → ↑ CVP → ↑ preload → ↑ PL
• Venous return
  • VR = (MSFP – RAP)/(SVR)
  • ↑VR → ↑ CVP → ↑ preload
• Pulmonary Artrial pressure (PAP)
  • ↑ afterload (or PAP) → ↓ SV → ↑ ESV → ↑ EDV (or ventricular preload) → ↑ RV output
• External compression
  • Pericardial effusion/tamponade → ↓ ventricular filling
• PEEP → ↑ ITP → ↓ VR

Afterload

AL = [(RVESP – ITP) RVESR]/2h

• RVESR (Vetricular Systolic Radius_
  • End-diastolic radius
• Transmural pressure (RVESP – ITP)
  • Intraventricular pressure (RVESP)
    • Outflow tract obstruction
• Myocardial wall thickness
  • Thin wall → ↑ compliance → ↓ AL

Transmural pressure (RVESP – ITP)

• Extra-cardiac pressure (ITP)
  • ↑ ITP → ↓ Δ P → ↓ AL
• Pulmonary Arterial pressure
  • Pulmonary vascular resistance (PVR) = (8 η L)/(π r^4)
    • ↑PEEP → ↓vessel radius → ↑AL
    • Position (Erect > PVR cf Supine)

• Afterload
  • “Anrep Effect”
• Preload
  • Initial myocyte stretch via Frank-starling
• Heart rate
  • “Bowditch effect” or “Treppe effect”

• ANS (main factor)
• Circulating catecholamines – Augments SNS adrenergic effect