Canonical Question

Cardiac Output

V5 D3.ii Historical V4 G3.iii 2 appearances

Master answer

Definitions of Preload:

The mean tension in the ventricular fibres or the initial myocardial fibre length prior to contraction.
Mathematically:

\[Preload \; = \; {{{(LVEDP \; – \; ITP)} \times LVEDR} \over {2h}} \]

where:
- ITP = intrathoracic pressure; LVEDP = left ventricular end-diastolic pressure; LVEDR = left ventricular end-diastolic radius (at midpoint of ventricle); h = thickness of ventricle

Extra-cardiac: End-diastolic filling pressure
- CVP
  - Venous compliance
  - Thoracic Blood Volume
    - Total Blood Volume
      - ↑ TBV → ↑MSFP → ↑ CVP → ↑ preload.
    - Blood Volume distribution
  - Venous return (VR)
    - ↑VR → ↑ CVP → ↑ preload
- Afterload factors
  - ↑Total peripheral resistance → ↓ VR → ↓ Preload
  - Compression
    - Pericardial effusion/tamponade → ↓ ventricular filling
    - PEEP → ↑ ITP → ↓ VR
  - Aortic pressures
    - ↑ afterload (or raised aortic pressure) → ↓ SV → ↑ ESV → ↑ EDV (or ventricular preload).

Intra-cardiac:
- End-diastolic radius
  - Ventricular and Pericardial compliance
  - ↑ compliance → ↑ filling for constant pressure → ↑preload
- Atrial contraction/coordination
  - ↑ atrial contraction/contractility → ↑ preload
    - eg ↑ SNS stimulation / fluid bolus
    - ↓ atrial contractile force/coordination in AF
- HR: ↓ HR → ↑ filling time → ↑ preload
- Myocardial Wall thickness
  - ↑Normal Growth → ↓ compliance and ↑ thickness (h) → ↓ Preload
  - ↑Compensatory hypertrophy

\[ VR \; = \; {{(MSFP \; – \; RAP)} \over {SVR}} \]

Venous Factors
- ↓Venomotor tone → ↓ VR
- Venous Values prevent retrograde flow → failure → ↓VR
Pump Factors
- Skeletal Muscle pump/Respiratory pump both ↑ VR
  - Inspiration
    - → ↓ ITP → ↓ RAP → ↑VR
    - → Diaphragmatic contraction → ↑IAP → ↑VR
- Effect of ventricular contraction and relaxation
  - → Systolic Rapid ejection phase → ↓RAP → ↑VR
  - → Early diastole → Rapid vetricular filling (open AV valve) → ↑VR
Posture
Intrapericardial pressure (tamponade)
Afterload
- ↑ Arteriolar tone → ↑ Resistance to VR → ↓ VR

↑ preload
- Ventricular systolic failure
- Outflow valve stenosis or regurgitation (AS or AR),
- Inflow valve regurgitation (MR/TR)
↓ preload
- Ventricular diastolic failure,
- Inflow valve stenosis (MS/TS)

Gladwin 2015

Answer by Gladwin

Exam	Exact wording	Relationship	Success
2010A Q21	Define preload and describe the determinants of preload.	historical_member	—
2015B Q15	Define cardiac preload and describe its determinants	historical_member	—