2021A Question 4
Describe the factors that affect afterload for each of the right and left ventricles.
Examiner Report
54.6% of candidates achieved a pass in this question.
The major domains assessed in this question were:
- A definition of afterload
- The differences between the left and right ventricles
- Extrinsic determinants of afterload as described by the Hagen-Poiseuille law
- Intrinsic determinants of afterload related to the Law of Laplace.
Typically answers that attempted to answer the questions “why?” and “how?” scored significantly better than those that presented simple lists. Credit was given for answers that demonstrated a better understanding of the factors and were able to integrate them into clinical examples, such as the effect of cardiac dilatation, ventricular hypertrophy and positive pressure ventilation. There were some excellent answers that illustrated in depth explanation of mechanisms and focused on factors that had opposite effects on each ventricle, highlighting the difference in afterload between the left and right side of the heart.
Common problems were:
- Incorrect recall of both the Hagen-Poiseuille law and the Law of Laplace
- Discussion of intrinsic factors (Laplace) without discussion of extrinsic factors (Hagen-Poiseuille) and vice versa
- Few candidates articulated that the radius is the most important determinant of resistance due to it’s fourth power
Model Answer
Structure:
- Introduction
- Determinants
- SVR
- PVR
Introduction
| Factor | Properties |
|---|---|
| Definition | - Sum of the factors contributing to total ventricular wall tension during systolic ejection - Determined primarily by vascular resistance in health |
| Equations | - - - |
| Left vs Right | - SVR is high (9-20 Wood units) - High pressure circuit facilitates selective distribution - Flow distribution determined by metabolic activity and SNS - PVR is low (0.25-2 Wood units) - Low pressure circuit minimizes transudation, preserves gas exchange - Flow distribution determined mostly by gravity and HPV - Many factors cause differential effects: - ↑ Temp, ↓ O2, ↑ CO2, ↓ pH, ↑ histamine, ↑ bradykinin (↓ SVR, ↑ PVR) - IPPV (↓ LV afterload, ↑ RV afterload) - Ventricular adaptation: - ↑↑ SVR → Thick-walled LV copes well - ↑↑ PVR → Thin-walled RV copes poorly |
Determinants
| Factor | Mechanism |
|---|---|
| Cardiac | Inflow tract: - AV valve regurgitation (MR, TR) → ↓ AL Ventricle: - ↑ Wall thickness (e.g. AS, PHTN) → ↓ Load on individual fibres - ↑ Cavity radius (e.g. Cardiomyopathy) → ↑ AL Outflow tract: - ↓ Radius (AS, PS, HCM) → ↑ AL |
| Vascular | - Resistance: See SVR and PVR - Factors increasing resistance: - ↓ Radius (power 4, hence most important factor) - ↑ Viscosity (↑ Hct, ↓ temp, ↓ flow rate) - ↑ Length (rarely important) - Compliance: E.g. Atherosclerosis → ↓ Windkessel effect → ↑ AL |
| Thoracic | - Intrathoracic pressure: E.g. IPPV → - ↓ LV afterload (↓ transmural pressure) - ↑ RV afterload (↑ SVR) |
SVR
| Factor | Mechanism |
|---|---|
| Physical | External compression → ↓ R - Intra-abdominal pressure: E.g. Pneumoperitoneum - Cross-clamp or tourniquet |
| Metabolic | Hyperactivity → ↑ R - ↑ Temp - ↓ O2 - ↑ CO2 - ↓ PH - ↑ Lactate Examples: - Exercise (muscle) - Pregnancy (uterus/placenta) - Simple obesity - Sepsis |
| Autonomic | Mechanisms: - Neural NAd → Α1 →↓ r - Hormonal Ad → Β2 → ↑ R Examples: - Pain - Anxiety - Exercise (↓ r in gut, skin, ↑ r in muscle) |
| Local | - Dilators: E.g. Histamine, bradykinin, NO - Constrictors: E.g. TXA2, 5HT |
PVR
Physical Factors
| Physical Factor | Mechanism |
|---|---|
| Blood pressure | - ↑ PAP or ↑ PVP → ↑ Radius - Recruitment of collapsed vessels (West zone 1 - Distension of patent vessels (West zones 2, 3) |
| Alveolar pressure | - IPPV, PEEP → ↑ Alveolar pressure → Vessel collapse - ↑ West Zone 1: Where PA > Pa > Pv - ↓ Total vascular surface area |
| Relative lung volume |
- ↓ Volume → ↓ Radial traction → Compress extra-alveolar vessels - ↑ Volume → Alveolar distension → Compress alveolar vessels - PVR lowest at FRC |
Metabolic Factors
| Metabolic Factor | Mechanism |
|---|---|
| O2 | Hypoxic pulmonary vasoconstriction (HPV) - - Serves to improve local V/Q matching |
| CO2 | ↑ CO2 → ↓ R |
| pH | ↓ PH → ↓ R |
| Temp | ↓ T → ↓ R |
Autonomic Factors
| Autonomic Factor | Mechanism |
|---|---|
| SNS | - Neural NAd → Α1 → ↓ R - Hormonal Ad → Β2→ ↑ R ACh → ↑ R |
| PSNS | ACh → ↑ NO release → ↑ R |
Local Factors
| Local Factor | Mechanism |
|---|---|
| Dilators | - NO: ↑ cGMP → Activate MLCP, ↓ Ca2+ influx, ↑ K+ efflux - PGI2 |
| Constrictors | e.g. ET, TXA2, 5HT, histamine, bradykinin |