G3iii / 14B19: Describe the factors that determine RV & LV afterload
14B19: Exam Report
Describe the factors that determine right and left ventricular afterload.
19% of candidates passed this question.
This is a big question and required some structure to cover the material required. A Definition of afterload, factors specific to left ventricle, right ventricle and both were required. The question asked to describe and not merely list factors affecting afterload.
Marks were not given for describing pathologies rather than physiological processes that affect afterload. Candidates seemed to lack depth and understanding on this topic.
G3iii / 14B19: Describe the factors that determine RV & LV afterload
Definition
Afterload = the sum of all factors required to overcome so that blood may be ejected from the heart to the arterial circulation
- AfterL gives rise to wall tension in the ventricle → ∴ afterL can be thought of as the wall tension required to overcome impedance to eject blood into the arterial circulation
- ∴ Factors affecting afterload are:
1. Factors Affecting Wall Tension
- Ventricle Transmural P
- Ventricle wall thickness
- Ventricle radius
- This is because ventricle is shaped like a sphere & obeys the Law of LaPlace:
(Transmural P = Intraventricular P – Intrapleural P)
2. Factors Affecting Outflow
- Vascular resistance
- Outflow tract resistance
- Root compliance & pressure
OVERALL:
- ↑ Wall tension = ↑ Afterload
- ↑ Outflow resistance = ↑ Afterload
Right Ventricle
- RV is unique because operates as a low pressure system due to low impedance of pulmonary vessels
Pulmonary Vascular Resistance
- Main determinant of RV afterload
- CO2, O2, pH, resp cycle all affect vessel radius
- ↑ Resistance = ↑ Afterload
Outflow Tract Resistance
- ↑ Outflow tract R = ↑ AfterL
- e. Saddle pulmonary embolus
Root Pressure/Compliance
- ↑Pulmonary Artery Pressure/↓pulmonary root compliance = ↑afterload
- Any cause of pulmonary HTN (LVF, MR/MS, chronic lung disease, multiple PEs)
- Pulmonary stenosis
Factors Affecting Wall Tension
- Thin walled RV is used to operating at a low resistance circuit ∴ not as affected by wall tension
- However any RV dilatation = ↑ wall T = ↑ afterL
- Susceptible to dilatation due to thin wall
- e. acute PE
- RV overload due to RV failure
Left Ventricle
SVR
- SVR is the main determinant of LV afterload
- Arterioles are the biggest resistors ∴the main determinants of SVR
- Based on H – P equation:
- ↑radius = ↓R = ↓afterL
- ↑viscosity = ↑R = ↑afterL
- ↑vessel length = ↑R = ↑afterL
Outflow Tract Resistance
- ↑R = ↑afterL
- Aortic stenosis
- SAM
- Tension PTX
Aortic Root Pressure/Compliance
- ↑aortic root P/↓compliance = ↑afterL
- e. Atherosclerosis Aortic Root
Wall Tension
- ∴↑transmural P / ↑radius = ↑WT = ↑afterL
- ↑Thickness = ↓wall T = ↓afterL e. WH because ↑ myofibrils share load
Ventricular Interdependence
- 20 – 40% RV contraction provided from LV via the IV septum
- ∴LV failure → RV dysfunction
- Conversely, significant ↑RV afterL causes RV dilatation → deviation of IV septum into LV → ↓LV filling → ↓SV → ↓CO
- Author: Krisoula Zahariou