F5ii: Understand pulmonary vascular resistance and the factors that affect this
Definition
- Vascular resistance describes the relationship between pressure & flow
- Based on → ΩHM’S LAW; V = IR
VOLTAGE (potential difference) = CURRENT (flow of e-) x RESISTANCE
∴ R = \( \frac{\text{V}}{\text{I}} = \frac{\text{PRESSURE}}{\text{FLOW}} \)
∴ PVR = \( \frac{\text{PULMONARY DRIVING PRESSURE}}{\text{CO}} \)
PVR = \( \frac{\text{MPAP – LAP}}{\text{CO}} \)
NB: All of these are measured by PAC 😊
- Normal PVR = 160 dynes/sec/cm5
Importance
- Determines BF to L) heart
∴PreL → ∴CO
2. R) heart exists in a low P environment
↑PVR = ↑P required by LV to maintain LV preL
Factors Affecting PVR
- HP equation: R = \( \frac{\text{8hL}}{\text{πr4}} \)
- r4 = largest determining factor
- ∴factors affecting vessel radius will have biggest influence on PVR
Passive Factors
1) Recruitment & Distention
- Recruitment & distention with low pulmonary circulation adapts to large ∆CO with minimal ∆PVR
- ↑Pulmonary BF \(
\begin{cases}
\text{Opening of previously closed capillaries = RECRUITMENT}\\
\text{↑diameter of open capillaries = DISTENTION}
\end{cases}
\)
2) Lung Volumes
- PVR is minimal at FRC
- @ low vol
- EXTRA-ALV vessels ↓diameter = ↑PVR
- @ high vol
- INTRA-ALV vessels ↓diameter = ↑PVR
- EXTRA-ALV vessels ↓ diameter = ↑ PVR


3) Gravity
- ↑gravity = ↑hydrostatic P
- BF ↓ from base → apex
- Pulmonary circulation is a low pressure system
- 30cm lungs with exert 30cm H2O pressure = 23mmHg!
- ∴there will be regional differences in BF depending on where you are in the lung
West’s Zones
APEX – WZ1:
- Alv P > PA P = nil BF
- ∴any ventilation is wasted (dead space)
- In normal lungs, this does not exist
- But if ↓PA P (hypoT) or ↑Alv P (PEEP)
PA > Pa > PV = NO FLOW
WZ2
- From apex → 10cm above heart
- PA P > PALV
- ∴BF determined by arterial – alveolar difference
- Capillaries flutter between open/closed state
WZ3
- From 10cm above heart → base of lung
- Both PA P & Pv P > PALV
- ∴caps always open
- BF determined by arterial – alveolar difference
Active Factors
HPVC
- NO = potent VD
- NO continuously synthesised by pulmonary vessel endothelium
- ↓Alveolar PO2 < 70mmHg = ↓NO synthesis
- Pulmonary vessel VC = ↑PVR
- Shunts blood away from poorly ventilated areas
- ↑V/Q matching
↑ CO2 & Acidosis
- Augments HPVC
ANS
Sympathetic
- α1 = VC
- β2 = VD
- α2
presynaptically inhibit NA release = VD
post synaptically ↑NO synthesis= VD
Parasympathetic
ACh → M3 receptor causes VD
Local Mediators
- VC – histamine, serotonin
- VD – NO, Prostacycline