Summary

·   Gravity creates a hydrostatic pressure gradient: base > apex

·   Lung divided into regions according to relationship between alveolar, vascular and interstitial pressures

Z1

·   PA > Pa > Pv

·   Minimal apical lung in health

·   = Alveolar dead space

·   No blood flow (Q)

·   ↑Z1 if

o ↑PA: e.g. IPPV, PEEP

o ↓Pa: e.g. pulmonary embolus, haemorrhage, ↓inotropy, pulmonary vasodilator)

Z2

·   Pa > PA > Pv

·   From 3cm above RV to near apex in health

·   Starling resistor:

o Diastole: Pa<PA hence Q = 0 and downstream Pv irrelevant

o Systole: Q begins when Pa >> PA; then Q ∝ (Pa-Pv)

o (↑↑ pressure required to re-open collapsed vessel)

Z3

·   Pa > Pv > PA

·   From near base to 3cm above RV

·   Majority of lung tissue

·   Q continuous.

Z4

·   Pa > Pi > Pv > PA

·   Q minimal?

·   Compression of extra-alveolar vessels:

o Base of lung at low volume in health (↓ radial traction)

o APO (↑ interstitial fluid)

Summary

·   Hydrostatic pressure gradient ∝ vertical height of lung

·   Erect > lateral decubitus > supine

Lateral decubitus

·   Z1: almost zero lateral non-dependent lung

·   Z2: almost whole non-dependent lug

·   Z3: almost dependent lung

·   Improved V/Q matching c.f. supine

Supine

·   Z1: minimal anterior both lungs

·   Z2: minimal mid-anterior both lungs

·   Z3: most of both lungs

·   No right-left inequality

Prone

·   More even distribution

o Gravity favours flow in non-dependent areas

o Lung architecture favours flow in dependent areas

Effect

·   ↑PAP or ↑PVP -> ↓ PVR

·   Unlike systemic circulation

Purpose

·   Allows high flow rate without high pressure (e.g. exercise)

·   Minimises transdation hence diffusion distance

·   Hence preserves gas exchange

Mechanisms

·   Recruitment: ↑ pressure -> re-open collapsed vessels (Z1)

·   Distension: ↑pressure -> ↑ radius of open vessels (Z2,3)

·   ↑Vascular surface area -> ↓PVR

o Q = (P₁-P₂)/R

o R = (8 x length x viscosity) / (π x radius₄) assuming laminar

Graph

Ventilation

Perfusion

Awake supine

Better ventrally:

·   Dorsal diaphragm displaced cephalad

o ↑Mechanical advantage

·   Dorsal lung compressed a bit by ventral lung/heart/abdo viscera

o ↓Volume -> ↑compliance

Better dorsally:

·      Lung architecture -> dorsal flow

·      Gravity -> dorsal flow

GA supine

·   Dorsal diaphragm displaced a lot

·   But now akinetic

·   Dorsal lung compressed a lot -> small airway collapse, atelectasis, ↓compliance

·   (Worse if ARDS)

Better dorsally (as above)

GA prone

More evenly spread

·   Lung/mediastinum/abdo viscera supported by sternum and ribs (not dorsal lung)

·   More homogeneity of intrapleural pressure / lung volume / lung compliance

More evenly spread

·   Lung architecture -> dorsal flow

·   Gravity -> ventral flow