Darcy–Weisbach equation

Given that the head loss h_f expresses the pressure loss Δp as the height of a column of fluid,

${\displaystyle \Delta p=\rho \cdot g\cdot h_{f}}$

where ρ is the density of the fluid. The Darcy–Weisbach equation can also be written in terms of pressure loss:

${\displaystyle \Delta p=f\cdot {\frac {L}{D}}\cdot {\frac {\rho V^{2}}{2}}}$

Lung compliance

In general, compliance is defined by the change in volume (ΔV) versus the associated change in pressure (ΔP), or ΔV/ΔP:

${\displaystyle Compliance={\frac {\Delta V}{\Delta P}}}$

During mechanical ventilation, compliance is influenced by three main physiologic factors:

1. Lung compliance
2. Chest wall compliance
3. Airway resistance

Lung compliance is influenced by a variety of primary abnormalities of lung parenchyma, both chronic and acute. Airway resistance is typically increased by bronchospasm and airway secretions. Chest wall compliance can be decreased by fixed abnormalities (e.g. kyphoscoliosis, morbid obesity) or more variable problems driven by patient agitation while intubated.^[1]

Calculating compliance on minute volume (V_E: ΔV is always defined by tidal volume (V_T), but ΔP is different for the measurement of dynamic vs. static compliance.