= Peak airway pressure
{title2=$P_{\rm peak}$}
\b[Peak airway pressure], $P_{\rm peak}$, is the largest measured airway <pressure> over the specified breath or inspiratory interval. Its meaning includes the measurement location, reference pressure and temporal response. At the airway opening during positive-pressure inflation, the peak can include the pressure needed for flow through resistance as well as the pressure distending the respiratory system.<Measured values for Ppeak and Pplateau by Hamilton Medical>[Definition of airway pressures]{ref}
For a passive single-compartment model with constant <airway resistance> $R$ and <respiratory-system compliance> $C$,
$$
P_{\rm aw}(t)=P_{\rm EEP,tot}+\frac{V(t)}{C}+RQ(t).
$$
Here $V$ is volume above the end-expiratory state and $Q$ is inspiratory flow. If constant flow persists to the end of inspiration, the largest volume and the resistive pressure occur together, giving
$$
P_{\rm peak}=P_{\rm EEP,tot}+\frac{V_T}{C}+RQ.
$$
With varying flow, the peak is instead the maximum of the time-dependent sum; separate maxima cannot automatically be added.
An original example takes total end-expiratory pressure 5 cm H$_2$O, <tidal volume> 0.50 L, $C=0.05$ L/cm H$_2$O and $Q=0.50$ L/s. For $R=10$ cm H$_2$O·s/L, peak pressure is $5+10+5=20$ cm H$_2$O. Doubling resistance raises the calculated peak to 25 cm H$_2$O while the equilibrated <plateau airway pressure> stays 15 cm H$_2$O. A larger peak can therefore occur without a larger static distending increment.
An <end-inspiratory hold> removes ongoing airway flow under appropriate conditions and helps separate these contributions. Active respiratory effort, leaks, nonlinear resistance or delayed redistribution complicate that separation. Peak pressure alone neither partitions lung and chest-wall loading nor establishes a unique cause of a pressure change.
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