Figure 17-12
Artificial ventilation by intermittent application of
constant pressure (square wave) followed by passive expiration. The pressure required
to overcome airway resistance (hatched lines, A)
and the airflow rate (of Equation 4, C) are proportional
to one another and decrease exponentially (assuming that resistance to airflow is
constant). The pressure required to overcome the elastic forces (height of the dashed
line, A) and lung volume (B)
are proportional to one another and increase exponentially. Values shown are typical
for an anesthetized supine paralyzed patient: total dynamic compliance, 50 mL/cm
H2
O; pulmonary resistance, 3 cm H2
O/L/sec; apparatus resistance,
7 cm H2
O/L/sec; total resistance, 10 cm H2
O/L/sec; time constant,
0.5 second. (Redrawn from Lumb AB: Artificial ventilation. In
Lumb AB [ed]: Nunn's Applied Respiratory Physiology, 5th ed. London, Butterworths,
2000, p 590.)
