Increase of functional residual capacity in hemorrhagic shock

Functional residual capacity (FRC) was measured by body plethysmography in anesthetized dogs subjected to hemorrhagic shock. This was elicited by bleeding the animals to a mean arterial blood pressure of 4 kPa. Reinfusion was done at 30% spontaneous uptake of the maximal bleeding volume. During the initial rapid bleed-out and early hypovolemia, the FRC increased proportionally to the bleeding volume by an average factor of 0.19. Subsequently FRC increased further to about 50% above the control level, and it remained elevated even after reinfusion. Minute ventilation reached a peak value of about 2.5 times of that of the control state, and then declined to almost control values in late hypovolemia when FRC was still very high. The mechanism of the significant increase in FRC is not clear, but it is suggested that the balance of its effects is unfavorable to the maintenance of adequate ventilation in late shock.