Venous admixture (Qva/Q) and true shunt (Qs/Qt) in ARF patients: effects of PEEP at constant FIO2

Venous admixture (Qva/Q) in ARF patients is due to both true right to left shunt (Qs/Qt: perfusion of truly unventilated areas) and to maldistribution [Qva-Qs)/Qt: effects of unevenness of ventilation/perfusion ratio). Using the retention rate of sulphur hexafluoride we determined the effects of PEEP on Qs/Qt and (Qva-Qs)/Qt at a constant FIO2 for each patient (0.57 +/- 0.19 SD, range 0.4-0.95). Eleven patients with ARF (treated either by CPPV or CPAP) were studied on 16 occasions. Each measurement was repeated at two levels of PEEP, 5 cm H2O below and 5 cm H2O above the patient's clinically determined PEEP level. The increase in PEEP resulted in: - a decrease in Qva/Q (from 0.37 +/- 0.13 to 0.27 +/- 0.12, p less than 0.01); - a parallel decrease in Qs/Qt (from 0.29 +/- 0.16 to 0.22 +/- 0.14, p less than 0.01); there was a positive correlation between Qva/Q and Qs/Qt changes (r = 0.53, p less than 0.05). No significant variation was demonstrated in (Qva-Qs)/Qt (from 0.074 +/- 0.045 to 0.054 +/- 0.048). On the other hand there was a negative correlation between the fraction of Qva/Q due to the maldistribution and FIO2: (Qva-Qs)/Qva = 0.75-0.86 FIO2 (r = 0.74, p less than 0.01). We conclude that: PEEP decreased Qva/Q mainly through changes in Qs/Qt but did not have a definite effect on (Qva-Qs)/Qt. Maldistribution was responsible for a significant portion of Qva/Q in those ARF patients tolerating a relatively low FIO2 (0.4-0.6).