Comparison of the P/V curve obtained by the supersyringe and the optoelectronic plethysmography

The pressure volume (P-V) curve of the total respiratory system is drawn assuming the changes of chest wall (?V_cw) equal to the volume displaced from the syringe (?V_gas). We compared ?V_gas and ?V_cw during P-V curves obtained by supersyringe and opto-electronic plethysmography [1]. Eight sedated paralysed intubated ALI/ARDS patients (5 M/3 F, age 70 ± 13 years, BMI 25.6 ± 3 kg/m², PaO₂/FiO₂ 220 ± 76) were studied. During the manoeuvre ?V_cw was recorded by optoelectronic plethysmography. The volume injected and withdrawn by the supersyringe step by step (100 ml) was corrected by temperature, humidity, pressure and gas exchange [2]. The discrepancy was computed as the difference between the volume of air inflated and the chest volume measured. The compliance of the total respiratory system was measured between zero and maximum airway pressure values on the inflation (Crs_inf) and deflation (Crs_def) limbs of the P-V curves. Hysteresis loops of the corrected P-V_gas and P-V_cw curves were calculated as the percentage ratio between the area of the P-V curves and the product of maximal volume by maximal pressure.

Even considering thermodynamics and gas exchange correction, ?V_gas values were systematically higher than ?V_cw probably due to blood shifts from the thorax to the extremities. As a consequence, the standard supersyringe method provides an overestimation of the inspiratory and expiratory compliance of the total respiratory system on the inflation limb and an overestimation of the hysteresis area.

Table 1.

	Discrepancy	Crsinf	Crs def	Hysteresis
	(ml)	(ml/cmH2O)	(ml/cmH2O)	(%)
Pvgas	130 ± 83	66 ± 21*	63 ± 21*	19.2 ± 5.9*
PVcw	-	59 ± 19	58 ± 18	15.2 ± 5.4

The data are expressed as mean ± SD.^* Paired t-test P < 0.05 vs PV_CW.