Effects of volume-targeted pressure-controlled inverse ratio ventilation on functional residual capacity and dead space in obese patients undergoing robot-assisted laparoscopic radical prostatectomy

Abstract

Background: The effect of inverse inspiration:expiration (I:E) ratio on functional residual capacity (FRC) during pneumoperitoneum is unclear. We hypothesised that volume-targeted pressure-controlled inverse ratio ventilation (vtPC-IRV) would increase FRC by increasing the level of auto-PEEP in low respiratory compliance situations.

Methods: During robot-assisted laparoscopic radical prostatectomy, 20 obese patients were sequentially ventilated with four different settings for 30 min in each setting: (1) control, I:E ratio of 1:2 and baseline airway pressure (BAP) of 5 cm H₂O; (2) IRV2, I:E ratio of 2:1 and BAP off; (3) IRV3, I:E ratio of 3:1 and BAP off; and (4) IRV4, I:E ratio of 4:1 and BAP off. The changes in FRC were identified and compared among these settings.

Results: The FRC significantly increased as the I:E ratio increased. The FRC values expressed as median (inter-quartile range) during control, IRV2, IRV3, and IRV4 were 1149 (898-1386), 1485 (1018-1717), 1602 (1209-1775), and 1757 (1337-1955) ml, respectively. Auto-PEEP increased significantly as the I:E ratio increased and correlated with FRC (rho=0.303; P=0.006). Shunt and physiological dead space were significantly lower in all IRV groups than in the control group; however, there were no significant differences among the IRV groups.

Conclusions: vtPC-IRV with shortened expiratory time and increased auto-PEEP effectively increases FRC during robot-assisted laparoscopic radical prostatectomy in obese patients. FRC increases progressively as the I:E ratio increases from 1:2 to 4:1; however, an I:E ratio higher than 2:1 does not further improve the dead space.

Clinical trial registration: UMIN000038989.

Keywords: dead space; functional residual capacity; pressure-controlled inverse ratio ventilation; robot-assisted laparoscopic radical prostatectomy; volumetric capnography.

Fig. 1

Novel theory of volumetric capnography. The expired tidal volume on the Y axis is plotted against the partial pressure of expired CO₂ on the X axis to express the curve of ‘Volume=VCO₂/FCO₂’. Phase I represents CO₂-free and pure dead space, phase II represents the transition between the airway and alveolar gas, and phase III represents the alveolar gas. Phase II ends at the inner square area maximum point. VD_aw is determined by applying Fowler's equal area method (area A equals to B). The volume from the start of expiration to the partial pressure of the mixed expired CO₂ point on the Y axis is defined as respiratory dead space (VD_resp). Thus, alveolar dead space (VD_alv) is calculated as VD_resp–VD_aw and shunt dead space (VD_shunt) is calculated as VD_phys–VD_resp. VD_phys, physiological dead space; VD_aw, airway dead space; VD_alv, alveolar dead space; VD_shunt, shunt dead space; V_TE, expired tidal volume; VCO₂, expired tidal volume of CO₂; FCO₂, fractional concentration of CO₂ [FCO₂=Pco₂ (P_B–P_H2O)⁻¹=Pco₂ (760–47)⁻¹; P_B, barometric pressure; P_H2O, water vapor pressure at 37°C]; F_ECO₂, mixed expired FCO₂; F_ETCO₂, end-tidal FCO₂; F_ACO₂, alveolar FCO₂; and FaCO₂, arterial FCO₂.

Fig. 2

CONSORT diagram for patient inclusion. CONSORT, Consolidated Standards of Reporting Trials; IRV, inverse-ratio ventilation; PC, pressure control; BAP, baseline airway pressure.

Fig. 3

Correlation of FRC with other variables, auto-PEEP, total PEEP, static compliance, Pao₂/F_Io₂, VD_phys, and VD_shunt. Spearman's rank correlation is used to assess the relationship between the variables. FRC, functional residual capacity; VD_phys, physiological dead space; VD_shunt, shunt dead space.