Predicting the effect of fresh gas flow on tidal volume in volume-controlled mechanically ventilated dogs

To determine if the tidal volume (V_T) delivered (V_TDEL) to canine patients being mechanically ventilated by a volume-controlled ventilator differed from the volume set on the ventilator (V_TSET) at three fresh gas flow (FGF) rates. To determine if V_TDEL could be accurately predicted by an FGF-based mathematical model.

Prospective proof-of-concept study.

A total of 23 adult client-owned dogs undergoing elective orthopedic surgery.

Dogs were anesthetized and ventilated with a volume-controlled mechanical ventilator with constant respiratory rate (f_R) of 10 breaths minute^-1, inspiratory-to-expiratory ratio of 1:2 [fraction of inspiratory time (T_I) in one respiratory cycle (T_tot) 1:3], and V_TSET as body weight (kg) × 15 (mL kg^-1). V_TDEL was measured in 20 dogs at three FGF (500, 1000 and 4000 mL minute^-1). A mathematical model was used to calculate predicted volume (V_TPRED) for each animal at each FGF: V_TSET + {FGF × [(T_I/T_tot)/f_R]}. Linear repeated measures models were fit comparing V_TDEL to V_TSET and to V_TPRED by FGF.

V_TDEL was significantly higher than V_TSET at every FGF (p < 0.05), and differences were larger at higher FGF (p < 0.001). There were no statistically significant differences between V_TDEL and V_TPRED at FGF rates of 500 and 4000 mL minute^-1 and, although the mean V_TDEL was statistically significantly higher than V_TPRED at FGF 1000 mL minute^-1 (p = 0.017), the mean difference of 9 mL was not clinically significant.

Dogs on volume-controlled ventilators may be ventilated at a higher V_TDEL than intended depending on the FGF settings. Ventilation of small animals at high FGF could inadvertently induce pulmonary damage. A mathematical equation can be used to achieve a desired V_TDEL by adjusting V_TSET values based on FGF, f_R and T_I/T_tot.