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

Objectives To determine if the tidal volume (VT) delivered (VTDEL) to canine patients being mechanically ventilated by a volume-controlled ventilator differed from the volume set on the ventilator (VTSET) at three fresh gas flow (FGF) rates. To determine if VTDEL could be accurately predicted by an FGF-based mathematical model. Study design Prospective proof-of-concept study. Animals A total of 23 adult client-owned dogs undergoing elective orthopedic surgery. Methods Dogs were anesthetized and ventilated with a volume-controlled mechanical ventilator with constant respiratory rate (fR) of 10 breaths minute -1, inspiratory-to -expiratory ratio of 1:2 [fraction of inspiratory time (TI) in one respiratory cycle (Ttot) 1:3], and VTSET as body weight (kg) x 15 (mL kg -1). VTDEL was measured in 20 dogs at three FGF (500, 1000 and 4000 mL minute -1). A mathe-matical model was used to calculate predicted volume (VTPRED) for each animal at each FGF: VTSET + {FGF x [(TI/ Ttot)/fR]}. Linear repeated measures models were fit comparing VTDEL to VTSET and to VTPRED by FGF. Results VTDEL was significantly higher than VTSET at every FGF (p < 0.05), and differences were larger at higher FGF (p < 0.001). There were no statistically significant differences between VTDEL and VTPRED at FGF rates of 500 and 4000 mL minute -1 and, although the mean VTDEL was statisti-cally significantly higher than VTPRED at FGF 1000 mL minute -1 (p = 0.017), the mean difference of 9 mL was not clinically significant. Conclusions and clinical relevance Dogs on volume -controlled ventilators may be ventilated at a higher VTDEL 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 VTDEL by adjusting VTSET values based on FGF, fR and TI/Ttot.