Dead space volumes in cats and dogs with small body mass ventilated with a fixed tidal volume

Objective: To compare the portion of tidal volume (VT) ventilating dead space volumes in nonbrachycephalic cats and dogs with small body mass receiving volume-controlled ventilation (VCV) with a fixed VT. Study design: Prospective, experimental study. Animals: A group of eight healthy adult cats and dogs [ideal body weight (IBW): 3.0 ± 0.5 and 3.8 ± 1.1 kg, respectively]. Methods: Anesthetized cats and dogs received VCV with a 12 mL kg–1 VT (inspiratory pause ≥ 0.5 seconds). Respiratory rate (fR) was adjusted to maintain normocapnia. Airway dead space (VDaw) and alveolar tidal volume (VTalv) were measured by volumetric capnography. Physiological dead space (VDphys) and VDphys/VT ratio were calculated using the Bohr–Enghoff method. Data recorded before surgery were compared by an unpaired t-test or Mann–Whitney U test (p < 0.05 considered significant). Results: The IBW (p = 0.07), PaCO2 (p = 0.40) and expired VT [VT(exp)] (p = 0.77) did not differ significantly between species. The VDaw (mL kg–1) was lower in cats (3.7 ± 0.4) than in dogs (7.7 ± 0.9) (p < 0.0001). The VTalv (mL kg–1) was larger in cats (8.3 ± 0.7) than in dogs (4.3 ± 0.7) (p < 0.0001). Cats presented a smaller VDphys/VT ratio (0.33 ± 0.03) and VDphys (4.0 ± 0.3 mL kg–1) than dogs (VDphys/VT: 0.60 ± 0.09; VDphys: 7.2 ± 1.4 mL kg–1) (p < 0.0001). The fR and minute ventilation (VT(exp) × fR) were lower in cats than in dogs (p = 0.048 and p = 0.038, respectively). Conclusions and clinical relevance: A fixed VT results in more effective ventilation in cats than in dogs with small body mass because of species-specific differences in and VDaw and VDphys. Because of the smaller VDaw and VDphys in cats than in dogs, a lower fR is required to maintain normocapnia in cats.