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

To compare the portion of tidal volume (V_T) ventilating dead space volumes in nonbrachycephalic cats and dogs with small body mass receiving volume-controlled ventilation (VCV) with a fixed V_T.

Prospective, experimental study.

A group of eight healthy adult cats and dogs [ideal body weight (IBW): 3.0 ± 0.5 and 3.8 ± 1.1 kg, respectively].

Anesthetized cats and dogs received VCV with a 12 mL kg^-1 V_T (inspiratory pause ≥ 0.5 seconds). Respiratory rate (f_R) was adjusted to maintain normocapnia. Airway dead space (V_Daw) and alveolar tidal volume (V_Talv) were measured by volumetric capnography. Physiological dead space (V_Dphys) and V_Dphys/V_T 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).

The IBW (p = 0.07), PaCO₂ (p = 0.40) and expired V_T [V_T(exp)] (p = 0.77) did not differ significantly between species. The V_Daw (mL kg^-1) was lower in cats (3.7 ± 0.4) than in dogs (7.7 ± 0.9) (p < 0.0001). The V_Talv (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 V_Dphys/V_T ratio (0.33 ± 0.03) and V_Dphys (4.0 ± 0.3 mL kg^-1) than dogs (V_Dphys/V_T: 0.60 ± 0.09; V_Dphys: 7.2 ± 1.4 mL kg^-1) (p < 0.0001). The f_R and minute ventilation (V_T(exp) × f_R) were lower in cats than in dogs (p = 0.048 and p = 0.038, respectively).

A fixed V_T results in more effective ventilation in cats than in dogs with small body mass because of species-specific differences in and V_Daw and V_Dphys. Because of the smaller V_Daw and V_Dphys in cats than in dogs, a lower f_R is required to maintain normocapnia in cats.