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 (V) ventilating dead space volumes in nonbrachycephalic cats and dogs with small body mass receiving volume-controlled ventilation (VCV) with a fixed V.

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 V (inspiratory pause ≥ 0.5 seconds). Respiratory rate (f) was adjusted to maintain normocapnia. Airway dead space (V) and alveolar tidal volume (V) were measured by volumetric capnography. Physiological dead space (V) and V/V 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), PaCO (p = 0.40) and expired V [V] (p = 0.77) did not differ significantly between species. The V (mL kg) was lower in cats (3.7 ± 0.4) than in dogs (7.7 ± 0.9) (p < 0.0001). The V (mL kg) was larger in cats (8.3 ± 0.7) than in dogs (4.3 ± 0.7) (p < 0.0001). Cats presented a smaller V/V ratio (0.33 ± 0.03) and V (4.0 ± 0.3 mL kg) than dogs (V/V: 0.60 ± 0.09; V: 7.2 ± 1.4 mL kg) (p < 0.0001). The f and minute ventilation (V × f) were lower in cats than in dogs (p = 0.048 and p = 0.038, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

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