Assessing recruitment of lung diffusing capacity in exercising guinea pigs with a rebreathing technique

Abstract

Noninvasive techniques for assessing cardiopulmonary function in small animals are limited. We previously developed a rebreathing technique for measuring lung volume, pulmonary blood flow, diffusing capacity for carbon monoxide (Dl(CO)) and its components, membrane diffusing capacity (Dm(CO)) and pulmonary capillary blood volume (Vc), and septal volume, in conscious nonsedated guinea pigs at rest. Now we have extended this technique to study guinea pigs during voluntary treadmill exercise with a sealed respiratory mask attached to a body vest and a test gas mixture containing 0.5% SF(6) or Ne, 0.3% CO, and 0.8% C(2)H(2) in 40% or 98% O(2). From rest to exercise, O(2) uptake increased from 12.7 to 25.5 ml x min(-1) x kg(-1) while pulmonary blood flow increased from 123 to 239 ml/kg. The measured Dl(CO), Dm(CO), and Vc increased linearly with respect to pulmonary blood flow as expected from alveolar microvascular recruitment; body mass-specific relationships were consistent with those in healthy human subjects and dogs studied with a similar technique. The results show that 1) cardiopulmonary interactions from rest to exercise can be measured noninvasively in guinea pigs, 2) guinea pigs exhibit patterns of exercise response and alveolar microvascular recruitment similar to those of larger species, and 3) the rebreathing technique is widely applicable to human ( approximately 70 kg), dog (20-30 kg), and guinea pig (1-1.5 kg). In theory, this technique can be extended to even smaller animals provided that species-specific technical hurdles can be overcome.

Fig. 1.

Schematic diagram of the experimental apparatus.

Fig. 2.

Representative disappearance curves show the exponential disappearance of C₂H₂ and CO and the stabilization of SF₆ and Ne dilution during rebreathing. F_A, fractional gas concentration during rebreathing; F_A0, initial fractional gas concentration.

Fig. 3.

Relationship between minute ventilation (V̇e) and O₂ uptake (V̇o₂) (A) and between CO₂ output (V̇co₂) and V̇o₂ (B). Linear regression equations through pooled data: V̇e = 14.15V̇o₂ + 426.0, R² = 0.74, and V̇co₂ = 0.920V̇o₂ + 0.383, R² = 0.93.

Fig. 4.

A: relationships of lung diffusing capacity for CO (Dl_CO) with respect to pulmonary blood flow (Q̇c) were measured while animals were rebreathing 40% or 98% O₂. Linear regression equations through pooled data: Dl_CO = 0.0026Q̇c − 0.0246, R² = 0.89, and Dl_CO = 0.0016 Q̇c − 0.0080, R² = 0.93. B: Dl_CO was standardized (Dl_CO-std) to a constant hematocrit (0.45) and alveolar O₂ tension (120 Torr) and plotted with respect to Q̇c for individual animals. Mean linear regression equation: Dl_CO-std = 0.0023Q̇c + 0.0211, R² =0.88.

Fig. 5.

A: membrane diffusing capacity (Dm_CO) plotted with respect to Q̇c. Each regression line belongs to an individual animal. Mean regression equation: Dm_CO = 0.0046Q̇c + 0.0695, R² = 0.53. B: pulmonary capillary blood volume (Vc) plotted with respect to Q̇c. Each regression line belongs to an individual animal. Mean regression equation: Vc = 0.0068Q̇c + 0.154, R² = 0.65. Both Dm_CO and Vc data were estimated with the rate of CO uptake (θ_CO) value for dog blood.

Fig. 6.

Body mass-specific relationships of standardized Dl_CO (hematocrit 0.45 and alveolar O₂ tension 120 Torr) to Q̇c for guinea pigs (present data), 8 normal dogs, and 17 healthy human subjects studied by a similar rebreathing technique in our laboratory. Linear regression equations through pooled data: guinea pig, Dl_CO = 0.0023Q̇c + 0.0224, R² = 0.88; dog, Dl_CO = 0.0019Q̇c + 0.3294, R² = 0.89; human, Dl_CO = 0.0017Q̇c + 0.2412, R² = 0.52. Slope of the relationship for guinea pig is significantly higher while the intercept was significantly lower than that in canine and humans subjects (both P < 0.05).