Functional residual capacity and body position in the dog

Area of Science:

• Physiology
• Respiratory Mechanics
• Comparative Anatomy

Background:

• Accurate measurement of resting lung volumes is crucial for understanding respiratory function.
• Previous studies often report lung volumes relative to body weight, which can lead to significant data variability.
• Standardizing measurements is essential for reliable comparisons across individuals and species.

Purpose of the Study:

• To determine resting lung volumes in dogs using the N2 washout method.
• To assess the variability of lung volumes when expressed in different units (ml/kg BW vs. %TLC40).
• To evaluate the relationship between supine and upright lung volumes and their predictability.

Main Methods:

• Nitrogen (N2) washout technique was employed to measure functional residual capacity (FRC) in anesthetized dogs.
• Lung volumes were measured in the supine position (FRCs) and the change to the upright position (deltaFRC) was calculated.
• Measurements were expressed in milliliters per kilogram of body weight (ml/kg BW) and as a percentage of total lung capacity at 40 cmH2O (TLC40).

Main Results:

• FRCs averaged 38.6 ± 8.2 and 42 ± 5.9 ml/kg BW across two dog groups.
• Expressing lung volumes as %TLC40 demonstrated marked uniformity for both supine (FRCs) and upright (FRCu) positions.
• This normalization allowed for accurate prediction of FRC from inspiratory capacity (IC).
• The relationship of upright FRC to total lung capacity in dogs was comparable to human data.

Conclusions:

• Expressing resting lung volumes as a percentage of TLC40 provides a more uniform and reliable measure in dogs than ml/kg BW.
• Normalization to %TLC40 facilitates accurate prediction of FRC from IC.
• Anesthetized dogs exhibit lower FRCs (%TLC40) compared to awake humans, likely due to anesthetic effects.