Partitioning of total pulmonary resistance in horses.

This research study measured both upper and lower airway resistance in thoroughbred horses, focusing on how total pulmonary resistance is divided between these two areas. The study also assessed three types of catheters used for internal pressure measurements in the horses.

Methodology

• The study involved 8 Thoroughbred geldings.
• Three different types of catheters were used to measure pressure under both static and dynamic conditions.
• A heated pneumotachograph (a device that measures the rate of air flow) attached to a mask over the horse’s nose was used to determine flow rate.
• The flow signal was electronically integrated to calculate the tidal volume (the volume of air displaced during normal inhalation and exhalation).
• Transpulmonary pressure (pressure in the lungs) was calculated by finding the difference between the pressure in the esophagus, measured using a balloon catheter, and the pressure in the mask.
• Lateral tracheal pressure (pressure in the trachea or windpipe) was measured with a polyethylene catheter inserted percutaneously (through the skin) into the middle portion of the trachea.
• Lower airway pressure was determined by finding the difference between the pressure in the esophagus and the lateral tracheal pressure. Upper airway pressure was calculated by finding the difference between the lateral tracheal pressure and mask pressure.
• Pressures were reported as the difference between maximal and minimal pressures recorded during a respiratory cycle.
• Airway resistance was calculated at 50% of the tidal volume using the isovolume method (a method which measures during periods when the volume is not changing).

Key Findings

• Both individual and group variations were observed in Pi (intra pleural pressure) and Pl/PL (lateral pressure to Transpulmonary pressure ratio), but Pi accounted for over 60% of PL in every horse.
• In 6 out of 8 horses, the lower airway resistance (Rl) was more than 50% of the total pulmonary resistance (RL). However, in the remaining 2 horses, Rl was only 30% and 34% of RL.
• The three different types of catheters showed minimal amplitude distortion under static conditions in in vitro (outside the body) tests. Under dynamic conditions though, the level of distortion varied based on the catheter used, the frequency, and the system’s resistance.