Regulation of respiratory muscle activities during chemoreceptor stimulation in adult horses.

The study investigates how the activity of certain respiratory muscles changes in adult horses when exposed to different levels of carbon dioxide (hypercapnia) and lower oxygen levels (hypocapnic hypoxia). Notably, despite changes in muscle activity patterns during hyperpnea (increased breathing), the lung volume at the end of an exhalation remained constant. The results suggest horses’ responses to such chemoreceptor stimulation are similar to those of awake dogs.

Research Methodology

• The researchers studied six fully-grown horses that had been previously prepared for the experiment while they were awake.
• Electromyographic activity, which measures the electrical activity produced by muscles, was monitored in three muscles related to breathing. The muscles examined were the costal portion of the diaphragm and the transverse abdominal and external oblique muscles.
• These measurements were recorded during normal breathing (eupneic breathing) and under two levels each of hypercapnia and hypocapnic hypoxia. Hypercapnia refers to an increase in the concentration of carbon dioxide in the body, while hypocapnic hypoxia is a condition characterized by low levels of oxygen.
• The scientists used the inert gas technique to measure the end-expiratory lung volumes (the volume of lungs at the end of expiration) during regular breathing, during a 6% carbon dioxide challenge, and during 12% oxygen breathing.

Findings

• Under normal breathing conditions, there was always electrical activity in all three observed muscles. This activity started before the commencement of breathing-related mechanical flow.
• As levels of hypercapnia rose, there was increased inspiratory and expiratory electrical activity. Specific attention was given to expiratory muscles, as their recruitment corresponded with significant increases in peak expiratory gastric pressure.
• However, under conditions of hypocapnic hypoxia, there was a varied recruitment of respiratory muscles. The diaphragm’s electrical activity increased earlier in relation to the beginning of mechanical flow and was more intense. However, there was no significant increase in the magnitude and beginning of abdominal expiratory activity, even when the animals’ breathing increased (hyperpnea).
• Regardless of the changes in muscle recruitment patterns during increased breaths, the end-expiratory lung volume remained the same.

Conclusion

• The research concludes that adult horses respond to both peripheral and central chemoreceptor stimulation in a similar way to awake dogs. Changes in conditions such as carbon dioxide and oxygen concentration level impact the activity of various respiratory muscles, but it does not alter the volume of the lungs at the end of an exhalation.