EVIDENCE LIBRARY
Body position shifted ventilation during a midazolam-containing anesthesia protocol in beagles
A veterinary study detected position-related shifts in the distribution of ventilation during combined anesthesia. Its design did not isolate midazolam’s contribution.
In nine healthy beagles, researchers detected small changes in where ventilation was distributed when the dogs were repositioned under anesthesia. They did not detect corresponding position effects while the dogs were awake. Midazolam was one component of the anesthetic protocol, alongside other drugs and procedural changes.
What the belt measured
The researchers used electrical impedance tomography, which estimates the regional distribution of ventilation from changes in electrical impedance around the chest. During anesthesia, the left-to-right center of ventilation shifted from 46.8% with the dogs on their backs to 49.8% with the dogs on their chests. The ventral-to-dorsal center moved from 49.8% on their backs to 51.8% on their left sides. These are shifts in the location of an imaging signal. The percentages do not describe the fraction of breathing lost.
Why the regimen matters
The same dogs were studied awake first, then received midazolam, medetomidine and butorphanol, followed by preoxygenation and propofol anesthesia. They were intubated and breathed spontaneously during the anesthetized measurements. Without a midazolam-only comparison, the study cannot assign the observed shift to midazolam or estimate its contribution.
Position was also a measurement condition
The researchers tested positions in a fixed sequence. Carryover from earlier positions or induction may therefore have influenced the measurements. They also noted that belt movement or changes in the position of internal organs could change what the belt imaged. For forensic interpretation, this experiment illustrates why a position-dependent signal under a multidrug protocol cannot establish a drug-specific respiratory effect in a person. A separate animal study illustrates another attribution problem when drugs are given in combination.
Source: Ambrisko TD, Schramel JP, Auer U, Moens YPS. PLOS ONE. 2017, 12:e0183340. doi:10.1371/journal.pone.0183340.