Causes, Effects and Methods of Monitoring Gas Exchange Disturbances during Equine General Anaesthesia

Elżbieta Stefanik, Olga Drewnowska, Barbara Lisowska, Bernard Turek, Elżbieta Stefanik, Olga Drewnowska, Barbara Lisowska, Bernard Turek

Abstract

Horses, due to their unique anatomy and physiology, are particularly prone to intraoperative cardiopulmonary disorders. In dorsally recumbent horses, chest wall movement is restricted and the lungs are compressed by the abdominal organs, leading to the collapse of the alveoli. This results in hypoventilation, leading to hypercapnia and respiratory acidosis as well as impaired tissue oxygen supply (hypoxia). The most common mechanisms disturbing gas exchange are hypoventilation, atelectasis, ventilation-perfusion (V/Q) mismatch and shunt. Gas exchange disturbances are considered to be an important factor contributing to the high anaesthetic mortality rate and numerous post-anaesthetic side effects. Current monitoring methods, such as a pulse oximetry, capnography, arterial blood gas measurements and spirometry, may not be sufficient by themselves, and only in combination with each other can they provide extensive information about the condition of the patient. A new, promising, complementary method is near-infrared spectroscopy (NIRS). The purpose of this article is to review the negative effect of general anaesthesia on the gas exchange in horses and describe the post-operative complications resulting from it. Understanding the changes that occur during general anaesthesia and the factors that affect them, as well as improving gas monitoring techniques, can improve the post-aesthetic survival rate and minimize post-operative complications.

Keywords: NIRS; equine anaesthesia; gas exchange; hypoxemia; pulse oximetry; respiratory monitoring; ventilation.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The haemoglobin dissociation curve shows the relationship between oxygen partial pressure (PaO2) and haemoglobin oxygen saturation (SaO2). P50 is the PaO2 value at which 50% of haemoglobin is saturated with O2 at a blood pH of 7.4 and temperature of 37 °C. Adapted with permission from [63] Copyright 2016 John Willey and Sons.
Figure 2
Figure 2
Sensor for measuring oxygenation of cerebral blood using the NIRS method (by Masimo®, Irvine, CA, USA), located above the dorsal sagittal sinus, which is a pool of venous blood from the cerebral hemispheres (horse is in the lateral recumbency).
Figure 3
Figure 3
Multifunctional patient monitor (Datex Ohmeda Cardiocap S5, Helsinki, Finland), used routinely during anaesthesia (monitor on the right) and the monitor for brain oximetry (Root with Sedline, Masimo), measured with NIRS (on the left). This device also measures density spectral array.
Figure 4
Figure 4
Special arterial catheter placed in the transversal facial artery, which allows one to take samples of arterial blood during a surgical procedure.
Figure 5
Figure 5
The compliance loop demonstrates the relationship between pressure and volume. Adapted with permission from [86] Copyright 2010 Y.P.S. Moens.
Figure 6
Figure 6
The resistance loop demonstrates the relationship between flow and volume. Loop A indicates typical resistance with a high peak flow. Loop B suggests increased airway resistance (decreased peak flow). Adapted with permission from [86] Copyright 2010 Y.P.S. Moens.
Figure 7
Figure 7
Normal capnogram. The description is included in the text. Adapted with permission from [91] Copyright 2013 Elsevier.

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