Gas Exchange In Horses Research Articles

Use of the SpO2/FiO2 Diagram to Assess Gas Exchange in Horses Under General Anaesthesia

The aim of this study was to evaluate the validity of the SpO2/FiO2 diagram in estimating gas exchange in horses under general anaesthesia. In this prospective, controlled clinical study were included 10 horses under general anaesthesia. FiO2 was progressively reduced with the following steps: 0.6, 0.4, 0.3 and 0.21; SpO2 was recorded at each step. An arterial blood sample was collected at the steps of 1.0 and 0.21, to calculate intrapulmonary shunt with the Fshunt formula. The Fshunt value calculated at 0.21 FiO2 was defined as “Fshunt 0.21”, the one calculated at 1.0 FiO2 as “Fshunt 1.0”. The FiO2 vs SpO2 data points were analyzed using a computer algorithm which uses the haemoglobin and a fixed value for arterial–venous oxygen difference of 3.5 mL/dL. The algorithm estimates a shunt value fitting the obtained data with an ideal SpO2/FiO2 curve. The value of shunt (Sshunt) was considered for the study. Correlation between “Fshunt 1.0”, “Fshunt0.21” and SShunt was determined using the Spearman Rank Correlation Coefficient test, the analysis of the regression curve and the coefficient of determination (r2). Values of P < .05 were considered statistically significant. A significant and strong correlation (P = .0069; r = 0.839; r2=0.6194) and a significant and moderate correlation (P = .0443; r = 0.644; r2=0.2336) was found between Sshunt and “Fshunt 1.0” and between Sshunt and “Fshunt 0.21”, respectively. The SpO2/FiO2 diagram proved to be a useful and non-invasive tool to characterize gas exchange in horses under general anaesthesia and mechanical ventilation.

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

Simple SummaryHorses are considered to be one of the most challenging domestic species to anaesthetize. Because of the compression of the abdominal visceral organs on the lungs when positioned in dorsal or lateral recumbency, general anaesthesia can cause significant changes to pulmonary function, blood circulation and gas exchange. Consequently, post-operative complications and anaesthetic mortality rates are higher for horses than for other commonly anaesthetized companion animals. There is no ideal method for monitoring respiratory gas concentrations during general anaesthesia, so it is important to know the advantages and limitations of individual methods and the factors that affect monitoring devices. The aim of this review is to summarize previously published studies regarding the causes and effects of intraoperative gas exchange disturbances as well as monitoring methods.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.

Case Studies in Physiology: The Case of the Giant Giraffe.

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Effect of breathing a heliox gas mixture on cardiopulmonary function in horses-anesthetized immediately following exercise

The purpose of this study was to determine the cardiopulmonary effects of breathing a heliox (60% He: 40% O2) carrier gas mixture in horses anesthetized immediately following treadmill exercise. Using a randomized crossover design, six healthy, Thoroughbred horses (five males, one female) weighing 517 ± 36.9 kg were instrumented to enable measurement of heart rate (HR), systolic (SAP), mean (MAP) and diastolic (DAP) carotid arterial pressures, mean pulmonary arterial pressure (PAP), central venous pressure (CVP), pulmonary arterial temperature (TEMP), and cardiac output. Horses were warmed up for 3 minutes at 4 m second−1, then exercised to the point of fatigue at 115% VO2max and immediately anesthetized using detomidine (0.04 mg kg−1 IV) followed by tiletamine-zolazepam (1.25 mg kg−1 IV). Anesthesia was maintained using 1.3 MAC isoflurane (ISO) vaporized in either 100% O2 (O), 60% N2 : 40% O2 (N), or 60% He : 40% O2 (H). End-tidal ISO concentration was measured using a RASCAL II anesthetic gas analyzer. Minute ventilation (MV) was measured during ISO anesthesia using a heated, calibrated pneumotachometer and differential pressure transducer. Ventilation during ISO anesthesia was controlled to maintain constant, similar minute ventilation between the O, N and H treatment groups. Cardiovascular data and samples for arterial and mixed venous blood gas analysis were recorded while the horse stood quietly prior to exercise, 30 seconds after the point of fatigue, immediately prior to isoflurane anesthesia and 15, 30, 45 and 60 minutes after initiating isoflurane anesthesia. Data were analyzed using repeated measures anova with posthoc differences between means identified using Bonferroni's method (p < 0.05). Exercise was associated with significant increases in HR, SAP, MAP, DAP, PAP, CVP, TEMP and significant decreases in pH, HCO3– and BE. Anesthesia was associated with a return of HR, SAP, MAP, DAP, PAP and CVP to pre-exercise values. MV measured during ISO was not different between groups. Horses in the O group had significantly higher values for PaO2 (156.7 mm Hg, 20.9 kPa) compared to the N group (93.6 mm Hg, 12.5 kPa) or the H group (90.4 mm Hg, 12.1 kPa). No other differences between treatments were identified. We conclude that use of H as a carrier gas does not improve gas exchange in horses anesthetized immediately following exercise.

Effect of prolonged heavy exercise on pulmonary gas exchange in horses.

During short-term maximal exercise, horses have impaired pulmonary gas exchange, manifested by diffusion limitation and arterial hypoxemia, without marked ventilation-perfusion (VA/Q) inequality. Whether gas exchange deteriorates progressively during prolonged submaximal exercise has not been investigated. Six thoroughbred horses performed treadmill exercise at approximately 60% of maximal oxygen uptake until exhaustion (28-39 min). Multiple inert gas, blood-gas, hemodynamic, metabolic rate, and ventilatory data were obtained at rest and 5-min intervals during exercise. Oxygen uptake, cardiac output, and alveolar-arterial PO2 gradient were unchanged after the first 5 min of exercise. Alveolar ventilation increased progressively during exercise, from increased tidal volume and respiratory frequency, resulting in an increase in arterial PO2 and decrease in arterial PCO2. At rest there was minimal VA/Q inequality, log SD of the perfusion distribution (log SDQ) = 0.20. This doubled by 5 min of exercise (log SDQ = 0.40) but did not increase further. There was no evidence of alveolar-end-capillary diffusion limitation during exercise. However, there was evidence for gas-phase diffusion limitation at all time points, and enflurane was preferentially overretained. Horses maintain excellent pulmonary gas exchange during exhaustive, submaximal exercise. Although VA/Q inequality is greater than at rest, it is less than observed in most mammals and the effect on gas exchange is minimal.