Impact of Trendelenburg (head down) and reverse Trendelenburg (head up) position on respiratory and cardiovascular function in anaesthetized horses

Healthy nonobese bitches maintain acceptable spontaneous ventilation during laparoscopic ovariectomies

OBJECTIVE

To determine whether end-tidal CO2 (Petco2), tidal volume (Vt), inspiratory minute volume (i), and respiratory rate (RR) remain within reference limits for spontaneously breathing dogs undergoing laparoscopic ovariectomy; characterize changes in these variables over time; and record the proportion of dogs requiring mechanical ventilation on the basis of predetermined criteria.

ANIMALS

22 client-owned dogs.

PROCEDURES

Between January and June 2019, dogs were anesthetized for abdominal insufflation (CO2; 10 mm Hg) and laparoscopic ovariectomy. Respiratory variables (Petco2, Vt, i, and RR) were recorded at 5 time points: preinsufflation (T2), immediately after insufflation (T3), immediately after first ovary extraction (T4), immediately after second ovary extraction (T5), and after abdominal deflation (T6). Results for variables were assessed to identify differences with reference limits and between time points.

RESULTS

At all time points (T2 to T6), Petco2 was within or above the reference limit, Vt was below or within reference limits, and i was either within or above reference limits. Significant temporal changes were detected in all recorded parameters. One dog had to be ventilated prior to abdominal insufflation due to inappropriately low Vt (< 6 mL/kg) and was excluded from further analysis. One of the 21 remaining dogs required rescue ventilation.

CLINICAL RELEVANCE

Following insufflation, there was a tendency toward inadequate ventilation over time characterized by increased Petco2. Twenty of 21 dogs maintained Petco2 within tolerance via a mild increase in RR and did not require assisted ventilation. Dogs undergoing short surgeries may not require assisted ventilation. Clinicians are advised to monitor for hypercapnia and be prepared to assist ventilation if required.

Effect of 15° Reverse Trendelenburg Position on Arterial Oxygen Tension during Isoflurane Anesthesia in Horses

Simple Summary

Horses commonly develop low blood oxygen levels during anesthesia, especially when they are placed on their backs. This study investigated whether a 15° head-up tilt, in a homogenous group of anesthetized horses positioned on their backs, would result in better blood oxygen levels as compared to no tilt. The results showed significantly greater blood oxygen levels with tilt compared to no tilt in five out of six horses tested. In one horse the effect was the opposite. The concurrent effect on cardiovascular function remains to be tested in detail. Further studies are needed to confirm these findings in a larger group of horses and to determine the effects on blood pressure and treatment options.

Abstract

Lower than expected arterial oxygen tension (PaO₂) continues to be an unresolved problem in equine anesthesia. The aim of this randomized, crossover, and prospective study using six adult horses is to determine if a 15° reverse Trendelenburg position (RTP) increases PaO₂ during inhalation anesthesia. Under constant-dose isoflurane anesthesia, dorsally recumbent horses were positioned either horizontally (HP) or in a 15° RTP for 2 h. Lungs were mechanically ventilated (15 mL/kg, 6 breaths/min). Arterial carbon dioxide tension (PaCO₂), PaO₂, inspired oxygen fraction (FiO₂), and end-tidal carbon dioxide tension (EtCO₂) were determined every 30 min during anesthesia. Indices of dead-space ventilation (Vd/Vt), oxygenation (P–F ratio), and perfusion (F–shunt) were calculated. Dobutamine and phenylephrine were used to support mean arterial pressure (MAP). Data are presented as median and range. In one horse, which was deemed an outlier due to its thoracic dimensions and body conformation, indices of oxygenation worsened in RTP compared to HP (median PaO₂ 438 vs. 568 mmHg; P–F ratio 454 vs. 586 mmHg, and F–shunt 13.0 vs. 5.7 mmHg). This horse was excluded from calculations. In the remaining five horses they were significantly better with RTP compared to HP. Results in remaining five horses showed that PaO₂ (502, 467–575 vs. 437, 395–445 mmHg), P-F ratio (518, 484–598 vs. 455, 407–458 mmHg), and F-shunt (10.1, 4.2–11.7 vs. 14.2, 13.8–16.0 mmHg) were significantly different between RTP and HP (p = 0.03). Other variables were not significantly different. In conclusion, the 15° RTP resulted in better oxygenation than HP in dorsally recumbent, isoflurane-anesthetized horses, although worsening of oxygenation may occur in individual horses. A study detailing the cardiovascular consequences of RTP is necessary before it can be recommended for clinical practice.

Risk factors associated with hypoxaemia in horses undergoing general anaesthesia: A retrospective study

BACKGROUND

Hypoxaemia is a common concern during equine general anaesthesia. The prevalence and predictors of hypoxaemia in horses undergoing elective anaesthesia in particular are poorly documented.

OBJECTIVES

To determine and compare the prevalence of hypoxaemia (defined as PaO₂  ≤ 80 mm Hg, on the lowest value recorded) in horses undergoing anaesthesia for exploratory laparotomy and elective procedures, and to identify risk factors in both subpopulations.

STUDY DESIGN

Retrospective cohort study.

METHODS

Data were collected from 774 records of 708 horses undergoing general anaesthesia between April 2017 and August 2020. Potential predictors of hypoxaemia in horses undergoing anaesthesia for exploratory laparotomy or elective procedures were investigated separately by univariable penalised maximum likelihood logistic regression, followed by multivariable analysis. The lowest recorded PaO₂ was used as a single data point for the determination of hypoxaemia and arterial oxygen partial pressure to fractional inspired oxygen ratio analysis.

RESULTS

Hypoxaemia was recorded in 23% horses undergoing exploratory laparotomy compared with 3.8% horses undergoing elective anaesthesia (P < .001). Multivariable analysis showed that weight above 550 kg, large intestinal lesions, and peak inspiratory pressure (PIP) above 30 cmH₂ O were significantly associated with hypoxaemia in horses undergoing exploratory laparotomy. Soft tissue procedures and PIP >30 cmH₂ O were significantly associated with hypoxaemia during elective anaesthesia.

MAIN LIMITATIONS

This study only considered the minimum PaO₂ recorded during anaesthesia. Duration and treatment of hypoxaemia were not considered.

CONCLUSIONS

Based on the lowest PaO₂ value during anaesthesia, in our population, horses undergoing anaesthesia for exploratory laparotomy were over six times more likely to become hypoxaemic than horses undergoing elective procedures. Bodyweight, type of procedure, and high PIP were predictors of hypoxaemia.

Arterial oxygenation in anesthetized horses placed in a 5-degree reverse Trendelenburg position

Low arterial oxygen is a common complication in anesthetized horses and placing the animal in reverse Trendelenburg (RT) position may treat hypoxemia. The objective of this study was to assess the arterial partial pressure of oxygen (PaO₂) in horses placed in a 5-degree RT compared to horizontal (H) position. Client-owned healthy horses (n = 60) undergoing elective surgeries were enrolled in a randomized controlled clinical study. Horses were sedated with butorphanol, an α₂-adrenoceptor agonist, ± acepromazine and induced with ketamine combined with a benzodiazepine, propofol, or guaifenesin. Anesthesia was maintained with isoflurane in oxygen with mechanical ventilation. Each group (RT and H) included 30 horses, 10 in each recumbency (dorsal, right and left lateral). Arterial blood gas analyses (aBG) were performed following arterial catheter placement then hourly. Time first-to-last aBG, changes in PaO₂, dynamic compliance (C_dyn), estimated pulmonary shunt fraction (F-shunt), and alveolar dead space to tidal volume ratio (V_D/V_T) were evaluated with a 2-way analysis of variance. Statistical significance was set at p < .05. Overall, PaO₂ increased in all groups; however no significant difference was found between recumbencies (dorsal, right and left lateral) and RT versus H in changes over time for PaO₂ (p = .064 and p = .070, respectively), C_dyn (p = .721 and p = .672, respectively), F-shunt (p = .055 and p = .054, respectively), or V_D/V_T (p = .616 and p = .064, respectively). In healthy anesthetized horses, 5-degree RT did not affect changes in PaO₂ as compared to H position.