Histopathological changes and mRNA expression in lungs of horses after inhalation anaesthesia with different ventilation strategies

Comparison of inhaled salbutamol and salmeterol for the treatment of arterial hypoxaemia in anaesthetized horses: a randomized clinical trial

OBJECTIVE

To compare the efficacy of inhaled salbutamol with salmeterol for the treatment of arterial hypoxaemia in anaesthetized horses.

STUDY DESIGN

Prospective, randomized, clinical study.

ANIMALS

A total of 108 client-owned horses (American Society of Anesthesiologists status I-V) anaesthetized for elective and emergency procedures.

METHODS

Horses were premedicated with acepromazine [intramuscularly 0.1 mg kg-1 or intravenously (IV) 0.05 mg kg-1] and xylazine (0.6 mg kg-1 IV). Midazolam (0.06 mg kg-1 IV) and ketamine (2.2 mg kg-1 IV) were combined to induce anaesthesia, and isoflurane in oxygen/air mixture (inspired oxygen fraction 0.7) was used for maintenance of anaesthesia. Mechanical ventilation was initiated without delay using the following ventilator settings: tidal volume 10 mL kg-1, respiratory rate 8 breaths minute-1, inspiratory-to-expiratory time ratio 1:2, no positive end-expiratory pressure. If arterial blood gas analysis revealed PaO2 < 100 mmHg (13.3 kPa), the administration of either inhaled salbutamol (2 μg kg-1) or salmeterol (0.5 μg kg-1) was randomly assigned Blood gas analysis was repeated 15 and 30 minutes after treatment. The intervention was considered successful when PaO2 after treatment ≥ 1.2 × PaO2 before treatment (i.e. ≥20% increase). PaO2 at 15 and 30 minutes was compared between groups using Mann-Whitney U test; p < 0.05 was considered significant.

RESULTS

Of the 108 horses, 60 were administered salbutamol, 65% and 60% responded successfully at 15 and 30 minutes, increasing their initial PaO2 by 38% and 44%, respectively. The other 48 horses were administered salmeterol, 35% responded successfully at 15 and 30 minutes, increasing their initial PaO2 by 3% and 4%, respectively. PaO2 was significantly higher after salbutamol than after salmeterol at 15 and 30 minutes.

CONCLUSIONS AND CLINICAL RELEVANCE

Using the described protocol, inhaled salbutamol was more effective than salmeterol in improving PaO2 in anaesthetized horses with value < 100 mmHg (13.3 kPa).

Effect of end-inspiratory pause on airway and physiological dead space in anesthetized horses

OBJECTIVE

To evaluate the impact of a 30% end-inspiratory pause (EIP) on alveolar tidal volume (V_Talv), airway (V_Daw) and physiological (V_Dphys) dead spaces in mechanically ventilated horses using volumetric capnography, and to evaluate the effect of EIP on carbon dioxide (CO₂) elimination per breath (Vco₂br^-1), PaCO_2, and the ratio of PaO₂-to-fractional inspired oxygen (PaO₂:FiO₂).

STUDY DESIGN

Prospective research study.

ANIMALS

A group of eight healthy research horses undergoing laparotomy.

METHODS

Anesthetized horses were mechanically ventilated as follows: 6 breaths minute^-1, tidal volume (V_T) 13 mL kg^-1, inspiratory-to-expiratory time ratio 1:2, positive end-expiratory pressure 5 cmH₂O and EIP 0%. Vco₂br^-1 and expired tidal volume (V_TE) of 10 consecutive breaths were recorded 30 minutes after induction, after adding 30% EIP and upon EIP removal to construct volumetric capnograms. A stabilization period of 15 minutes was allowed between phases. Data were analyzed using a mixed-effect linear model. Significance was set at p < 0.05.

RESULTS

The EIP decreased V_Daw from 6.6 (6.1-6.7) to 5.5 (5.3-6.1) mL kg^-1 (p < 0.001) and increased V_Talv from 7.7 ± 0.7 to 8.6 ± 0.6 mL kg^-1 (p = 0.002) without changing the V_TE. The V_Dphys to V_TE ratio decreased from 51.0% to 45.5% (p < 0.001) with EIP. The EIP also increased PaO₂:FiO₂ from 393.3 ± 160.7 to 450.5 ± 182.5 mmHg (52.5 ± 21.4 to 60.0 ± 24.3 kPa; p < 0.001) and Vco₂br^-1 from 0.49 (0.45-0.50) to 0.59 (0.45-0.61) mL kg^-1 (p = 0.008) without reducing PaCO₂.

CONCLUSIONS AND CLINICAL RELEVANCE

The EIP improved oxygenation and reduced V_Daw and V_Dphys, without reductions in PaCO₂. Future studies should evaluate the impact of different EIP in healthy and pathological equine populations under anesthesia.

The optimal PEEP after alveolar recruitment maneuver assessed by electrical impedance tomography in healthy horses

Background

Electrical impedance tomography (EIT) has been an essential tool for assessing pulmonary ventilation in several situations, such as the alveolar recruitment maneuver (ARM) in PEEP titration to maintain the lungs open after atelectasis reversion. In the same way as in humans and dogs, in horses, this tool has been widely used to assess pulmonary aeration undergoing anesthesia, mechanical ventilation, recruitment maneuver, standing horses, or specific procedures.

Objectives

The present study aimed to evaluate the distribution of regional ventilation during ARM based on lung monitoring assessment by EIT, with a focus on better recruitment associated with less or no overdistention.

Methods

Fourteen horses of 306 ± 21 kg undergoing isoflurane anesthesia in dorsal recumbency were used. The animals were mechanically ventilated with a tidal volume of 14 ml kg^-1 and a respiratory rate of 7-9. An alveolar recruitment maneuver was instituted, increasing the PEEP by five cmH₂O every 5 min until 32 cmH₂O and decreasing it by five cmH₂O every 5 min to 7 cmH₂O. At each step of PEEP, arterial blood samples were collected for blood gas analysis, EIT images, hemodynamic, and respiratory mechanics.

Results

Associated with the CoV-DV increase, there was a significant decrease in the DSS during the ARM and a significant increase in the NSS when PEEP was applied above 12 cmH₂O compared to baseline. The Compl_ROI showed a significant increase in the dependent area and a significant decrease in the non-dependent area during ARM, and both were compared to their baseline values. The driving pressure decreased significantly during the ARM, and Cst, PaO₂, and PaO₂/FiO₂ ratio increased significantly. The V_D/V_T decreased significantly at DEPEEP17 and DEPEEP12. There was an HR increase at INPEEP27, INPEEP 32, and DEPEEP17 (p < 0.0001; p < 0.0001; and p < 0.05, respectively), those values being above the normal reference range for the species. The SAP, MAP, DAP, CI, and DO₂I significantly decreased INPEEP32 (p < 0.05).

Conclusion

The ARM by PEEP titration applied in the present study showed better ventilation distribution associated with better aeration in the dependent lung areas, with minimal overdistention between PEEP 17 and 12 cmH₂O decreasing step. Those changes were also followed by improvements in static and regional compliance associated with increased oxygenation and pulmonary ventilation. ARM promoted a transitory decrease in arterial blood pressure and depression in CI with a concomitant drop in oxygen delivery, which should be best investigated before its routine use in clinical cases.

Comparison of single-breath continuous positive airway pressure manoeuvre with inhaled salbutamol to improve oxygenation in horses anaesthetized for laparotomy

OBJECTIVE

To compare the efficacy of single-breath continuous positive airway pressure manoeuvre (CPAP-M) with inhaled salbutamol, and a combination of both.

STUDY DESIGN

Randomized, clinical study.

ANIMALS

A total of 62 client-owned horses (American Society of Anesthesiologists status III-V) anaesthetized for laparotomy.

METHODS

Horses were premedicated with intravenous (IV) xylazine (0.4-0.6 mg kg^-1), anaesthesia was induced with midazolam (0.06 mg kg^-1 IV) and ketamine (2.2 mg kg^-1 IV) and maintained with isoflurane in oxygen using volume-controlled ventilation without positive end-expiratory pressure. If PaO₂ was < 100 mmHg (13.3 kPa), either a CPAP-M (50 cmH₂O for 45 seconds) or salbutamol (0.002 mg kg^-1) was administered. The intervention was considered successful if PaO₂ reached 100 mmHg (13.3 kPa). If PaO₂ remained < 100 mmHg (13.3 kPa), treatments were switched. PaO₂/FiO₂ ratio and estimated shunt fraction (F-shunt) were derived from data obtained from arterial blood gas measurements. Dynamic compliance (C_dyn) was calculated from variables recorded at the moment of arterial blood analysis. Fisher's exact tests compared success rates between treatments, and linear models were performed to test whether the treatment modified the values of the measurements; p < 0.05.

RESULTS

Salbutamol was the first intervention in 28 horses and was effective in 22 horses. CPAP-M was the first intervention in 34 horses and was effective in 26 horses. CPAP-M after salbutamol was performed in six horses, with four responders, and salbutamol after CPAP-M was administered to eight horses, with one responder. Salbutamol, but not CPAP-M, significantly decreased F-shunt. Both salbutamol and CPAP-M significantly increased C_dyn.

CONCLUSIONS AND CLINICAL RELEVANCE

Salbutamol and CPAP-M were comparably effective in improving oxygenation and C_dyn in anaesthetized horses with PaO₂ < 100 mmHg (13.3 kPa). Whether combining both treatments might be beneficial needs to be confirmed on a larger number of horses.

Prolonged Recovery From General Anesthesia Possibly Related to Persistent Hypoxemia in a Draft Horse

Horses are susceptible to developing large areas of pulmonary atelectasis during recumbency and anesthesia. The subsequent pulmonary shunt is responsible for significant impairment of oxygenation. Since ventilation perfusion mismatch persists into the post-operative period, hypoxemia remains an important concern in the recovery stall. This case report describes the diagnosis and supportive therapy of persistent hypoxemia in a 914 kg draft horse after isoflurane anesthesia. It highlights how challenging it can be to deal with hypoxemia after disconnection from the anesthesia machine and how life-threatening it can become if refractory to treatment. Furthermore, it stresses the point on the interactions between hypoxemia and other factors, such as residual drug effects and hypothermia, that should also be considered in the case of delayed recovery from general anesthesia.