Effects of an alveolar recruitment maneuver on cardiovascular and respiratory parameters during total intravenous anesthesia in ponies

Cardiopulmonary effects of apneustic anesthesia ventilation in anesthetized pigs: a new mode of ventilation for anesthetized veterinary species

Objective

To compare the cardiopulmonary effects of apneustic anesthesia ventilation (AAV) and conventional mechanical ventilation (CMV) in anesthetized pigs and to describe a new mode of ventilation for anesthetized veterinary species.

Study design

Randomized, crossover design without washout.

Animals

Twelve healthy, female white Landrace pigs.

Methods

Following ketamine-midazolam premedication and anesthetic induction with propofol, the trachea was intubated, and each pig was positioned in dorsal recumbency. Anesthesia was maintained with propofol and sufentanil infusions. Pigs were instrumented and their lungs were sequentially ventilated with each mode, in random order, for 1 h according to predefined criteria [fraction of inspired oxygen (FiO2) = 0.21, 10 mL kg-1 tidal volume (VT), and arterial carbon dioxide tension (PaCO2) within 40-45 mmHg]. Cardiopulmonary data were collected at baseline, 30 and 60 min. In 8 pigs, thoracic computed tomography (CT) was performed following the 60 min time point for each mode of ventilation and images were analyzed to quantify lung aeration. The effects of ventilation mode, time, and order were analyzed using repeated measures ANOVA. Paired t-tests were used to compare lung aeration between modes. Significance was defined as p < 0.05.

Results

Data from 12 pigs were analyzed. A significant effect of mode was found for heart rate, mean arterial pressure (MAP), pulmonary artery occlusion pressure, cardiac index (CI), stroke volume index, systemic vascular resistance, pulmonary vascular resistance, oxygen delivery index (DO2I), oxygen extraction ratio (O2ER), VT, arterial oxygen tension, arterial hemoglobin saturation, PaCO2, end-tidal carbon dioxide tension, alveolar dead space (VDalv/VTalv), venous admixture (Q . s / Q . t ), mean airway pressure, and dynamic compliance index (CRSI). Order effects were also observed for some cardiovascular and respiratory variables. For the eight pigs that underwent thoracic CT, AAV resulted in significantly larger proportions of normally and hyperaerated lung while CMV resulted in larger proportions of hypoaerated and atelectatic lung.

Conclusions

In dorsally recumbent anesthetized pigs, ventilated with FiO2 = 0.21, both modes of ventilation supported adequate oxygenation while AAV resulted in higher CRSI, and lower VDalv/VTalv andQ . s / Q . t , compared with CMV. AAV was also associated with lower MAP, CI, and DO2I and higher O2ER compared with CMV. Further investigation of AAV in anesthetized animals is warranted.

Flow-controlled expiration reduces positive end-expiratory pressure requirement in dorsally recumbent, anesthetized horses

Introduction

Equine peri-anesthetic mortality is higher than that for other commonly anesthetized veterinary species. Unique equine pulmonary pathophysiologic aspects are believed to contribute to this mortality due to impairment of gas exchange and subsequent hypoxemia. No consistently reliable solution for the treatment of peri-anesthetic gas exchange impairment is available. Flow-controlled expiration (FLEX) is a ventilatory mode that linearizes gas flow throughout the expiratory phase, reducing the rate of lung emptying and alveolar collapse. FLEX has been shown to improve gas exchange and pulmonary mechanics in anesthetized horses. This study further evaluated FLEX ventilation in anesthetized horses positioned in dorsal recumbency, hypothesizing that after alveolar recruitment, horses ventilated using FLEX would require a lower positive end-expiratory pressure (PEEP) to prevent alveolar closure than horses conventionally ventilated.

Methods

Twelve adult horses were used in this prospective, randomized study. Horses were assigned either to conventional volume-controlled ventilation (VCV) or to FLEX. Following induction of general anesthesia, horses were placed in dorsal recumbency mechanically ventilated for a total of approximately 6.5 hours. Thirty-minutes after starting ventilation with VCV or FLEX, a PEEP-titration alveolar recruitment maneuver was performed at the end of which the PEEP was reduced in decrements of 3 cmH₂O until the alveolar closure pressure was determined. The PEEP was then increased to the previous level and maintained for additional three hours. During this time, the mean arterial blood pressure, pulmonary arterial pressure, central venous blood pressure, cardiac output (CO), dynamic respiratory system compliance and arterial blood gas values were measured.

Results

The alveolar closure pressure was significantly lower (6.5 ± 1.2 vs 11.0 ± 1.5 cmH₂O) and significantly less PEEP was required to prevent alveolar closure (9.5 ± 1.2 vs 14.0 ± 1.5 cmH₂O) for horses ventilated using FLEX compared with VCV. The CO was significantly higher in the horses ventilated with FLEX (37.5 ± 4 vs 30 ± 6 l/min).

Discussion

We concluded that FLEX ventilation was associated with a lower PEEP requirement due to a more homogenous distribution of ventilation in the lungs during expiration. This lower PEEP requirement led to more stable and improved cardiovascular conditions in horses ventilated with FLEX.

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.

Flow-controlled expiration improves respiratory mechanics, ventilation, and gas exchange in anesthetized horses

OBJECTIVE

Mechanical ventilation is usually achieved by active lung inflation during inspiration and passive lung emptying during expiration. By contrast, flow-controlled expiration (FLEX) ventilation actively reduces the rate of lung emptying by causing linear gas flow throughout the expiratory phase. Our aim was to evaluate the effects of FLEX on lung compliance and gas exchange in anesthetized horses in dorsal recumbency.

ANIMALS

8 healthy horses.

PROCEDURES

All animals were anesthetized twice and either ventilated beginning with FLEX or conventional volume-controlled ventilation in a randomized, crossover design. Total anesthesia time was 3 hours, with the ventilatory mode being changed after 1.5 hours. During anesthesia, cardiac output (thermodilution), mean arterial blood pressures, central venous pressure, and pulmonary arterial pressure were recorded. Further, peak, plateau, and mean airway pressures and dynamic lung compliance (Cdyn) were measured. Arterial blood gases were analyzed every 15 minutes. Data were analyzed using ANOVA (P < 0.05).

RESULTS

FLEX ventilation resulted in significantly higher arterial oxygen partial pressures (521 vs 227 mm Hg) and Cdyn (564 vs 431 mL/cm H2O) values compared to volume-controlled ventilation. The peak and plateau airway pressure were lower, but mean airway pressure was significantly higher (4.8 vs 9.2 cm H2O) in FLEX ventilated horses. No difference for cardiovascular parameters were detected.

CLINICAL RELEVANCE

The results of this study showed a significant improvement of the Pao2 and Cdyn without compromising the cardiovascular system when horses were ventilated by use of FLEX compared to conventional ventilation.

Assessment of Regional Ventilation During Recruitment Maneuver by Electrical Impedance Tomography in Dogs

Background

During protective mechanical ventilation, electrical impedance tomography (EIT) is used to monitor alveolar recruitment maneuvers as well as the distribution of regional ventilation. This technique can infer atelectasis and lung overdistention during mechanical ventilation in anesthetized patients or in the ICU. Changes in lung tissue stretching are evaluated by monitoring the electrical impedance of lung tissue with each respiratory cycle.

Objective

This study aimed to evaluate the distribution of regional ventilation during recruitment maneuvers based on the variables obtained in pulmonary electrical impedance tomography during protective mechanical ventilation, focusing on better lung recruitment associated with less or no overdistention.

Methods

Prospective clinical study using seven adult client–owned healthy dogs, weighing 25 ± 6 kg, undergoing elective ovariohysterectomy or orchiectomy. The animals were anesthetized and ventilated in volume-controlled mode (7 ml.kg⁻¹) with stepwise PEEP increases from 0 to 20 cmH₂O in steps of 5 cmH₂O every 5 min and then a stepwise decrease. EIT, respiratory mechanics, oxygenation, and hemodynamic variables were recorded for each PEEP step.

Results

The results show that the regional compliance of the dependent lung significantly increased in the PEEP 10 cmH₂O decrease step when compared with baseline (p < 0.027), and for the nondependent lung, there was a decrease in compliance at PEEP 20 cmH₂O (p = 0.039) compared with baseline. A higher level of PEEP was associated with a significant increase in silent space of the nondependent regions from the PEEP 10 cmH₂O increase step (p = 0.048) until the PEEP 15 cmH₂O (0.019) decrease step with the highest values at PEEP 20 cmH₂0 (p = 0.016), returning to baseline values thereafter. Silent space of the dependent regions did not show any significant changes. Drive pressure decreased significantly in the PEEP 10 and 5 cmH₂O decrease steps (p = 0.032) accompanied by increased respiratory static compliance in the same PEEP step (p = 0.035 and 0.018, respectively).

Conclusions

The regional ventilation distribution assessed by EIT showed that the best PEEP value for recruitment maintenance, capable of decreasing areas of pulmonary atelectasis in dependent regions promoting less overinflation in nondependent areas, was from 10 to 5 cmH₂O decreased steps.

Effects of pulsed inhaled nitric oxide delivery on the distribution of pulmonary perfusion in spontaneously breathing and mechanically ventilated anesthetized ponies

OBJECTIVE

To measure changes in pulmonary perfusion during pulsed inhaled nitric oxide (PiNO) delivery in anesthetized, spontaneously breathing and mechanically ventilated ponies positioned in dorsal recumbency.

ANIMALS

6 adult ponies.

PROCEDURES

Ponies were anesthetized, positioned in dorsal recumbency in a CT gantry, and allowed to breathe spontaneously. Pulmonary artery, right atrial, and facial artery catheters were placed. Analysis time points were baseline, after 30 minutes of PiNO, and 30 minutes after discontinuation of PiNO. At each time point, iodinated contrast medium was injected, and CT angiography was used to measure pulmonary perfusion. Thermodilution was used to measure cardiac output, and arterial and mixed venous blood samples were collected simultaneously and analyzed. Analyses were repeated while ponies were mechanically ventilated.

RESULTS

During PiNO delivery, perfusion to aerated lung regions increased, perfusion to atelectatic lung regions decreased, arterial partial pressure of oxygen increased, and venous admixture and the alveolar-arterial difference in partial pressure of oxygen decreased. Changes in regional perfusion during PiNO delivery were more pronounced when ponies were spontaneously breathing than when they were mechanically ventilated.

CLINICAL RELEVANCE

In anesthetized, dorsally recumbent ponies, PiNO delivery resulted in redistribution of pulmonary perfusion from dependent, atelectatic lung regions to nondependent aerated lung regions, leading to improvements in oxygenation. PiNO may offer a treatment option for impaired oxygenation induced by recumbency.

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.

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.

Effects of positive end-expiratory pressure alone or an open-lung approach on recruited lung volumes and respiratory mechanics of mechanically ventilated horses

OBJECTIVE

To evaluate the effects of positive end-expiratory pressure (PEEP) alone and PEEP preceded by lung recruitment manoeuvre (LRM) on lung volumes and respiratory system mechanics in healthy horses undergoing general anaesthesia.

STUDY DESIGN

Controlled, prospective clinical study.

ANIMALS

A group of 15 horses undergoing arthroscopy.

METHODS

Following anaesthetic induction, initial ventilatory settings were: tidal volume 15 mL kg^-1, inspiratory:expiratory ratio 1:2, respiratory rate to maintain end-tidal CO₂ between 5.3-6.6 kPa (40-50 mmHg). The following settings were implemented sequentially: zero PEEP (ZEEP); PEEP 10 cmH₂O (PEEP); LRM (50 cmH₂O for 20 seconds) followed by 10 cmH₂O of PEEP (LRM + PEEP). Static compliance (C_st), driving pressure, delta end-expiratory (ΔEELV) and recruited lung volumes (RLV) were obtained 30 minutes after initiating each ventilatory strategy. Data were analyzed with paired t test or analysis of variance followed by Tukey's post hoc test. Data are shown as mean ± standard deviation; p < 0.05 was considered significant.

RESULTS

PEEP induced ΔEELV of 6.68 ± 3.36 mL kg^-1; ΔEELV during LRM + PEEP was 14.28 ± 5.59 mL kg^-1 (p < 0.0001). The RLV was greater during the LRM + PEEP phase (12.30 ± 5.85 mL kg^-1) than during PEEP (4.47 ± 3.97 mL kg^-1; p < 0.0001). The C_st was unchanged from ZEEP to PEEP (0.75 ± 0.21 and 0.85 ± 0.22 mL cmH₂O^-1 kg^-1, respectively, p = 0.36) but increased using LRM + PEEP (1.11 ± 0.25 mL cmH₂O^-1 kg^-1, p = 0.0004). Driving pressure was lower during LRM + PEEP than during PEEP and ZEEP (16 ± 2, 19 ± 2 and 21 ± 4 cmH₂O, respectively, p < 0.0001).

CONCLUSIONS AND CLINICAL RELEVANCE

Unlike PEEP alone, PEEP preceded by LRM increased RLV and C_st and reduced driving pressure in horses under anaesthesia.

Determination of reference intervals for equine arterial blood-gas, acid-base and electrolyte analysis

OBJECTIVE

To establish reference intervals for arterial blood-gas (ABG), acid-base and electrolyte values from a healthy equine population.

STUDY DESIGN

Retrospective clinical study.

ANIMALS

A total of 139 client-owned, systemically healthy horses, 1 year of age and older, presented for elective surgical procedures.

METHODS

Blood samples were collected anaerobically from the transverse facial or common carotid artery of horses breathing room air, prior to administration of preanaesthetic medication. Samples were analysed immediately, without correction for body temperature, using an automated bench-top analyser. Variables analysed included pH, arterial partial pressure of carbon dioxide (PaCO₂) and arterial partial pressure of oxygen (PaO₂) and plasma concentrations of sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺) and chloride (Cl^-). Actual and standardized plasma bicarbonate concentration [HCO₃^- (P) and HCO₃^- (P, st)], blood and extracellular fluid base excess [base (B) and base (ECF)] and anion gap (AG) were calculated by the machine from preprogrammed algorithms. Methods used for determination of PaCO₂, PaO₂, HCO₃^- (P), HCO₃^- (P, st), base (B) and base (ECF) met the guidelines of the Clinical and Laboratory Standards Institute. Reference intervals were determined with the nonparametric or the standard parametric method dependent on data distribution.

RESULTS

Reference intervals were determined for pH, 7.37-7.49; PaCO₂, 4.84-7.20 kPa (36.3-54.0 mmHg); PaO₂, 11.01-14.97 kPa (82.6-112.3 mmHg); Na⁺, 133-141 mmol L^-1; K⁺, 3.05-4.65 mmol L^-1; Ca²⁺, 1.34-1.72 mmol L^-1; Cl^-, 100-110 mmol L^-1; HCO₃^- (P), 23.55-33.90 mmol L^-1; HCO₃^- (P, st), 23.87-32.45 mmol L^-1; base (B), 0.51-8.80 mmol L^-1; base (ECF), -0.53 to 9.39 mmol L^-1 and AG, 1.5-11.5 mEq L^-1.

CONCLUSIONS AND CLINICAL RELEVANCE

These data were derived from the largest group of horses reported in a single study and may aid in interpretation of ABG, acid-base and electrolyte measurements in clinical practice.

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.

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

Inappropriate mechanical ventilation can lead to ventilator-induced lung injury (VILI). Aim of this study was to evaluate the effects of inhalation anaesthesia and ventilation with and without recruitment (RM) and PEEP titration on alveolar integrity in horses. Twenty-three horses were divided into 4 groups (group OLC ventilated with OLC, group IPPV ventilated with intermittent positive pressure ventilation, group NV non-ventilated, and group C non-anaesthetized control group). After sedation with xylazine and induction with diazepam and ketamine anaesthetized horses were under isoflurane anaesthesia for 5.5h. The horses were euthanized and tissue samples of the dependent and non-dependent lung areas were collected. Histopathological examinations of the lung tissue as well as relative quantification of mRNA of IL-1β, IL-6, iNOS, MMP1 and MMP9 by PCR were performed. Horses of group OLC had significantly less alveolar congestion and atelectasis but greater alveolar overdistension compared to groups NV and IPPV. In groups OLC and group IPPV an increase in IL-1β/6 and MMP1/9 was detected compared to groups NV and C. In conclusion, in breathing spontaneously or IPPV-ventilated horses a higher degree of atelectasis was detected, whereas in OLC-ventilated horses a higher degree of overdistention was present. Elevated levels in IL and MMP might be early signs of VILI in ventilated horses.

Continuous positive airway pressure (CPAP) decreases pulmonary shunt in anaesthetized horses

OBJECTIVE

To evaluate the effects of continuous positive airway pressure (CPAP) on intrapulmonary shunt, cardiac output and oxygen delivery in horses subjected to a 6 hour period of general anaesthesia.

STUDY DESIGN

Randomized, experimental, crossover study.

ANIMALS

Ten healthy adult horses.

METHODS

Following medetomidine, diazepam and ketamine administration, orotracheal intubation was performed and horses positioned in dorsal recumbency. Anaesthesia was maintained with isoflurane carried in an oxygen and air mix (FiO₂ 0.5) combined with a medetomidine infusion. Horses were anaesthetized twice and either CPAP (8 cmH₂ O) or physiologic airway pressure (NO CPAP) was applied to the lungs for 6 hours; the order of treatments was randomly assigned. Following induction of anaesthesia, cardiovascular and respiratory variables (including arterial blood gas analysis) were recorded every 30 minutes, cardiac output was measured every 60 minutes using the lithium dilution technique and oxygen delivery calculated. If PaCO₂ exceeded 100 mmHg (13.3 kPa), controlled ventilation was initiated and horses excluded from further data collection. Groups were compared using a general linear model.

RESULTS

Data from eight horses were analysed. PaO₂ was 15-56 mmHg (2.00-7.45 kPa) higher (p < 0.001) and shunt fraction 6-14% lower (p < 0.001) in the CPAP group. No differences were seen for cardiac output and oxygen delivery. The lack of difference in oxygen delivery was attributed to lower haemoglobin levels in the CPAP group than in the NO CPAP group.

CONCLUSIONS AND CLINICAL RELEVANCE

CPAP of 8 cmH₂ O can be used in dorsally recumbent horses to decrease pulmonary shunt fraction without causing a decrease in cardiac output during longterm anaesthesia.

Effects of dexmedetomidine and xylazine on cardiovascular function during total intravenous anaesthesia with midazolam and ketamine and recovery quality and duration in horses

OBJECTIVES

To compare cardiovascular effects and recovery quality and duration of total intravenous anaesthesia (TIVA) with xylazine-ketamine-midazolam or dexmedetomidine-ketamine-midazolam.

STUDY DESIGN

Prospective, randomized experimental cross-over trial.

ANIMALS

Eight adult warmblood horses.

METHODS

After sedation with acepromazine and either xylazine [0.5 mg kg(-1) , intravenously (IV)] or dexmedetomidine (3.5 μg kg(-1) IV) anaesthesia was induced with ketamine and midazolam and maintained with a constant rate infusion (CRI) of xylazine (1 mg kg(-1)  hour(-1) ) [XKM] or dexmedetomidine (7 μg kg(-1)  hour(-1) ) [DKM] in combination with midazolam (0.1 mg kg(-1)  hour(-1) ), and ketamine infusion (initially 3 mg kg(-1)  hour(-1) ) for 120 minutes. Ketamine infusion rate was increased in response to positive reactions to electrical nociceptive stimulation performed every 30 minutes. Heart rate (HR), mean arterial blood pressure (MAP) and cardiac output (Q˙t) were measured before treatment (baseline), after sedation (not Q˙t), and during anaesthesia. Xylazine, dexmedetomidine, midazolam and ketamine kinetics were calculated, from plasma drug concentrations. Twenty minutes after end of TIVA, flumazenil (0.01 mg kg(-1) IV) was administered. Recovery quality and duration were assessed. Two-way analysis of variance with repeated measurements or Wilcoxon signed rank test as relevant were used to analyse data with an alpha of 5%.

RESULTS

Compared to baseline, MAP did not change, while similar, but limited, decreases in HR and Q˙t were observed in both TIVA's. Mean ketamine doses of 3.7 mg kg(-1)  hour(-1) were required with both treatments. Plasma concentrations of dexmedetomidine and xylazine showed high intra- and inter-individual changes with elimination half-lifes of 46 ± 7 minutes and 64 ± 13 minutes, respectively. Recovery quality was good to excellent with mean duration of 37 ± 16 and 46 ± 21 minutes after stopping TIVA with XKM and DKM, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Both drug combinations are suitable to maintain anaesthesia for two hours, with good cardiovascular and good to excellent recovery conditions.

Effect of 50% and maximal inspired oxygen concentrations on respiratory variables in isoflurane-anesthetized horses

Background

The purpose of this study was to compare the effects of 0.5 fraction of inspired oxygen (FiO₂) and >0.95 FiO₂ on pulmonary gas exchange, shunt fraction and oxygen delivery (DO₂) in dorsally recumbent horses during inhalant anesthesia. The use of 0.5 FiO₂ has the potential to reduce absorption atelectasis (compared to maximal FiO₂) and augment alveolar oxygen (O₂) tensions (compared to ambient air) thereby improving gas exchange and DO₂. Our hypothesis was that 0.5 FiO₂ would reduce ventilation-perfusion mismatching and increase the fraction of pulmonary blood flow that is oxygenated, thus improving arterial oxygen content and DO₂.

Results

Arterial partial pressures of O₂ were significantly higher than preanesthetic levels at all times during anesthesia in the >0.95 FiO₂ group. Arterial partial pressures of O₂ did not change from preanesthetic levels in the 0.5 FiO₂ group but were significantly lower than in the >0.95 FiO₂ group from 15 to 90 min of anesthesia. Alveolar to arterial O₂ tension difference was increased significantly in both groups during anesthesia compared to preanesthetic values. The alveolar to arterial O₂ tension difference was significantly higher at all times in the >0.95 FiO₂ group compared to the 0.5 FiO₂ group. Oxygen delivery did not change from preanesthetic values in either group during anesthesia but was significantly lower than preanesthetic values 10 min after anesthesia in the 0.5 FiO₂ group. Shunt fraction increased in both groups during anesthesia attaining statistical significance at varying times. Shunt fraction was significantly increased in both groups 10 min after anesthesia but was not different between groups. Alveolar dead space ventilation increased after 3 hr of anesthesia in both groups.

Conclusions

Reducing FiO₂ did not change alveolar dead space ventilation or shunt fraction in dorsally recumbent, mechanically ventilated horses during 3 hr of isoflurane anesthesia. Reducing FiO₂ in dorsally recumbent isoflurane anesthetized horses does not improve oxygenation or oxygen delivery.