Effect of different inspired fractions of oxygen on F-shunt and arterial partial pressure of oxygen in isoflurane-anaesthetized and mechanically ventilated Shetland ponies

ObjectiveTo determine the effect of fraction of inspired oxygen (FiO₂) on intrapulmonary shunt fraction as measured by F-shunt in ponies during isoflurane anaesthesia.Study designProspective, randomized clinical study.AnimalsA group of 23 adult Shetland ponies undergoing a total of 32 anaesthetic procedures.MethodsPonies were premedicated intravenously (IV) with detomidine (0.01 mg kg^–1) and either morphine (0.1 mg kg^–1) or butorphanol (0.02 mg kg^–1). Anaesthesia was induced with ketamine (2.2 mg kg^–1) and midazolam (0.07 mg kg^–1) administered IV. Ponies were randomly allocated to maintenance of anaesthesia with isoflurane in oxygen (group TH; FiO₂ = 0.95) or a mixture of oxygen and medical air (group TL; FiO₂ = 0.65); all ponies were given a constant rate of infusion of detomidine. Animals were mechanically ventilated to maintain PaCO₂ between 40 and 50 mmHg. Arterial blood gas analysis was performed every 30 minutes. The F-shunt equation was calculated for each time point T0, T30, T60 and T90. Data were analysed using linear mixed model analysis and presented as mean ± standard deviation (p < 0.05).ResultsPaO₂ was greater in group TH than in group TL (TH: 406 ± 90, 438 ± 83, 441 ± 69 and 464 ± 53 mmHg versus TL: 202 ± 90, 186 ± 84, 172 ± 85 and 191 ± 98 mmHg at T0, T30, T60 and T90, respectively; p < 0.0001). In TH, F-shunt was < TL. Significant differences were found at T60 (TH: 13.2% ± 4.3 versus TL: 19.4% ± 8.3; p = 0.016) and T90 (TH: 11.7% ± 3.5 versus TL: 18.6% ± 9.5; p = 0.036).Conclusions and clinical relevanceOur findings do not support a beneficial effect of using a reduced FiO₂ to improve oxygenation in anaesthetized and mechanically ventilated Shetland ponies.     

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

ObjectiveTo 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 designProspective research study.AnimalsA group of eight healthy research horses undergoing laparotomy.MethodsAnesthetized 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.ResultsThe 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 relevanceThe 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.     

Effects of intraoperative positive end-expiratory pressure and fraction of inspired oxygen on postoperative oxygenation in dogs undergoing stifle surgery

ObjectiveTo compare the effects of fraction of inspired oxygen (FiO₂) with the addition of positive end-expiratory pressure (PEEP) during anesthesia on arterial oxygenation in the first 4 postoperative hours in dogs. We hypothesized that compared with dogs breathing FiO₂ ≥ 0.95 and no PEEP (ZEEP), the use of intraoperative PEEP would improve postoperative oxygenation, and that the use of PEEP combined with an FiO₂ of 0.4 would further improve it.Study designProspective, randomized study.AnimalsA total of 30 dogs undergoing unilateral stifle surgery.MethodsUsing a standardized anesthetic protocol, dogs were assigned to either FiO₂ ≥ 0.95 and ZEEP, FiO₂ ≥ 0.95 and 5 cmH₂O PEEP or FiO₂ 0.4 and 5 cmH₂O PEEP. All dogs were mechanically ventilated with a tidal volume of 12 mL kg^–1. Dogs breathed room air after recovery from anesthesia. Arterial blood gases were measured during surgical closure and 10, 120 and 240 minutes after extubation. Demographic characteristics were compared with Kruskal–Wallis tests. The effects of treatment and time on the PaO₂, PaCO₂, PaO₂:FiO₂ and shunt fraction (F-shunt) were assessed with mixed-effect models.ResultsThe PaO₂ and F-shunt were lower during anesthesia for dogs breathing FiO₂ 0.4. No differences among groups were measured after extubation for any variable.Conclusions and clinical relevanceCompared with dogs ventilated with FiO₂ ≥ 0.95 and ZEEP, application of 5 cmH₂O PEEP did not improve intraoperative gas exchange. The combination of 5 cmH₂O PEEP and FiO₂ 0.4 resulted in lower intraoperative F-shunt values. However, no benefits from those maneuvers on postoperative PaO₂ and F-shunt were recorded after extubation, suggesting that alterations in pulmonary function imposed by anesthesia were reversed soon after extubation.     

Detomidine and the combination of detomidine and MK‐467, a peripheral alpha‐2 adrenoceptor antagonist,as premedication in horses anaesthetized with isoflurane

ObjectiveTo investigate MK-467 as part of premedication in horses anaesthetized with isoflurane.Study designExperimental, crossover study with a 14 day wash-out period.AnimalsSeven healthy horses.MethodsThe horses received either detomidine (20 μg kg⁻¹ IV) and butorphanol (20 μg kg⁻¹ IV) alone (DET) or with MK-467 (200 μg kg⁻¹ IV; DET + MK) as premedication. Anaesthesia was induced with ketamine (2.2 mg kg⁻¹) and midazolam (0.06 mg kg⁻¹) IV and maintained with isoflurane. Heart rate (HR), mean arterial pressure (MAP), end-tidal isoflurane concentration, end-tidal carbon dioxide tension, central venous pressure, fraction of inspired oxygen (FiO₂) and cardiac output were recorded. Blood samples were taken for blood gas analysis and to determine plasma drug concentrations. The cardiac index (CI), systemic vascular resistance (SVR), ratio of arterial oxygen tension to inspired oxygen (P_aO₂/FiO₂) and tissue oxygen delivery (DO₂) were calculated. Repeated measures anova was applied for HR, CI, MAP, SVR, lactate and blood gas variables. The Student's t-test was used for pairwise comparisons of drug concentrations, induction times and the amount of dobutamine administered. Significance was set at p < 0.05.ResultsThe induction time was shorter, reduction in MAP was detected, more dobutamine was given and HR and CI were higher after DET+MK, while SVR was higher with DET. Arterial oxygen tension and P_aO₂/FiO₂ (40 minutes after induction), DO₂ and venous partial pressure of oxygen (40 and 60 minutes after induction) were higher with DET+MK. Plasma detomidine concentrations were reduced in the group receiving MK-467. After DET+MK, the area under the plasma concentration time curve of butorphanol was smaller.Conclusions and clinical relevanceMK-467 enhances cardiac function and tissue oxygen delivery in horses sedated with detomidine before isoflurane anaesthesia. This finding could improve patient safety in the perioperative period. The dosage of MK-467 needs to be investigated to minimise the effect of MK-467 on MAP.     

Postoperative oxygenation in healthy dogs following mechanical ventilation with fractions of inspired oxygen of 0.4 or >0.9

ObjectiveTo evaluate arterial oxygenation during the first 4 postoperative hours in dogs administered different fractions of inspired oxygen (FiO₂) during general anesthesia with mechanical ventilation.Study designProspective, randomized clinical trial.AnimalsA total of 20 healthy female dogs, weighing >15 kg and body condition scores 3–7/9, admitted for ovariohysterectomy.MethodsDogs were randomized to breathe an FiO₂ >0.9 or 0.4 during isoflurane anesthesia with intermittent positive pressure ventilation. The intraoperative PaO₂:FiO₂ ratio was recorded during closure of the linea alba. Arterial blood was obtained 5, 60 and 240 minutes after extubation for measurement of PaO₂ and PaCO₂ (FiO₂ = 0.21). Demographic characteristics, duration of anesthesia, PaO₂:FiO₂ ratio and anesthetic agents were compared between groups with Wilcoxon tests. The postoperative PaO₂, PaCO₂, rectal temperature, a visual sedation score and events of hypoxemia (PaO₂ < 80 mmHg) were compared between groups with mixed-effects models or generalized linear mixed models.ResultsGroups were indistinguishable by demographic characteristics, duration of anesthesia, anesthetic agents administered and intraoperative PaO₂:FiO₂ ratio (all p > 0.08). Postoperative PaO₂, PaCO₂, rectal temperature or sedation score were not different between groups (all p > 0.07). During the first 4 postoperative hours, hypoxemia occurred in three and seven dogs that breathed FiO₂ >0.9 or 0.4 during anesthesia, respectively (p = 0.04).Conclusions and clinical relevanceThe results identified no advantage to decreasing FiO₂ to 0.4 during anesthesia with mechanical ventilation with respect to postoperative oxygenation. Moreover, the incidence of hypoxemia in the first 4 hours after anesthesia was higher in these dogs than in dogs breathing FiO₂ >0.9.     

Comparison of the effects of propofol or alfaxalone for anaesthesia induction and maintenance on respiration in cats

ObjectiveTo compare the effects of propofol and alfaxalone on respiration in cats.Study designRandomized, ‘blinded’, prospective clinical trial.AnimalsTwenty cats undergoing ovariohysterectomy.MethodsAfter premedication with medetomidine 0.01 mg kg⁻¹ intramuscularly and meloxicam 0.3 mg kg⁻¹ subcutaneously, the cats were assigned randomly into two groups: group A (n = 10) were administered alfaxalone 5 mg kg⁻¹ minute⁻¹ followed by 10 mg kg⁻¹ hour⁻¹ intravenously (IV) and group P (n = 10) were administered propofol 6 mg kg⁻¹ minute⁻¹ followed by 12 mg kg⁻¹hour⁻¹ IV for induction and maintenance of anaesthesia, respectively. After endotracheal intubation, the tube was connected to a non-rebreathing system delivering 100% oxygen. The anaesthetic maintenance drug rate was adjusted (± 0.5 mg kg⁻¹ hour⁻¹) every 5 minutes according to a scoring sheet based on physiologic variables and clinical signs. If apnoea > 30 seconds, end-tidal carbon dioxide (Pe′CO₂) > 7.3 kPa (55 mmHg) or arterial haemoglobin oxygen saturation (SpO₂) < 90% occurred, manual ventilation was provided. Methadone was administered postoperatively. Data were analyzed using independent-samples t-tests, Fisher's exact test, linear mixed-effects models and binomial test.ResultsManual ventilation was required in two and eight of the cats in group A and P, respectively (p = 0.02). Two cats in both groups showed apnoea. Pe′CO₂ > 7.3 kPa was recorded in zero versus four and SpO₂ < 90% in zero versus six cats in groups A and P respectively. Induction and maintenance dose rates (mean ± SD) were 11.6 ± 0.3 mg kg⁻¹ and 10.7 ± 0.8 mg kg⁻¹ hour⁻¹ for alfaxalone and 11.7 ± 2.7 mg kg⁻¹ and 12.4 ± 0.5 mg kg⁻¹ hour⁻¹ for propofol.Conclusion and clinical relevanceAlfaxalone had less adverse influence on respiration than propofol in cats premedicated with medetomidine. Alfaxalone might be better than propofol for induction and maintenance of anaesthesia when artificial ventilation cannot be provided.     

The cardiorespiratory and anesthetic effects of clinical and supraclinical doses of alfaxalone in cats

ObjectiveTo determine the cardiorespiratory and anesthetic effects of 0, 5, 15, and 50 mg kg^?1 intravenous (IV) alfaxalone in hydroxypropyl beta cyclodextrin (Alfaxan; Jurox Pty Ltd, Rutherford, NSW, Australia) in cats.Study designFour treatments of alfaxalone were administered in sequential order.AnimalsEight healthy adult cats (four male; four female) weighing between 3.71 and 5.91 kg.MethodsCats were instrumented for hemodynamic measurements. Four (0, 5, 15, and 50 mg kg^?1) IV doses of alfaxalone were administered over one minute, with a 3-hour washout period between doses 0, 5, and 15 mg kg^?1 on Day 0. The 50 mg kg^?1 treatment was administered 24 hours later. Measurements of heart rate, aortic systolic, mean, and diastolic blood pressures, pulmonary arterial and right atrial mean pressures, cardiac output, respiratory rate, tidal and minute volumes, and arterial blood pH and blood gases (PaO₂, PaCO₂) were performed at pre-determined intervals. Systemic vascular resistance and rate pressure product were calculated. The quality of induction, maintenance, and recovery from anesthesia and the response to noxious stimulation were categorically scored.ResultsAlfaxalone administration resulted in dose-dependent cardiorespiratory depression. Decreases in arterial blood pressure and increases in heart rate occurred at higher doses. Most variables returned to baseline by 15-30 minutes. Respiratory rate, minute volume, and PaO₂ decreased. Apnea was the most common side effect. Induction and maintenance quality were judged to be good to excellent at all doses and quality of recovery good to excellent at all but the 50 mg kg^?1 dose. The duration of anesthesia and unresponsiveness to noxious stimulation increased with dose. The administration of the 50 mg kg^?1 dose produced marked cardiorespiratory depression and apnea.Conclusions and clinical relevanceAlfaxalone produced dose-dependent anesthesia, cardiorespiratory depression and unresponsiveness to noxious stimulation in unpremedicated cats. Hypoventilation and apnea were the most common side effects.     

Computed tomographic analysis of the effects of two inspired oxygen concentrations on pulmonary aeration in anesthetized and mechanically ventilated dogs

OBJECTIVE: To compare the effect of 2 concentrations of oxygen in inspired gas (fraction of inspired oxygen [FIO(2)] 1.0 or 0.4) on pulmonary aeration and gas exchange in dogs during inhalation anesthesia. ANIMALS: 20 healthy dogs. PROCEDURES: Following administration of acepromazine and morphine, anesthesia was induced in each dog with thiopental and maintained with isoflurane in 100% oxygen (100% group; n = 10) or a mixture of 40% oxygen and air (40% group; 10). Dogs were placed in dorsal recumbency and were mechanically ventilated. After surgery, spiral computed tomography (CT) of the thorax was performed and PaO(2), PaCO(2), and the alveolar-arterial oxygen tension difference (P([A-a])O(2)) were assessed. The lung CT images were analyzed, and the extent of hyperinflated (-1,000 to -901 Hounsfield units [HUs]), normally aerated (-900 to -501 HUs), poorly aerated (-500 to -101 HUs), or nonaerated (-100 to +100 HUs) areas was determined. RESULTS: Compared with the 100% oxygen group, the normally aerated lung area was significantly greater and the poorly aerated and nonaerated areas were significantly smaller in the 40% oxygen group. The time to CT (duration of surgery) was similar in both groups. Although PaCO(2) was similar in both groups, PaO(2) and P((A-a))O(2) were significantly higher in the 100% oxygen group. In both groups, pulmonary atelectasis developed preferentially in caudal lung fields. CONCLUSION AND CLINICAL RELEVANCE: In isoflurane-anesthetized dogs, mechanical ventilation with 40% oxygen appeared to maintain significantly better lung aeration and gas exchange than ventilation with 100% oxygen.     

Transcutaneous monitor approximates PaCO(2) but not PaO(2) in anesthetized rabbits

ObjectiveTo compare the accuracy of transcutaneous (tc) to arterial partial pressure of carbon dioxide (PaCO₂) and partial pressure of oxygen (PaO₂) in anesthetized rabbits.Study designProspective, randomized, experimental study.AnimalsEight healthy adult female New Zealand white rabbits weighing 4.05 ± 0.30 kg.MethodsIsoflurane anesthetized rabbits received six treatments in random order; PaCO₂ < 35, 35-45, and >45 mmHg and PaO₂ < 80, 100-200, >200 mmHg. Arterial and transcutaneous measurements were taken after 15 minutes of stabilization at each condition. Linear regression, correlation and Bland-Altman analysis were performed to compare PtcCO₂ to PaCO₂ and PtcO₂ to PaO₂.ResultsOver a range of measured PaCO₂ values from 21 to 67 mmHg (n = 24) mean bias for PtcCO₂ was -1 mmHg and the 95% limits of agreement were -7 to 5 mmHg. The correlation between PtcCO₂ and PaCO₂ was strong with R² value of 0.9454. Over the entire range of measured PaO₂ values (46-508 mmHg) mean bias for PtcO₂ was -61 mmHg and the 95% limits of agreement were -226 to 104 mmHg. Correlation was poor with R² = 0.5969. Comparing PtcO₂ to PaO₂ over a narrower range [PaO₂ < 150 mmHg (n = 13)] improved the correlation, with an R² value of 0.8518, mean bias of -7 mmHg and 95% limits of agreement from -33 to 19 mmHg.Conclusions and clinical relevanceIn healthy anesthetized rabbits, PtcCO₂ closely approximated PaCO₂. In contrast PtcO₂ underestimated PaO₂, particularly at high values. The PtcCO₂ sensor may be a useful noninvasive way to assess adequacy of ventilation in anesthetized rabbits.     

Physiologic and blood gas effects of xylazine–ketamine versus xylazine–tiletamine–zolazepam immobilization of white-tailed deer before and after oxygen supplementation: a preliminary study

ObjectiveTo compare oxygenation and ventilation in white-tailed deer (Odocoileus virginianus) anesthetized with two treatments with and without oxygen supplementation.Study designRandomized, blinded, crossover study.AnimalsA total of eight healthy adult white-tailed deer weighing 49–62 kg.MethodsEach deer was anesthetized twice intramuscularly: 1) treatment XK, xylazine (2 mg kg^–1) and ketamine (6 mg kg^–1) and 2) treatment XTZ, xylazine (2 mg kg^–1) and tiletamine–zolazepam (4 mg kg^–1). With the deer in sternal position, arterial and venous blood was collected before and at 30 minutes during administration of oxygen at 1 L minute^–1 through a face mask. PaO₂ and heart rate (HR) were compared using two-way repeated measures anova. pH, PaCO₂ and lactate concentration were analyzed using mixed-effects linear models, p < 0.05.ResultsWhen breathing air, PaO₂ was < 80 mmHg (10.7 kPa) in six and seven deer with XK and XTZ, respectively, and of these, PaO₂ was < 60 mmHg (8.0 kPa) in three and five deer, respectively. With oxygen supplementation, PaO₂ increased to 128 ± 4 and 140 ± 5 mmHg (17.1 ± 0.5 and 18.7 ± 0.7 kPa), mean ± standard error, with XK and XTZ, respectively (p < 0.001). PaO₂ was not significantly different between treatments at either time point. HR decreased during oxygen supplementation in both treatments (p < 0.001). Lactate was significantly lower (p = 0.047) with XTZ than with XK (2.2 ± 0.6 versus 3.5 ± 0.6 mmol L^–1) and decreased (p < 0.001) with oxygen supplementation (4.1 ± 0.6 versus 1.6 ± 0.6 mmol L^–1). PaCO₂ increased in XTZ during oxygen breathing.Conclusions and clinical relevanceTreatments XK and XTZ resulted in hypoxemia, which responded to oxygen supplementation. Both treatments are suitable for immobilization of white-tailed deer under the study circumstances.     

Evaluation of chemical restraint,isoflurane anesthesia and methadone or tramadol as preventive analgesia in spotted pacas (Cuniculus paca) subjected to laparoscopy

ObjectiveTo evaluate the efficacy and cardiopulmonary effects of ketamine–midazolam for chemical restraint, isoflurane anesthesia and tramadol or methadone as preventive analgesia in spotted pacas subjected to laparoscopy.Study designProspective placebo-controlled blinded trial.AnimalsA total of eight captive female Cuniculus paca weighing 9.3 ± 0.9 kg.MethodsAnimals were anesthetized on three occasions with 15 day intervals. Manually restrained animals were administered midazolam (0.5 mg kg^–1) and ketamine (25 mg kg^–1) intramuscularly. Anesthesia was induced and maintained with isoflurane 30 minutes later. Tramadol (5 mg kg^–1), methadone (0.5 mg kg^–1) or saline (0.05 mL kg^–1) were administered intramuscularly 15 minutes prior to laparoscopy. Heart rate (HR), respiratory rate, mean arterial pressure (MAP), peripheral oxygen saturation (SpO₂), end-tidal CO₂ partial pressure (Pe′CO₂), end-tidal concentration of isoflurane (Fe′Iso), pH, PaO₂, PaCO₂, bicarbonate (HCO₃^?), anion gap (AG) and base excess (BE) were monitored after chemical restraint, anesthesia induction and at different laparoscopy stages. Postoperative pain was assessed by visual analog scale (VAS) for 24 hours. Variables were compared using anova or Friedman test (p < 0.05).ResultsChemical restraint was effective in 92% of animals. Isoflurane anesthesia was effective; however, HR, MAP, pH and AG decreased, whereas Pe′CO₂, PaO₂, PaCO₂, HCO₃^? and BE increased. MAP was stable with tramadol and methadone treatments; HR, Fe′Iso and postoperative VAS decreased. VAS was lower for a longer time with methadone treatment; SpO₂ and AG decreased, whereas Pe′CO₂, PaCO₂ and HCO₃^? increased.Conclusions and clinical relevanceKetamine–midazolam provided satisfactory restraint. Isoflurane anesthesia for laparoscopy was effective but resulted in hypotension and respiratory acidosis. Tramadol and methadone reduced isoflurane requirements, provided postoperative analgesia and caused hypercapnia, with methadone causing severe respiratory depression. Thus, the anesthetic protocol is adequate for laparoscopy in Cuniculus paca; however, methadone should be avoided.     

Hypoventilation following oxygen administration associated with alfaxalone–dexmedetomidine–midazolam anesthesia in New Zealand White rabbits

ObjectiveTo investigate the relationship between oxygen administration and ventilation in rabbits administered intramuscular alfaxalone–dexmedetomidine–midazolam.Study designProspective, randomized, blinded study.AnimalsA total of 25 New Zealand White rabbits, weighing 3.1–5.9 kg and aged 1 year.MethodsRabbits were anesthetized with intramuscular alfaxalone (4 mg kg^–1), dexmedetomidine (0.1 mg kg^–1) and midazolam (0.2 mg kg^–1) and randomized to wait 5 (n = 8) or 10 (n = 8) minutes between drug injection and oxygen (100%) administration (facemask, 1 L minute^–1). A control group (n = 9) was administered medical air 10 minutes after drug injection. Immediately before (PRE_oxy/air5/10) and 2 minutes after oxygen or medical air (POST_oxy/air5/10), respiratory rate (f_R), pH, PaCO₂, PaO₂, bicarbonate and base excess were recorded by an investigator blinded to treatment allocation. Data [median (range)] were analyzed with Wilcoxon, Mann–Whitney U and Kruskal–Wallis tests and p < 0.05 considered significant.ResultsHypoxemia (PaO₂ < 88 mmHg, 11.7 kPa) was observed at all PRE times: PRE_oxy5 [71 (61–81) mmHg, 9.5 (8.1–10.8) kPa], PRE_oxy10 [58 (36–80) mmHg, 7.7 (4.8–10.7) kPa] and PRE_air10 [48 (32–64) mmHg, 6.4 (4.3–8.5) kPa]. Hypoxemia persisted when breathing air: POST_air10 [49 (33–66) mmHg, 6.5 (4.4–8.8) kPa]. Oxygen administration corrected hypoxemia but was associated with decreased f_R (>70%; p = 0.016, both groups) and hypercapnia (p = 0.016, both groups). Two rabbits (one per oxygen treatment group) were apneic (no thoracic movements for 2.0–2.5 minutes) following oxygen administration. f_R was unchanged when breathing air (p = 0.5). PaCO₂ was higher when breathing oxygen than air (p < 0.001).Conclusions and clinical relevanceEarly oxygen administration resolved anesthesia-induced hypoxemia; however, f_R decreased and PaCO₂ increased indicating that hypoxemic respiratory drive is an important contributor to ventilation using the studied drug combination.     

Blood Gas Values During Intermittent Positive Pressure Ventilation and Spontaneous Ventilation in 160 Anesthetized Horses Positioned in Lateral or Dorsal Recumbency

One hundred sixty horses were anesthetized with xylazine, guaifenesin, thiamylal, and halothane for elective soft tissue and orthopedic procedures. Horses were randomly assigned to one of four groups. Group 1 (n = 40): Horses positioned in lateral (LR_G1,; n = 20) or dorsal (DR_G1,; n = 20) recumbency breathed spontaneously throughout anesthesia. Group 2 (n = 40): Intermittent positive pressure ventilation (IPPV) was instituted throughout anesthesia in horses positioned in lateral (LR_G2; n = 20) or dorsal (DR_G2; n = 20) recumbency. Group 3 (n = 40): Horses positioned in lateral (LR_G3; n = 20) or dorsal (DR_G3; n = 20) recumbency breathed spontaneously for the first half of anesthesia and intermittent positive pressure ventilation was instituted for the second half of anesthesia. Group 4 (n = 40): Intermittent positive pressure ventilation was instituted for the first half of anesthesia in horses positioned in lateral (LR_G4; n = 20) or dorsal (DR_G4; n = 20) recumbency. Spontaneous ventilation (SV) occured for the second half of anesthesia. The mean time of anesthesia was not significantly different within or between groups. The mean time of SV and IPPV was not significantly different in groups 3 and 4. Variables analyzed included pH, PaCO₂, PaO₂, and P(A-a)O₂ (calculated). Spontaneous ventilation resulted in significantly higher PaCO₂ and P(A-a)O₂ values and significantly lower PaO₂ values in LR_G1, and DR_G1, horses compared with LR_G2 and DR_G2 horses. Intermittent positive pressure ventilation resulted in normocarbia and significantly lower P(A-a)O₂ values in LR_G2 and DR_G2 horses. In LR_G2 the Pao₂ values significantly increased from 20 minutes after induction to the end of anesthesia. The PaO₂ and P(A-a)O₂ values were not significantly different from the beginning of anesthesia after IPPV in DR_G2 or DR_G3. The PaO₂ values significantly decreased and the P(A-a)O₂ values significantly increased after return to SV in horses in LR_G4, and DR_G4. The PaO₂ values were lowest and the P(A-a)O₂ values were highest in all horses positioned in dorsal recumbency compared with lateral recumbency and in SV horses compared with IPPV horses. The pH changes paralleled the changes in PaCO₂. Blood gas values during right versus left lateral recumbency in all groups were also evaluated. The PaO₂ values were significantly lower and the P(A-a)O₂ values were significantly higher during SV in horses positioned in left lateral (LR_LG1) compared with right lateral (LR_RG1) recumbency. No other significant changes were found comparing left and right lateral recumbency. Arterial hypoxemia (PaO₂ < 60 mm Hg) developed in 35% of DR_G1 horses and 20% of DR_G2 horses at the end of anesthesia. Arterial hypercarbia (PaCO₂= 50–60 mm Hg) developed in DRoi horses. Arterial hypoxemia that developed in 20% of DR_G3 horses was not improved with IPPV. Arterial hypoxemia developed in 55% of DR_G4 horses after return to SV. Some DR_G4 horses with hypoxemia also developed hypercarbia, whereas some had PaCO₂ values within normal limits. Arterial hypoxemia developed in one LR_G1, and two LR_G4, horses. Hypercarbia developed in onlv one LR_G4 horse.     

Arterial blood gas tensions during recovery in horses anesthetized with apneustic anesthesia ventilation compared with conventional mechanical ventilation

ObjectiveTo compare PaO₂ and PaCO₂ in horses recovering from general anesthesia maintained with either apneustic anesthesia ventilation (AAV) or conventional mechanical ventilation (CMV).Study designRandomized, crossover design.AnimalsA total of 10 healthy adult horses from a university-owned herd.MethodsDorsally recumbent horses were anesthetized with isoflurane in oxygen [inspired oxygen fraction = 0.3 initially, with subsequent titration to maintain PaO₂ ≥ 85 mmHg (11.3 kPa)] and ventilated with AAV or CMV according to predefined criteria [10 mL kg^–1 tidal volume, PaCO₂ 40–45 mmHg (5.3–6.0 kPa) during CMV and < 60 mmHg (8.0 kPa) during AAV]. Horses were weaned from ventilation using a predefined protocol and transferred to a stall for unassisted recovery. Arterial blood samples were collected and analyzed at predefined time points. Tracheal oxygen insufflation at 15 L minute^–1 was provided if PaO₂ < 60 mmHg (8.0 kPa) on any analysis. Time to oxygen insufflation, first movement, sternal recumbency and standing were recorded. Data were analyzed using repeated measures anova, paired t tests and Fisher’s exact test with significance defined as p < 0.05.ResultsData from 10 horses were analyzed. Between modes, PaO₂ was significantly higher immediately after weaning from ventilation and lower at sternal recumbency for AAV than for CMV. No PaCO₂ differences were noted between ventilation modes. All horses ventilated with CMV required supplemental oxygen, whereas three horses ventilated with AAV did not. Time to first movement was shorter with AAV. Time to oxygen insufflation was not different between ventilation modes.ConclusionsAlthough horses ventilated with AAV entered the recovery period with higher PaO₂, this advantage was not sustained during recovery. Whereas fewer horses required supplemental oxygen after AAV, the use of AAV does not preclude the need for routine supplemental oxygen administration in horses recovering from general anesthesia.     

Effect of pressure support ventilation during weaning on ventilation and oxygenation indices in healthy horses recovering from general anesthesia

ObjectiveTo determine if pressure support ventilation (PSV) weaning from general anesthesia affects ventilation or oxygenation in horses.Study designProspective randomized clinical study.AnimalsTwenty client‐owned healthy horses aged 5 ± 2 years, weighing 456 ± 90 kg.MethodsIn the control group (CG; n = 10) weaning was performed by a gradual decrease in respiratory rate (f_R) and in the PSV group (PSVG; n = 10) by a gradual decrease in f_R with PSV. The effect of weaning was considered suboptimal if PaCO₂ > 50 mmHg, arterial pH < 7.35 plus PaCO₂ > 50 mmHg or PaO₂ < 60 mmHg were observed at any time after disconnection from the ventilator until 30 minutes after the horse stood. Threshold values for each index were established and the predictive power of these values was tested.ResultsPressure support ventilation group (PSVG) had (mean ± SD) pH 7.36 ± 0.02 and PaCO₂ 41 ± 3 mmHg at weaning and the average lowest PaO₂ 69 ± 6 mmHg was observed 15 minutes post weaning. The CG had pH 7.32 ± 0.02 and PaCO₂ 57 ± 6 mmHg at weaning and the average lowest PaO₂ 48 ± 5 mmHg at 15 minutes post weaning. No accuracy in predicting weaning effect was observed for f_R (p = 0.3474), minute volume (p = 0.1153), SaO₂ (p = 0.1737) and PaO₂/PAO₂ (p = 0.1529). A high accuracy in predicting an optimal effect of weaning was observed for V_T > 10 L (p = 0.0001), f_R/V_T ratio ≤ 0.60 breaths minute^?1 L^?1 (p = 0.0001), V_T/bodyweight > 18.5 mL kg^?1 (p = 0.0001) and PaO₂/FiO₂ > 298 (p = 0.0002) at weaning. A high accuracy in predicting a suboptimal effect of weaning was observed for V_T < 10 L (p = 0.0001), f_R/V_T ratio ≥ 0.60 breaths minute^?1 L^?1 (p = 0.0001) and Pe′CO₂ ≥ 38 mmHg (p = 0.0001) at weaning.Conclusions and clinical relevancePressure support ventilation (PSV) weaning had a better respiratory outcome. A higher V_T, V_T/body weight, PaO₂/FiO₂ ratio and a lower f_R/V_T ratio and Pe′CO₂ were accurate in predicting the effect of weaning in healthy horses recovering from general anesthesia.     

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

ObjectiveTo compare the efficacy of single-breath continuous positive airway pressure manoeuvre (CPAP-M) with inhaled salbutamol, and a combination of both.Study designRandomized, clinical study.AnimalsA total of 62 client-owned horses (American Society of Anesthesiologists status III–V) anaesthetized for laparotomy.MethodsHorses 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.ResultsSalbutamol 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 relevanceSalbutamol 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.     

Effect of intravenous butorphanol infusion on the minimum alveolar concentration of isoflurane in cats

ObjectiveTo determine the effect of butorphanol, administered by intravenous (IV) infusion, on the minimum alveolar concentration of isoflurane (MAC_ISO) in cats and to examine the dosage dependence of this effect.Study designRandomized, placebo-controlled, crossover experimental study.AnimalsA group of six healthy adult male neutered cats.MethodsCats were anesthetized with isoflurane in oxygen. A venous catheter was placed for fluid and drug administration, and an arterial catheter was placed for measurement of arterial pressure and blood sampling. Four treatments were administered at random with at least 2 week interval between treatments: saline (control), butorphanol low dosage (treatment LD; 0.25 mg kg^–1 IV bolus followed by 85 μg kg^–1 minute^–1 for 20 minutes, then 43 μg kg^–1 minute^–1 for 40 minutes, then 19 μg kg^–1 minute^–1), medium dosage (treatment MD, double the dosages in LD) and high dosage (treatment HD, quadruple the dosages in LD). MAC_ISO was determined in duplicate using the bracketing technique and tail clamping. Pulse rate, arterial pressure, hemoglobin oxygen saturation, end-tidal partial pressure of carbon dioxide and arterial blood gas and pH were measured.ResultsButorphanol reduced MAC_ISO in a dosage-dependent manner, by 23 ± 8%, 37 ± 12% and 68 ± 10% (mean ± standard deviation) in treatments LD, MD and HD, respectively. The main cardiopulmonary effect observed was a decrease in pulse rate, significant in treatment HD compared with control.Conclusions and clinical relevanceButorphanol caused a dosage-dependent MAC_ISO reduction in cats. IV infusion of butorphanol may be of interest for partial IV anesthesia in cats.     

Effects of changing body position on oxygenation and arterial blood pressures in foals anesthetized with guaifenesin, ketamine, and xylazine

ObjectiveTo investigate the impact of a change in body position on blood gases and arterial blood pressures in foals anesthetized with guaifenesin, ketamine, and xylazine.Study designProspective, randomized experimental study.AnimalsTwelve Quarter Horse foals, age of 5.4 ±0.9 months and weighing 222 ± 48 kg.MethodsFoals were anesthetized with guaifenesin, ketamine, and xylazine for 40 minutes in lateral recumbency and then assigned to a change in lateral recumbency after hoisting (Group 1, n = 6), or no change (Group 2, n = 6). Oxygen 15 L minute^?1 was insufflated into the endotracheal tube throughout anesthesia. Arterial blood pressure, heart rate, respiratory rate (f_R), inspired fraction of oxygen (FiO₂), and end-tidal carbon dioxide (Pe’CO₂) were measured every 5 minutes. Arterial pH and blood gases [arterial partial pressure of oxygen (PaO₂), arterial partial pressure of carbon dioxide (PaCO₂)] were measured at 10, 30, and 40 minutes after induction, and 5 minutes after hoisting. Alveolar dead space ventilation and PaO₂/FiO₂ were calculated. Two repeated measures models were used. All hypothesis tests were two-sided and significance level was α = 0.05. All values are presented as least square means ± SE.ResultsValues at time-matched points from the two groups were not significantly different so they were combined. Arterial partial pressure of oxygen decreased significantly from 149 ± 14.4 mmHg before hoisting to 92 ± 11.6 mmHg after hoisting (p=0.0013). The PaO₂/FiO₂ ratio decreased from 275 ± 30 to 175 ± 24 (p=0.0055). End-tidal carbon dioxide decreased significantly from 48.7 ± 1.6 to 44.5 ± 1.2 mmHg (p=0.021). Arterial partial pressure of carbon dioxide, blood pressures and heart rates measured 5 minutes after hoisting were not different from measurements obtained before hoisting.Conclusion and clinical relevanceHoisting decreased PaO₂ in anesthetized healthy foals. Administration of supplemental oxygen is recommended to counter the decrease in oxygenation and PaO₂ measurement is necessary to detect early changes.     

The effect of the inspired oxygen fraction on arterial blood oxygenation in spontaneously breathing,isoflurane anaesthetized horses: a retrospective study

ObjectivesTo investigate the influence of two inspired oxygen fractions (FIO₂) on the arterial oxygenation in horses anaesthetized with isoflurane.Study DesignRetrospective, case-control clinical study.AnimalsTwo hundred equine patients undergoing non-abdominal surgery (ASA class 1–2), using a standardized anaesthetic protocol and selected from anaesthetic records of a period of three years, based on pre-defined inclusion criteria.MethodsIn group O (n = 100), medical oxygen acted as carrier gas, while in group M (n = 100), a medical mixture of oxygen and air (FIO₂ 0.60) was used. Demographic data, FIO₂, arterial oxygen tension (PaO₂) and routinely monitored physiologic data were recorded. The alveolar-arterial oxygen tension difference [P(A-a)O₂] and PaO₂/FIO₂ ratio were calculated. The area under the curve, standardized to the anaesthetic duration, was calculated and statistically compared between groups using t-tests or Mann–Whitney tests as appropriate. Categorical data were compared using Chi-square tests.ResultsNo significant differences in age, body weight, sex, breed, surgical procedure, position, anaesthetic duration or arterial carbon dioxide tension were found. Mean FIO₂ was 0.78 in group O and 0.60 in group M. Compared to group O, significantly lower values for PaO₂ and for P(A-a)O₂ were found in group M. In contrast, the PaO₂/FIO₂ ratio and the percentage of horses with a PaO₂ <100 mmHg (13.33 kPa) were comparable in both groups.ConclusionsAlthough a reduction of the inspired oxygen fraction resulted in a lower PaO₂, the P(A-a)O₂ was also lower and the number of horses with PaO₂ values <100 mmHg was comparable.Clinical relevanceIn healthy isoflurane anaesthetized horses, the use of a mixture of oxygen and air as carrier gas seems acceptable, but further, prospective studies are needed to confirm whether it results in a lower degree of ventilation/perfusion mismatching.