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.     

Effects of Carbon Dioxide Insufflation and Body Position on Blood Gas Values in Horses Anesthetized for Laparoscopy

The objective of the study was to describe the effects of carbon dioxide pneumoperitoneum and Trendelenburg position on arterial blood gas values in horses anesthetized for laparoscopy. The study design was a prospective case series using 14 healthy adult horses anesthetized for elective laparoscopic surgery. All horses in the study were maintained under anesthesia with halothane in oxygen with intermittent positive-pressure ventilation. A pneumoperitoneum of 15 mmHg or less was achieved with carbon dioxide, and horses were tilted to a 35-degree Trendelenburg position to allow the completion of laparoscopic cryptorchidectomy (n = 13) or ovariectomy (n = 1). Heart rate, mean arterial pressure, and arterial blood gases were recorded at six time intervals throughout the procedure. Results of the study indicated a pH that decreased and partial pressure of carbon dioxide (PaCO₂) and mean arterial pressure that increased over time and differed significantly from baseline during Trendelenburg position. Partial pressure of oxygen (PaO₂) was significantly lower than baseline after assumption of Trendelenburg position and did not improve on return to normal recumbency and abdominal pressure. As body weight increased, pH and PaO₂ decreased and PaCO₂ increased. We concluded that horses placed in Trendelenburg position have changes that are transient, with the exception of PaO₂. Heavier horses have a greater change in pH, PaCO₂, and PaO₂ than lighter horses during abdominal insufflation and Trendelenburg position. The changes incurred during CO₂ abdominal insufflation and Trendelenburg position are transient, with the exception of a decreased PaO₂. Heavy horses undergoing abdominal insufflation and Trendelenburg position should be closely monitored for critical cardiopulmonary values.     

Comparison of the efficacy of two ventilatory strategies in improving arterial oxygen tension and content in anaesthetized sheep

ObjectiveTo compare F-shunt and oxygen content indices in sheep ventilated with a positive end-expiratory pressure (PEEP) of 5 cmH₂O alone or preceded by a stepwise alveolar recruitment manoeuvre (ARM).Study designRandomized crossover design.AnimalsA total of six nonpregnant Brogna ewes weighing 34–47 kg, undergoing thoracolumbar magnetic resonance scan.MethodsIn medetomidine-sedated sheep, anaesthesia was induced with propofol and maintained with isoflurane 1.1% ± 0.1% and an inspired oxygen fraction (FiO₂) of 0.4. Animals were placed in left lateral recumbency and, after 10 minutes of spontaneous breathing, mechanically ventilated with 5 cmH₂O of PEEP with (group ARM) or without (group PEEP) a stepwise recruitment manoeuvre. Maintaining a fixed driving pressure of 15 cmH₂O, PEEP was increased from 0 to 20 cmH₂O every 3 minutes in 5 cmH₂O increments. In each sheep, arterial blood samples were collected to measure arterial gases and to calculate F-shunt, PaO₂/alveolar oxygen partial pressure (PAO₂) and PaO₂/FIO₂ during spontaneous breathing before mechanical ventilation (T₀), after 20 minutes of ventilation (T₂₀) and during spontaneous breathing at extubation (T_ext).ResultsBoth ventilatory strategies improved the arterial oxygen content although four animals in group PEEP showed oxygen content compatible with hypoxia compared with group ARM. F-shunt values were not statistically different at any time point in sheep that underwent only PEEP ventilation while they decreased at T₂₀ and T_ext compared with T₀ in group ARM. At extubation F-shunt was statistically lower in sheep that underwent an ARM. Mechanical ventilation improved PaO₂/PAO₂ and PaO₂/FIO₂ but they did not differ between groups.Conclusionsand clinical relevance The stepwise ARM evaluated in this study improved oxygenation indices and decreased F-shunt. This effect was maintained at extubation compared with sheep that were ventilated with only PEEP 5 cmH₂O.     

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.     

Cardiovascular effects of medetomidine-ketamine anaesthesia in sheep, with and without 100% oxygen, and its reversal with atipamezole.

The cardiovascular effects of intravenously (iv) administered medetomidine 20 μg/kg bodyweight (bwt) and ketamine (2 mg/kgbwt), with and without 100% inspired oxygen, were investigated in six domestic sheep. A second dose of medetomidine and ketamine was administered iv, at dose 10 μg/kg bwt and 1 mg/kg bwt respectively, 25 minutes after the initial injection. Heart rate, PaO₂ pH and haemoglobin saturation decreased whereas PaCO₂ and base excess increased post-injection. Transient hypertension and an increase in respiration rate were evident within the first 10 minutes of anaesthesia. Significant hypoxaemia (P<0.01) developed in sheep breathing room air. Inspired 100% oxygen improved PaO₂ (but the difference was not significant), and improved haemoglobin saturation significantly (P<0.05), however, this effect varied between individuals. One sheep breathing room air suffered a cardiac arrest immediately post-injection and had to be resuscitated. Atipamezole 125 μg/kg given intramuscularly 45 minutes after the initial injection rapidly reversed the effects of medetomidine. Recovery times did not significantly differ although time to extubation and standing tended to be longer in sheep breathing room air compared to the sheep breathing 100% oxygen. The quality of the recovery did not differ.     

The cardiopulmonary effects of clenbuterol when administered to dorsally recumbent halothane-anaesthetised ponies - failure to increase arterial oxygenation

Clenbuterol (0·8 μg kg ⁻¹ intravenously) was investigated in ponies (small horses) anaesthetised with acepromazine, detomidine and thiopentone, then halothane in oxygen alone (hyperoxic group) or with nitrous oxide (hypoxic group). Following instrumentation, ponies were placed in dorsal recumbency for 60 minutes, clenbuterol (both groups) or a saline control (hyperoxic group) given, and cardiopulmonary parameters monitored for a further 60 minutes. In the hyperoxic group, clenbuterol administration resulted in a transitory (<five minutes) 15 per cent fall in arterial blood pressure and 78 per cent rise in intramuscular blood flow. Heart rate increased from a mean of 42 (SD 4) to 54 (12) beats per minute, the rise being significant for 15 minutes. Cardiac index increased from 2·1 (0·7) to 3·-9 (0·7) litres m⁻² and remained significantly elevated for the remainder of the measurement period. Cardiovascular changes in the hypoxic group were similar. 30 minutes after clenbuterol administration, PaO₂ had changed non-significantly from 32·.3 (19·2) to 33·.4 (17) kPa in the hyperoxic group and from 7·9 (1·8) to 8·.6 (1·3) kPa in the hypoxic group. The study concludes that under these experimental conditions, clenbuterol does not cause significant improvement in arterial oxygenation, but its cardiovascular effects are minimal or advantageous.     

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.     

Effect of body position on the arterial partial pressures of oxygen and carbon dioxide in spontaneously breathing,conscious dogs in an intensive care unit

Objective – To evaluate the effect of body position on the arterial partial pressures of oxygen and carbon dioxide (PaO₂, PaCO₂), and the efficiency of pulmonary oxygen uptake as estimated by alveolar‐arterial oxygen difference (A‐a difference). Design – Prospective, randomized, crossover study. Setting – University teaching hospital, intensive care unit. Animals – Twenty‐one spontaneously breathing, conscious, canine patients with arterial catheters placed as part of their management strategy. Interventions – Patients were placed randomly into lateral or sternal recumbency. PaO₂ and PaCO₂ were measured after 15 minutes in this position. Patients were then repositioned into the opposite position and after 15 minutes the parameters were remeasured. Measurements and Main Results – Results presented as median (interquartile range). PaO₂ was significantly higher (P=0.001) when patients were positioned in sternal, 91.2 mm Hg (86.0–96.1 mm Hg), compared with lateral recumbency, 86.4 mm Hg (73.9–90.9 mm Hg). The median change was 5.4 mm Hg (1.1–17.9 mm Hg). All 7 dogs with a PaO₂<80 mm Hg in lateral recumbency had improved arterial oxygenation in sternal recumbency, median increase 17.4 mm Hg with a range of 3.8–29.7 mm Hg. PaCO₂ levels when patients were in sternal recumbency, 30.5 mm Hg (27.3–32.7 mm Hg) were not significantly different from those in lateral recumbency, 32.2 mm Hg (28.3–36.0 mm Hg) (P=0.07). The median change was ?1.9 mm Hg (?3.6–0.77 mm Hg). A‐a differences were significantly lower (P=0.005) when patients were positioned in sternal recumbency, 21.7 mm Hg (17.3–27.7 mm Hg), compared with lateral recumbency, 24.6 mm Hg (20.4–36.3 mm Hg). The median change was ?3.1 mm Hg (?14.6–0.9 mm Hg). Conclusions – PaO₂ was significantly higher when animals were positioned in sternal recumbency compared with lateral recumbency, predominantly due to improved pulmonary oxygen uptake (decreased A‐a difference) rather than increased alveolar ventilation (decreased PaCO₂). Patients with hypoxemia (defined as PaO₂<80 mm Hg) in lateral recumbency may benefit from being placed in sternal recumbency. Sternal recumbency is recommended to improve oxygenation in hypoxemic patients.     

Preliminary Results of Behavioral and Cardiopulmonary Effects of a Constant Rate Infusion of Remifentanil–Xylazine for Sedation in Horses

Standing surgical procedures are performed commonly in horses under sedation. The use of a xylazine and remifentanil combination has not been investigated in horses. We proposed to evaluate behavioral and cardiopulmonary effects of an intravenous (IV) infusion of xylazine with remifentanil for sedation in horses. Xylazine (0.8 mg/kg IV) followed in 3 minutes by remifentanil (0.0005 mg/kg IV), and a constant rate infusion of xylazine and remifentanil (0.65 mg/kg/h; 0.0225 mg/kg/h, respectively) was administered in three horses. Heart rate, respiratory rate (RR), arterial blood pressures, quality of sedation, pH, partial pressure of arterial CO₂ (PaCO₂), partial pressure of arterial O₂ (PaO₂), ataxia, sedation, and sedation overall outcome were assessed. Heart rate and RR remained within normal values during sedation without significant changes from baseline. Systolic, mean, and diastolic arterial blood pressures were increased during sedation. There were no significant changes in pH, PaCO₂, and PaO₂. Sedation developed immediately after injection of xylazine in the three horses but did not increase after remifentanil bolus or IV infusion of both drugs. None of the mares had ataxia. Adverse effects during and after sedation were present: excitement, increase in locomotor activity, and decrease in the gastrointestinal motility. The combination of xylazine and remifentanil sedation protocol produces adverse effects. This protocol cannot be recommended for clinical conditions, at the described doses.     

Cardiopulmonary effects associated with head-down position in halothane-anesthetized ponies with or without capnoperitoneum

Objective To compare the cardiopulmonary effects of the head‐down position, with or without capnoperitoneum, in halothane‐anesthetized horses. Study design Prospective randomized study. Animals Five ponies (four mares, one stallion; bodyweight 302 ± 38.4 kg [mean ± SD]) were used. Methods The ponies were anesthetized with xylazine, guiafenesin, ketamine, and maintained with halothane/oxygen and lungs were ventilated to 40 ± 2 mm Hg (5.3 ± 0.3 kPa) end‐tidal CO₂ tension. After baseline cardiopulmonary measurements, ponies were kept in horizontal position for 30 minutes, then tilted head‐down 30° to the horizontal position for 60 minutes, and then returned to a horizontal position for final measurements. Capnoperitoneum (intra‐abdominal pressure: 12 mm Hg [1.6 kPa]) was introduced after baseline cardiopulmonary measurements, until 5 minutes before the final measurements (treatment INS). Ponies in the control treatment (CON) did not receive capnoperitoneum. Cardiopulmonary data were collected every 10 minutes following the baseline measurements until recovery. Results In the head‐down position, in both treatments, significant decreases were observed in PaO₂, and significant increases were observed in PaCO₂, right atrial blood pressure, arterial to end‐tidal CO₂ gradient, calculated V_d/V_t and ratios. During the head‐down position, in CON there was decreased cardiac index, and in INS, there were decreases in arterial plasma pH and increases in mean systemic arterial and airway pressures. Treatment INS developed ventilation–perfusion mismatch earlier in the study, and had longer recovery times compared to CON. Conclusion Cardiac index and systemic blood pressure appeared to be preserved in INS during the head‐down position, but ventilation–perfusion mismatch appeared earlier with head‐down position and capnoperitoneum. Clinical relevance Healthy ponies tolerate capnoperitoneum at 12 mm Hg (1.6 kPa) intra‐abdominal pressure when tilted head down 30° to the horizontal position.     

An evaluation of apnea or spontaneous ventilation in early recovery following mechanical ventilation in the anesthetized horse

Objective To compare arterial oxygen and carbon dioxide tensions in apneic and spontaneously ventilating horses recovering from anesthesia. Study design Randomized clinical trial. Animal population Forty‐two healthy horses averaging 466 ± 106 kg and 6 ± 5 years of age. Methods Anesthetized horses undergoing a variety of surgical procedures and receiving positive pressure ventilation (IPPV) were divided into two equal groups. One group was allowed to return to spontaneous ventilation prior to disconnection from the anesthetic circuit (weaned). The other group remained apneic during transport to a recovery stall. Arterial blood gas data were collected at five time points: 20 minutes before moving to a recovery stall (t = ? 20); at the time the anesthetic circuit was disconncted (t = 0); at 3 and 5 minutes post‐disconnection (t = 3 and t = 5) and at the time of the first spontaneous breath (t = sv). The data were analyzed using an anova method for repeated measures and paired, two‐tailed t‐tests. Significance was assumed when p < 0.05. Results The apneic group took a mean of 5 minutes 18 seconds (± 135 seconds) before starting spontaneous ventilation. This group maintained significantly higher PaO₂ levels at intermediate time points (t = 0 and t = 3) but no difference was noted after 5 minutes. PaCO₂ levels were higher in the weaned group at time 0 minutes, returning to a comparable level to the apneic group at t = 3 minutes. Conclusions and clinical relevance Horses can survive a short period of apnea during transport from the surgery suite to recovery stall and may benefit from a reduced incidence of transient hypoxemia compared with spontaneously ventilating horses. This information has practical implications for the anesthetist evaluating the options for discontinuing IPPV when horses are moved to a recovery stall.     

Effects of Intermittent Positive Pressure Ventilation on Cardiopulmonary Function in Horses Anesthetized with Total Intravenous Anesthesia Using Combination of Medetomidine,Lidocaine, Butorphanol and Propofol (MLBP-TIVA)

Effects of intermittent positive pressure ventilation (IPPV) on cardiopulmonary function were evaluated in horses anesthetized with total intravenous anesthesia using constant rate infusions of medetomidine (3.5 µg/kg/hr), lidocaine (3 mg/kg/hr), butorphanol (24 µg/kg/hr) and propofol (0.1 mg/kg/min) (MLBP-TIVA). Five horses were anesthetized twice using MLBP-TIVA with or without IPPV at 4-week interval (crossover study). In each occasion, the horses breathed 100% oxygen with spontaneous ventilation (SB-group, n=5) or with IPPV (CV-group, n=5), and changes in cardiopulmonary parameters were observed for 120 min. In the SB-group, cardiovascular parameters were maintained within acceptable ranges (heart rate: 33–35 beats/min, cardiac output: 27–30 l/min, mean arterial blood pressure [MABP]: 114–123 mmHg, mean pulmonary arterial pressure [MPAP]: 28–29 mmHg and mean right atrial pressure [MRAP]: 19–21 mmHg), but severe hypercapnea and insufficient oxygenation were observed (arterial CO₂ pressure [PaCO₂]: 84–103 mmHg and arterial O₂ pressure [PaO₂]: 155–172 mmHg). In the CV-group, normocapnea (PaCO₂: 42–50 mmHg) and good oxygenation (PaO₂: 395–419 mmHg) were achieved by the IPPV without apparent cardiovascular depression (heart rate: 29–31 beats/min, cardiac output: 17–21 l /min, MABP: 111–123 mmHg, MPAP: 27–30 mmHg and MRAP: 15–16 mmHg). MLBP-TIVA preserved cardiovascular function even in horses artificially ventilated.     

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.     

Comparative effects of three different ventilatory treatments on arterial blood gas values and oxygen extraction in healthy anaesthetized dogs

ObjectiveTo compare the effect of invasive continuous positive airway pressure (CPAP), pressure-controlled ventilation (PCV) with positive end-expiratory pressure (PEEP) and spontaneous breathing (SB) on PaO₂, PaCO₂ and arterial to central venous oxygen content difference (CaO₂-CcvO₂) in healthy anaesthetized dogs.Study designProspective randomized crossover study.AnimalsA group of 15 adult male dogs undergoing elective orchidectomy.MethodsDogs were anaesthetized [buprenorphine, medetomidine, propofol and isoflurane in an air oxygen (FiO₂= 0.5)]. All ventilatory treatments (CPAP: 4 cmH₂O; PCV: 10 cmH₂O driving pressure; PEEP, 4 cmH₂O; respiratory rate of 10 breaths minute^–1 and inspiratory-to-expiratory ratio of 1:2; SB: no pressure applied) were applied in a randomized order during the same anaesthetic. Arterial and central venous blood samples were collected immediately before the start and at 20 minutes after each treatment. Data were compared using a general linear mixed model (p < 0.05).ResultsMedian PaO₂ was significantly higher after PCV [222 mmHg (29.6 kPa)] than after CPAP [202 mmHg (26.9 kPa)] and SB [208 mmHg (27.7 kPa)] (p < 0.001). Median PaCO₂ was lower after PCV [48 mmHg (6.4 kPa)] than after CPAP [58 mmHg (7.7 kPa)] and SB [56 mmHg (7.5 kPa)] (p < 0.001). Median CaO₂-CcvO₂ was greater after PCV (4.36 mL dL^–1) than after CPAP (3.41 mL dL^–1) and SB (3.23 mL dL^–1) (p < 0.001). PaO₂, PaCO₂ and CaO₂-CcvO₂ were no different between CPAP and SB (p > 0.99, p = 0.697 and p = 0.922, respectively).Conclusions and clinical relevanceCPAP resulted in similar arterial oxygenation, CO₂ elimination and tissue oxygen extraction to SB. PCV resulted in improved arterial oxygenation and CO₂ elimination. Greater oxygen extraction occurred with PCV than with CPAP and SB, offsetting its advantage of improved arterial oxygenation. The benefit of invasive CPAP over SB in the healthy anaesthetized dog remains uncertain.     

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.     

Pulsed delivery of inhaled nitric oxide counteracts hypoxaemia during 2.5 hours of inhalation anaesthesia in dorsally recumbent horses

Objective The study aimed to investigate the effect of varying pulse lengths of inhaled nitric oxide (iNO), and 2.5 hours of continuous pulse‐delivered iNO on pulmonary gas exchange in anaesthetized horses. Study Design Experimental study. Animals Six Standardbred horses. Methods Horses received acepromazine, detomidine, guaifenesin, thiopentone and isoflurane in oxygen, were positioned in dorsal recumbency and were breathing spontaneously. iNO was on average pulsed during the first 20, 30, 43 or 73% of the inspiration in 15 minute steps. The pulse length that corresponded to the highest (peak) partial pressure of arterial oxygen (PaO₂) in the individual horses was determined and delivered for a further 1.5 hours. Data measured or calculated included arterial and mixed venous partial pressures of O₂ and CO₂, heart rate, respiratory rate, expired minute ventilation, pulmonary and systemic arterial mean pressures, cardiac output and venous admixture. Data (mean ± SD) was analysed using anova with p < 0.05 considered significant. Results Although the pulse length of iNO that corresponded to peak PaO₂ varied between horses, administration of all pulse lengths of iNO increased PaO₂ compared to baseline. The shortest pulse lengths that resulted in the peak PaO₂ were 30 and 43% of the inspiration. Administration of iNO increased PaO₂ (12.6 ± 4.1 kPa [95 ± 31 mmHg] at baseline to a range of 23.0 ± 8.4 to 25.3 ± 9.0 kPa [173 to 190 mmHg]) and PaCO₂ (8.5 ± 1.2 kPa [64 ± 9 mmHg] to 9.8 ± 1.5 kPa [73 ± 11 mmHg]) and decreased venous admixture from 32 ± 6% to 25 ± 6%. The increase in PaO₂ and decrease in venous admixture was sustained for the entire 2.5 hours of iNO delivery. Conclusions The improvement in arterial oxygenation during pulsed delivery of iNO was significant and sustained throughout 2.5 hours of anaesthesia. Clinical relevance Pulsed iNO potentially could be used clinically to counteract hypoxemia in anaesthetized horses.     

A comparison of the effects of carbon dioxide and medical air for abdominal insufflation on respiratory parameters in xylazine-sedated sheep undergoing laparoscopic artificial insemination

AIMS: To determine if abdominal insufflation with medical air will improve oxygenation and ventilation parameters when compared to insufflation with CO₂ in xylazine-sedated sheep undergoing laparoscopic artificial insemination (AI).

METHODS: Forty-seven sheep underwent oestrus synchronisation and were fasted for 24 hours prior to laparoscopic AI. Each animal was randomised to receive either CO₂ or medical air for abdominal insufflation. An auricular arterial catheter was placed and utilised for serial blood sampling. Respiratory rates (RR) and arterial blood samples were collected at baseline, after xylazine (0.1?mg/kg I/V) sedation, 2 minutes after Trendelenburg positioning, 5 minutes after abdominal insufflation, and 10 minutes after being returned to a standing position. Blood samples were collected in heparinised syringes, stored on ice, and analysed for arterial pH, partial pressure of arterial O₂ (PaO₂), and CO₂ (PaCO₂). The number of ewes conceiving to AI was also determined.

RESULTS: Repeated measures ANOVA demonstrated temporal effects on RR, PaO₂, PaCO₂ and arterial pH during the laparoscopic AI procedure (p<0.001), but no difference between insufflation groups (p>0.01). No sheep experienced hypercapnia (PaCO₂>50?mmHg) or acidaemia (pH<7.35). Hypoxaemia (PaO₂<70?mmHg) was diagnosed during the procedure in 14/22 (64%) ewes in the CO₂ group compared with 8/23 (35%) ewes in the medical air group (p=0.053). Overall, 15/20 (75%) ewes in the CO₂ group conceived to AI compared with 16/22 (72.7%) in the medical air group (p=0.867).

CONCLUSIONS AND CLINICAL RELEVANCE: There were no statistical or clinical differences in RR, PaO₂, PaCO₂, pH, or conception to AI when comparing the effects of CO₂ and medical air as abdominal insufflation gases. None of the sheep experienced hypercapnia or acidaemic, yet 42% (19/45) of sheep developed clinical hypoxaemia, with a higher percentage of ewes in the CO₂ group developing hypoxaemia than in the medical air group. Based on the overall analysis, medical air could be utilised as a comparable alternative for abdominal insufflation during laparoscopic AI procedures.     

The influence of body mass and thoracic dimensions on arterial oxygenation in anaesthetized horses and ponies

ObjectiveTo examine the relationship between body mass and thoracic dimensions on arterial oxygen tensions (PaO₂) in anaesthetized horses and ponies positioned in dorsal recumbency.Study designProspective clinical study.AnimalsThirty six client-owned horses and ponies, mean [±SD (range)] age 8.1 ± 4.8 (1.5–20) years and mean body mass 467 ± 115 (203–656) kg.MethodsBefore general anaesthesia, food and water were withheld for 12 and 1 hours respectively. Body mass (kg), height at the withers (H), thoracic circumference (C), thoracic depth (length between dorsal spinous process and sternum; D), thoracic width (between point of shoulders; W), and thoracic diagonal length (point of shoulder to last rib; L) were measured. Pre-anaesthetic medication was with intravenous (IV) romifidine (0.1 mg kg⁻¹). Anaesthesia was induced with an IV ketamine (2.2 mg kg⁻¹) and diazepam (0.05 mg kg⁻¹) combination and maintained with halothane in 1:1 oxygen:nitrous oxide (N₂O) mixture. Animals were positioned in dorsal recumbency and allowed to breathe spontaneously. Nitrous oxide was discontinued after 10 minutes, and arterial blood samples obtained and analysed for gas tensions at 15, 30 and 60 minutes after connection to the anaesthetic breathing circuit. Data were analysed using anova and Pearson's correlation co-efficient.ResultsThe height per unit body mass (H kg⁻¹) and thoracic circumference per unit body mass (C kg⁻¹) correlated strongly (r = 0.85, p < 0.001 and r = 0.82, p < 0.001 respectively) with arterial oxygen tensions (PaO₂) at 15 minutes.ConclusionsThere is a strong positive correlation between H kg⁻¹ and C kg⁻¹ and PaO₂ after 15 minutes of anaesthesia in halothane-anaesthetized horses positioned in dorsal recumbency.Clinical relevanceReadily obtained linear measurements (height and thoracic circumference) and body mass may be used to predict the ability of horses to oxygenate during anaesthesia.     

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.     

Evaluation of lung ventilation distribution using electrical impedance tomography in standing sedated horses with capnoperitoneum

ObjectiveTo determine changes in distribution of lung ventilation with increasing intra-abdominal pressure (IAP) from carbon dioxide (CO₂) insufflation in standing sedated horses.Study designProspective experimental study.AnimalsA group of six healthy adult horses.MethodsEach horse was sedated with acepromazine, detomidine and butorphanol and sedation maintained with a detomidine infusion. The horse was restrained in a stocks system and a 32 electrode electrical impedance tomography (EIT) belt was wrapped around the thorax at the fifth–sixth intercostal space. EIT images and arterial blood samples for PaO₂ and PaCO₂, pH and lactate concentration were obtained during capnoperitoneum at 0 (baseline A), 5, 8 and 12 mmHg as IAP increased and at 8, 5, 0 (baseline B) mmHg as IAP decreased. At each IAP, after a 2 minute stabilization period, EIT images were recorded for ≥ 2 minutes to obtain five consecutive breaths. Statistical analysis was performed using anova for repeated measures with Geisser-Greenhouse correction and a Tukey’s multiple comparison test for parametric data. The relationship between PaO₂ and the center of ventilation in the ventral-dorsal (CoV-VD) and right-left (CoV-RL) directions or total impedance change as a surrogate for tidal volume (ΔZV_T) were tested using linear regression analysis. Significance was assumed when p ≤ 0.05.ResultsThere were no significant changes in CoV-VD, CoV-RL, PaO₂, PaCO₂, lactate concentration, pH, heart rate and respiratory rate with targeted IAP. There was a significant decrease in ΔZV_T compared with baseline A at 5 mmHg IAP as IAP was increased.Conclusions and clinical relevanceCapnoperitoneum causes a significant decrease in ΔZV_T in standing sedated horses with increasing IAP.