Oxygenation,oxygen delivery and anaesthesia in the horse

Horses are the most difficult of the common companion animals to anaesthetise. Hypoxaemia or inadequate oxygen delivery to peripheral tissues during anaesthesia would seem a potential cause of increased mortality, but no direct link has been established. A number of methods of increasing oxygenation and oxygen delivery have been reported, with varying results and potential applicability. The purpose of this article is to review the literature with regard to oxygenation, oxygen delivery and methods to improve each and to make recommendations for clinical application.     

Effect of reducing inspired oxygen concentration on oxygenation parameters during general anaesthesia in horses in lateral or dorsal recumbency

Objective

To compare the effects of two concentrations of oxygen delivered to the anaesthetic breathing circuit on oxygenation in mechanically ventilated horses anaesthetised with isoflurane and positioned in dorsal or lateral recumbency.

Methods

Selected respiratory parameters and blood lactate were measured and oxygenation indices calculated, before and during general anaesthesia, in 24 laterally or dorsally recumbent horses. Horses were randomly assigned to receive 100% or 60% oxygen during anaesthesia. All horses were anaesthetised using the same protocol and intermittent positive pressure ventilation (IPPV) was commenced immediately following anaesthetic induction and endotracheal intubation. Arterial blood gas analysis was performed and oxygenation indices calculated before premedication, immediately after induction, at 10 and 45 min after the commencement of mechanical ventilation, and in recovery.

Results

During anaesthesia, the arterial partial pressure of oxygen was adequate in all horses, regardless of position of recumbency or the concentration of oxygen provided. At 10 and 45 min after commencing IPPV, the arterial partial pressure of oxygen was lower in horses in dorsal recumbency compared with those in lateral recumbency, irrespective of the concentration of oxygen supplied. Based on oxygenation indices, pulmonary function during general anaesthesia in horses placed in dorsal recumbency was more compromised than in horses in lateral recumbency, irrespective of the concentration of oxygen provided.

Conclusion

During general anaesthesia, using oxygen at a concentration of 60% instead of 100% maintains adequate arterial oxygenation in horses in dorsal or lateral recumbency. However, it will not reduce pulmonary function abnormalities induced by anaesthesia and recumbency.     

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.     

Effect of various levels of dissolved oxygen on reactive oxygen species and cryocapacitation‐like changes in bull sperm

The present investigation was carried out to study the effect of various levels of dissolved oxygen (DO) on reactive oxygen species (ROS) and cryocapacitation‐like changes in bull sperm. Egg yolk–Tris–glycerol (EYTG) extender was split into four subextenders; viz., Extender I (control; no flushing with liquid nitrogen (LN₂)), Extender II, Extender III and Extender IV were flushed with LN₂ for 40, 16 and 8 min, respectively. The DO levels were standardized to 11.7, 2, 4 and 8 ppm, respectively, in control (Extender I), Extender II, Extender III and Extender IV. Ejaculates with mass motility of ≥ 3+ were divided into group I (diluted with Extender I), group II (diluted with Extender II), group III (diluted with Extender III) and group IV (diluted with Extender IV) up to 80 × 10⁶ sperm/ml. Extended semen samples were packed in French mini straws (0.25 ml), equilibrated and cryopreserved. Semen samples were evaluated at prefreeze and post‐thaw stage for various parameters (DO, progressive motility (PM), viability (VIB), acrosomal integrity (AI), hypo‐osmotic swelling (HOS) test, ROS, cholesterol (C) and phospholipid (P). The percentage of PM, VIB, AI, HOS test, cholesterol (C) and phospholipid (P) levels, and capacitated sperm were significantly (p < 0.05) higher in groups III and IV as compared to groups I and II. However, the acrosome‐reacted sperm (%; pattern AR) were significantly (p < 0.05) decreased in group III as compared to all other groups. Besides the proportion of sperm displaying tyrosine‐phosphorylated pattern, EA (fluorescence at both equatorial and anterior acrosomal regions, i.e. high capacitation level) was significantly (p < 0.05) reduced in group III compared to all other groups. In conclusion, varying DO levels in the extender significantly affect sperm quality, ROS production and capacitation‐like changes in bulls.     

Effects of isovolemic resuscitation with hemoglobin-based oxygen carrier Hemoglobin glutamer-200 (bovine) on systemic and mesenteric perfusion and oxygenation in a canine model of hemorrhagic shock: a comparison with 6% hetastarch solution and shed blood

OBJECTIVE: To study Hemoglobin glutamer-200 bovine (Hb-200), 6% hetastarch (HES) and shed whole blood (WB) resuscitation in canine hemorrhagic shock. STUDY DESIGN: Prospective laboratory investigation. Animals Twelve adult dogs [29 +/- 1 kg (mean +/- SD)]. METHODS: Anesthetized dogs were instrumented for recording systemic and mesenteric hemodynamic parameters and withdrawal of arterial, mixed and mesenteric venous blood, in which hematological, oxygenation, blood gas and acid-bases variables were determined. Recordings were made before [baseline (BL)], after 1 hour of hypovolemia and immediately and 3 hours post-resuscitation with 30 mL kg(-1) of either Hb-200, HES, or WB. RESULTS: Blood withdrawal (average 34 +/- 2 mL kg(-1)) caused significant hemodynamic changes, metabolic acidosis and hyperlactatemia characteristic for hemorrhagic shock. Only WB transfusion restored all variables. Hemoglobin glutamer-200 bovine infusion returned most hemodynamic parameters including cardiac output and mesenteric arterial blood flow to BL but increased mean arterial pressure above BL (p < 0.05). However, Hb-200 failed to restore total Hb and arterial oxygen content (CaO2), leaving systemic (DO2I) and mesenteric O2 delivery (DO2Im) below BL (p < 0.05). Nevertheless, acid-base variables recovered completely after Hb-200 resuscitation, and met-hemoglobin (Met-Hb) levels increased (p < 0.05). Hetastarch resuscitation returned hemodynamic variables to or above BL but further decreased total Hb and CaO2, preventing recovery of sDO2I and mDO2I (p < 0.05). Thus, systemic and mesenteric O2 extraction stayed above BL (p < 0.05) while acid-base variables recovered to BL, although slower than in Hb-200 and WB groups (p < 0.05). CONCLUSIONS AND CLINICAL RELEVANCE: Resuscitation with Hb-200 seemed to resolve metabolic acidosis and lactatemia more rapidly than HES, but not WB; yet it is not superior to HES in improving DO2I and DO2Im. The hyperoncotic property of solutions like Hb-200 that results in rapid volume expansion with more homogenous microvascular perfusion and the ability to facilitate diffusive O2 transfer accelerating metabolic recovery may be the key mechanisms underlying their beneficial effects as resuscitants.     

Roles of reactive oxygen species in the corpus luteum

Cells living under aerobic conditions always face the oxygen paradox. Oxygen is necessary for cells to maintain their lives. However, reactive oxygen species such as superoxide radicals, hydroxyl radicals and hydrogen peroxide are generated from oxygen and damage cells. Oxidative stress occurs as a consequence of the excessive production of reactive oxygen species and impaired antioxidant defense systems. Antioxidant enzymes include superoxide dismutase (SOD), which is a specific enzyme to scavenge superoxide radicals; copper‐zinc SOD, located in the cytosol and Mn‐SOD, located in the mitochondria. Both types of SOD belong to the first enzymatic step to scavenge superoxide radicals. It has been reported that a number of local factors such as cytokines, growth factors and eicosanoids are involved in the regulation of the corpus luteum (CL) function in addition to gonadotropins. Since reactive oxygen species are generated and SOD is expressed in the CL, there is a possibility that reactive oxygen species and SOD work as local regulators of the CL function. The present review reports that reactive oxygen species and their scavenging systems play important roles in the regulation of the CL function.     

Pulmonary gas exchange in anaesthetised horses mechanically ventilated with oxygen or a helium/oxygen mixture

Reason for performing study: It is unknown whether administration of gas‐mixtures high in inspired fraction of oxygen (FiO₂) under general anaesthesia may increase formation of pulmonary atelectasis and impair gas exchange. Objective: To evaluate the effects of different FiO₂ on pulmonary gas exchange in isoflurane‐anaesthetised horses breathing a helium/oxygen (He/O₂) mixture. Methods: Thirty healthy mature horses were sedated with i.v. acepromazine (0.02 mg/kg bwt), detomidine (0.002 mg/kg bwt) and xylazine (0.2‐0.4 mg/kg bwt). General anaesthesia was induced with i.v. 5% guaifenesin to effect, diazepam (0.1 mg/kg bwt) and ketamine (2 mg/kg bwt), and maintained with isoflurane. Fifteen horses (Group HX) were ventilated mechanically with gas mixtures of successively increasing FiO₂ (0.25‐0.30, 0.50‐0.55, >0.90), obtained by blending O₂with Heliox (70% He/30% O₂). The other 15 horses (Group O) were ventilated immediately with 100% O₂(FiO₂>0.90). After 20 min of ventilation at the different FiO₂levels in Group HX and after 60 min in Group O, PaO₂ and PaCO₂ were measured and the alveolar to arterial PO₂gradient (P(_A‐a)O₂) was calculated. Data analysis included robust categorical regression with clustering on horse (P<0.05). Results: Inhalation of a He/O₂ mixture with FiO₂ as low as 0.25‐0.30 ensured adequate arterial oxygenation and was associated with a smaller P_(A‐a)O₂ gradient than inhalation of pure O₂ (P<0.05). In Group HX, PaO₂ increased with each rise in FiO2 and so did P(A‐a)O₂ (P<0.05). The PaO₂ was significantly lower and the P_(A‐a)O₂ higher in Group O compared to Group HX at a FiO₂ >0.90 (P<0.05). Conclusions and potential relevance: Administration of a He/O₂gas mixture low in FiO₂ can better preserve lung function than ventilation with pure oxygen. A step‐wise increase of FiO₂ using a He/O₂ gas mixture might offer advantages with respect to pulmonary gas exchange over an immediate exposure to 100% O₂.     

Comparison of unilateral versus bilateral nasal catheters for oxygen administration in dogs

Objective: To determine the effect of bilateral nasal oxygen supplementation on tracheal airway and arterial blood gas parameters. Design: Original research. Setting: Research Laboratory. Animals: Eight normal dogs. Interventions: None. Measurements: Intra‐tracheal oxygen concentration and arterial oxygen partial pressure at three different oxygen flow rates given through either unilateral or bilateral nasal catheters. Main results: F_IO₂ and PaO₂ were significantly increased with higher total oxygen flow rates, but the increase was the same whether the higher flow was delivered through one nasal catheter or divided and administered though two nasal catheters. The use of bilateral nasal catheters allowed a tracheal F_IO₂ as high as 0.60 with minimal patient discomfort. Conclusions: The benefit of bilateral nasal catheters for oxygen supplementation is the ability to provide high total oxygen flows with decreased risk of patient discomfort. If the desired oxygen flow can be achieved with a unilateral nasal catheter, then the only benefit of bilateral catheters is increased patient comfort. The use of bilateral nasal oxygen catheters for oxygen supplementation can result in an F_IO₂ that is high enough to produce oxygen toxicity with prolonged administration.     

Calculation of bovine haemoglobin oxygen saturation by algorithms integrating age,haemoglobin content,blood pH,partial pressures of oxygen and carbon dioxide in the blood,and temperature

In human and veterinary medicine, arterial and venous haemoglobin oxygen saturations are often used to estimate the severity of a disease and to guide therapeutic decisions. In veterinary medicine, haemoglobin oxygen saturation (SO(2)) is usually calculated using a blood gas analyser and algorithms developed for humans. It is possible, therefore, that the values obtained in animals may be distorted, particularly in animals with a high haemoglobin oxygen affinity, like young calves. In order to verify this hypothesis, we compared the arterial (SaO(2)) and venous (SvO(2)) haemoglobin oxygen saturations calculated using three different algorithms, and the oxygen exchange fraction (OEF) at the tissue level, which is the degree of haemoglobin desaturation between arterial and venous blood (SaO(2)-SvO(2)), with the values obtained from the whole bovine oxygen equilibrium curve (OEC) determined by a reference method. The blood gas analysers underestimated SvO(2) values; consequently, the OEF was overestimated (by about 10%). Two methods of reducing these errors were assessed. As the haemoglobin oxygen affinity decreases during the first month of life in calves a relationship between PO(2) at 50% haemoglobin saturation (P50) and age was established in order to correct the calculated values of venous and arterial SO(2), taking into account the estimated position of the OEC. This method markedly reduced the error for SvO(2) and OEF. Secondly, the SO(2) was calculated using a mathematical model taking into account the age of the animal and the specific effects of pH, PCO(2), and temperature on the bovine OEC. Using this method, the mean difference between the OEF values calculated using the mathematical model and those calculated by the reference method was close to zero. The errors produced by blood gas analysers can thus be minimised in two ways: firstly, by simply introducing a P50 estimated from the age of the calf into the analyser before the measurement; and secondly, by calculating the SO(2) using a mathematical model applied to the bovine OEC.     

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.     

Accuracy of oxygen delivery through bubble humidifiers and nasal catheters

ObjectiveTo evaluate the equipment used for nasal insufflation of oxygen and determine its accuracy.Study designOriginal study.MethodsOxygen delivery assemblies consisting of a flowmeter, bubble humidifier, oxygen delivery tubing and nasal insufflation catheters were assembled. Single and double catheter assemblies were made for four sizes of nasogastric feeding tubes (3.5 Fr, 5.0 Fr, 8.0 Fr and 10.0 Fr) resulting in 64 individual assemblies. A gas flow analyzer measured oxygen flow at the tip of the nasal catheter assemblies and from the pressure relief valve (PRV) of the bubble humidifiers. Statistical analyses were conducted to assess the functionality of assemblies. For functional assemblies, the accuracy of oxygen flow relative to the prescribed flow settings was determined.ResultsCatheter size was significantly associated with the functionality of assemblies. Probability (95% confidence interval) of 3.5 Fr, 5.0 Fr and 8.0 Fr assemblies being functional was estimated at 0.53 (0.14, 0.89), 0.83 (0.36, 0.98) and 0.98 (0.76, 0.99), respectively. All 10.0 Fr assemblies were functional. Functional assemblies, in general, consistently under-delivered the prescribed flow because a large portion of set flow was diverted through the bubble humidifier PRV.ConclusionsLeaks through the PRV cause significant diversion of oxygen prior to it reaching the catheter tips. Smaller patients are particularly susceptible, as small catheters limit oxygen delivery creating proportionally greater leaks through the PRV.Clinical relevanceIt was not possible to accurately deliver oxygen because of leaks through the PRV. Targeting a specific outcome (e.g., oxyhemoglobin saturation > 94%, PaO₂ 80–120 mmHg; 11–16 kPa) and avoiding unnecessarily high fractions of inspired oxygen cannot be done if flow delivery cannot be accurately assured. One possible solution would be to use a bubble humidifier with a 6 psi PRV that does not leak prior to reaching the opening pressure.     

Proper theoretical analysis of the oxygen wash-in kinetics of circle breathing systems

Objective To correctly define the theoretical treatment of oxygen wash‐in kinetics in circle breathing systems and reevaluate previously published results for the rate of change of oxygen concentration in a large animal circle breathing system. Study design Theoretical analysis of previously published data. Animals None. Methods Previously published data for the rate of change of oxygen concentration in a large animal circle breathing system with two reservoir bag sizes (40 and 20 L) at three different flow rates (3, 6, and 10 L minute^?1) were obtained from the original publication, via digital extraction, and analyzed to determine system time constants. The results of this analysis were compared to those originally reported and the level of mathematical agreement between experiment and theory was quantified. Results Theoretical time constants for the system with 40 L reservoir bag with flow rates of 3, 6, and 10 L minute^?1 were 16.4, 8.2, and 4.9 minutes, respectively. Experimentally derived time constants for this system with flow rates of 3, 6, and 10 L minute^?1 were 18.1, 9.2, and 5.4 minutes, respectively. Percent differences between experimental and theoretical time constants for this system with flow rates of 3, 6, and 10 L minute^?1 were 10.4, 12.2, and 10.2%, respectively. For the system with a 20 L reservoir bag and 6 L minute^?1 flow rate, the theoretical and experimentally derived time constants were 5.5 and 5.6 minutes, respectively, with a 1.8% difference. The average relative deviations between theory and experiment for the system at 6 L minute^?1 flow with a 40 or 20 L reservoir bag were 1.3% and 3.0%, respectively. Conclusions and clinical relevance Proper theoretical analysis of experimentally obtained data for the wash‐in kinetics of oxygen into a large animal circle breathing system leads to improved mathematical agreement between theory and experiment when compared to the originally published results. Application of this method should allow more accurate prediction of the rate of change of oxygen concentration in anesthetic circuits.     

The effect of conglutinin on production of reactive oxygen species in bovine granulocytes

Conglutinin is a high molecular-weight lectin originally detected in bovine serum. It belongs to the family of collectins that bind sugar residues in a Ca²⁺-dependent manner and are effector molecules in innate immunity. Conglutinin appears to play an important role in immune defense mechanisms, showing antiviral and antibacterial activities when tested in vivo and in vitro. The present study evaluated the effect of conglutinin on the respiratory bursts in bovine peripheral phagocytes. Using nitroblue tetrazolium and hydrogen peroxide assays, we showed that sugar ligand-bound conglutinin stimulated the production of superoxide and H₂O₂ in granulocytes whereas the non-sugar-bound form of conglutinin inhibited these processes. These results indicate that both forms of conglutinin are able to interact with surface leukocyte receptors but have opposite effects on phagocytic activity. Our findings suggest that conglutinin bound to sugar residues on microbial surfaces can induce oxygen burst in phagocytes, and thereby mediates the elimination of pathogens and prevents the spread of infection.