Anesthesia for the horse with colic

Uncontrolled pain: a call for better study design

Studies assessing animal pain in veterinary research are often performed primarily for the benefit of animals. Frequently, the goal of these studies is to determine whether the analgesic effect of a novel treatment is clinically meaningful, and therefore has the capacity to improve the welfare of treated animals. To determine the treatment effect of a potential analgesic, control groups are necessary to allow comparison. There are negative control groups (where pain is unattenuated) and positive control groups (where pain is attenuated). Arising out of animal welfare concerns, there is growing reluctance to use negative control groups in pain studies. But for studies where pain is experimentally induced, the absence of a negative control group removes the opportunity to demonstrate that the study methods could differentiate a positive control intervention from doing nothing at all. For studies that are controlled by a single comparison group, the capacity to distinguish treatment effects from experimental noise is more difficult; especially considering that pain studies often involve small sample sizes, small and variable treatment effects, systematic error and use pain assessment measures that are unreliable. Due to these limitations, and with a focus on farm animals, we argue that many pain studies would be enhanced by the simultaneous inclusion of positive and negative control groups. This would help provide study-specific definitions of pain and pain attenuation, thereby permitting more reliable estimates of treatment effects. Adoption of our suggested refinements could improve animal welfare outcomes for millions of animals globally.

Arterial Blood Gas, Electrolyte and Acid-Base Values as Diagnostic and Prognostic Indicators in Equine Colic

The study aimed to investigate if arterial blood analysis in conscious horses presenting with signs of colic and breathing ambient air had diagnostic or prognostic value. Arterial blood samples from 352 horses presenting with colic at a university equine referral hospital were analysed for pH, partial pressure of carbon dioxide (PaCO2), partial pressure of oxygen (PaO2), concentrations of sodium (Na+), potassium (K+), ionised calcium (Ca2+) and chloride (Cl-), actual and standardised plasma bicarbonate concentration (HCO3- (P) and HCO3- (P, st)), blood and extracellular fluid base excess (Base (B) and Base (ecf)) and anion gap (AG). Results were compared to previously reported values for healthy horses, and comparisons were made between final diagnosis, treatment and survival to hospital discharge. Significant differences were found between colic cases and healthy reference values between some primary aetiologies. Overall, surgical and non-surgical colic cases differed in Ca2+ and Cl- concentrations and Ca2+ differed between cases that survived to discharge and those that did not. PaO2 differed between small intestinal surgical cases that survived and those that did not. From these results, we developed regression models that demonstrated excellent or good predictive value in identifying the likelihood of surgical versus medical management and survival to hospital discharge.

Xylazine Infusion during Equine Colic Anesthesia with Isoflurane and Lidocaine: A Retrospective Study

This retrospective study investigated the effect of a xylazine infusion on heart rate; mean arterial pressure; blood gases; anesthetic and dobutamine requirements; recovery quality and duration; percentage of death/survival; and days to die/discharge in horses after colic surgery under partial intravenous anesthesia with isoflurane and lidocaine infusion. Anesthetic records of equine colic surgery were reviewed from similar periods in 2020-2021 and 2021-2022. In both groups, after sedation with xylazine 0.7 mg/kg intravenously (IV) and induction with ketamine 2.2 mg/kg and midazolam 0.06 mg/kg IV, anesthesia was maintained with isoflurane and lidocaine (bolus 1.5 mg/kg IV, infusion 2 mg/kg/h). Group L (2020-2021, n = 45) received xylazine 0.2 mg/kg IV before recovery, group XL (2021-2022, n = 44) received xylazine 0.5 mg/kg/h IV intraoperatively. In group XL, minimal (p = 0.04) and average (p = 0.04) heart rate, intraoperative hematocrit (p = 0.001), minimal (p = 0.002) and maximal (p = 0.04) dobutamine administration rate, animals requiring ketamine top-ups (p = 0.04), and the number of days to discharge (p = 0.02), were significantly lower compared to group L. During recovery in group XL, the time to sternal recumbency (p = 0.03) and time to first attempt (p = 0.04) were significantly longer. This retrospective study suggests that a xylazine infusion may have beneficial effects on horses undergoing colic surgery. Further prospective studies are necessary.

Pathophysiology of intraabdominal hypertension and abdominal compartment syndrome and relevance to veterinary critical care

BACKGROUND

Increased intraabdominal pressure, termed intraabdominal hypertension (IAH), is reported as an independent cause of morbidity and mortality in the human ICU but, until recently, has been rarely described in veterinary species outside of experimental models. Failure to identify severe IAH leads to organ dysfunction, termed abdominal compartment syndrome, and rapidly becomes fatal without therapeutic intervention. Although the veterinary community has been slow to address the concept of IAH and associated comorbidities, recent companion and large animal case series and experimental studies suggest IAH may also be common in veterinary species and correlates well with risk factors and grading systems already described in the human literature.

PATHOPHYSIOLOGY

Increasing abdominal pressures exert deleterious local effects through visceral ischemia and reperfusion injury as well as systemic effects on the cardiovascular, pulmonary, renal, and central nervous systems. Even mild grades of IAH increase systemic vascular resistance, impede venous return, increase pulmonary wedge pressure, and decrease pulmonary function. More severe grades cause azotemia, oliguria, decreased coronary blood flow, hypoxia, increased intracranial pressure, and death.

IMPORTANCE

Many of the common diseases in veterinary patients are associated with IAH, including gastric dilatation-volvulus, colon volvulus, closed pyometra, hemoperitoneum, ascites, uroperitoneum, and hydrops. Monitoring of the veterinary patient is difficult, but several experimental studies validate both the presence of IAH and the ability to monitor abdominal pressures in large and small animal species. Moreover, prompt recognition of IAH and subsequent treatment is feasible in the veterinary ICU.

KEY POINTS

Increasing abdominal pressures exert deleterious local effects through visceral ischemia and reperfusion injury as well as systemic effects on the cardiovascular, pulmonary, renal, and central nervous systems. Increases in central venous pressure, systemic vascular resistance, pulmonary wedge pressure, and a decreased cardiac output by way of both decreased preload and increased afterload have been documented as a result of intraabdominal hypertension (IAH). Direct diagnosis of IAH is achieved by blind or ultrasound-guided abdominal needle puncture attached to a water manometer or direct pressure monitoring transducer. Transvesicular measurement of intraabdominal pressure (IAP) is relatively noninvasive, and many patients that would benefit from rapid diagnosis of IAH and abdominal compartment syndrome already have indwelling bladder catheters. Recommendations for interventions are based on the assigned grade of IAH (mild, moderate, severe).

KEY POINTS

If IAH is strongly suspected or diagnosed, abdominal wall compliance may be improved through judicious use of neuromuscular blockers and sedation. Decompression, either minimally invasive or surgical, is absolutely recommended for IAPs consistently above 20 mm Hg, especially in the presence of signs attributed to secondary organ dysfunction.

Evaluation of phenylephrine and exercise with or without trocarization for treatment of suspected nephrosplenic entrapment in horses

OBJECTIVE To report the outcomes of horses with suspected nephrosplenic entrapment (NSE) of the large colon treated by IV phenylephrine administration and exercise with and without trocarization (ie, medical management). DESIGN Retrospective, observational study. ANIMALS 134 horses. PROCEDURES Electronic medical records were searched to identify horses that underwent medical management for suspected NSE at a veterinary teaching hospital between 1995 and 2014. Demographic information, physical and ultrasonographic examination findings, treatment information (including the number of times the treatment was performed and patient response), surgical findings if applicable, complications, and patient outcome were recorded. Descriptive statistics were reported. RESULTS 72 horses had suspected NSE that resolved with medical treatment; 59 of 62 horses underwent laparotomy when medical management failed, and 3 were euthanized without surgery. Twenty-five of the 59 horses had confirmed NSE that was surgically corrected, and 34 had lesions other than or in addition to NSE. All horses that had surgically corrected NSE and 18 of 34 horses that had other lesions survived to hospital discharge. The odds of resolution of NSE with medical management were greater for horses that underwent ≤ 2 (vs > 2) treatments. The treatment success rate for horses that underwent trocarization was not greater than that for horses that did not have the procedure. CONCLUSIONS AND CLINICAL RELEVANCE Suspected NSE resolved with the described medical management for most horses. However, results indicated the potential for misdiagnosis was high. Timely surgical intervention is recommended for horses that fail to respond to medical treatment.

Dynamic prediction of fluid responsiveness during positive pressure ventilation: a review of the physiology underlying heart-lung interactions and a critical interpretation

OBJECTIVE

Cardiovascular responses to hypovolemia and hypotension are depressed during general anesthesia. A considerable number of anesthetized and critically ill animals may not benefit hemodynamically from a fluid bolus; therefore, it is important to have measures for accurate prediction of fluid responsiveness. Static measures of preload, such as central venous pressure, do not provide accurate prediction of fluid responsiveness, whereas dynamic measures of cardiovascular function, obtained during positive pressure ventilation, are highly predictive. This review describes key physiological concepts behind heart-lung interactions during positive pressure ventilation, factors that can modify this relationship and provides the basis for a rational interpretation of the information obtained from dynamic measurements, with a focus on pulse pressure variation (PPV).

DATABASE USED

PubMed. Search items used were: heart-lung interaction, positive pressure ventilation, pulse pressure variation, dynamic index of fluid therapy, goal-directed hemodynamic therapy, dogs, cats, pigs, horses and rabbits.

CONCLUSIONS

The veterinary literature suggests that targeting specific PPV thresholds should guide fluid therapy in lieu of conventional assessments. Understanding the physiology of heart-lung interactions during intermittent positive pressure ventilation provides a rational basis for interpreting the literature on dynamic indices of fluid responsiveness, including PPV. Clinical trials are needed to evaluate whether goal-directed fluid therapy based on PPV results in improved outcomes in veterinary patient populations.

Effects of pulsed inhaled nitric oxide on arterial oxygenation during mechanical ventilation in anaesthetised horses undergoing elective arthroscopy or emergency colic surgery

BACKGROUND

Administration of pulsed inhaled nitric oxide (PiNO) improves arterial oxygenation in spontaneously breathing anaesthetised healthy horses and in horses undergoing colic surgery. However, because hypoventilation commonly occurs, horses are often mechanically ventilated to prevent hypercarbia.

OBJECTIVES

To evaluate the effects of PiNO on arterial oxygenation during anaesthesia in mechanically ventilated healthy horses and horses undergoing colic surgery.

STUDY DESIGN

Prospective nonblinded clinical trial.

METHODS

Fifty horses undergoing elective arthroscopy (Group A) and 30 horses undergoing colic surgery (Group C) in dorsal recumbency were included in the study. Every second horse in each group received PiNO (A-INO, C-INO), the others served as controls (A-CN, C-CN). All horses were mechanically ventilated and anaesthesia was maintained with isoflurane. PiNO was mechanically delivered at the proximal end of the endotracheal tube as a pulse during the first part of each inspiration. Data were collected at the start (baseline, before PiNO) and at the end of inhalation anaesthesia. The Tukey method was used to compare baseline and end values for each parameter.

RESULTS

Arterial oxygen tension (PaO₂ ) increased from (median [IQR]) 13.6 (9.3, 30.1) at baseline to 24.2 (18.6, 37.0) kPa at the end of anaesthesia in A-INO (P = 0.005) and from 7.7 (6.4, 8.5) to 15.5 (9.9, 26.9) kPa in C-INO (P = 0.007). Mean (95% CI) difference in F-shunt between baseline and end were -6 (-10; -1) and -11 (-22; -1) % in A-INO (P = 0.005) and C-INO (P = 0.04) respectively. There was no change in PaO₂ or F-shunt from baseline to end of anaesthesia in A-CN or C-CN.

MAIN LIMITATIONS

Cardiac output was not measured, thus O₂ delivery could not be calculated.

CONCLUSIONS

The combination of mechanical ventilation and PiNO improved pulmonary gas exchange during anaesthesia by a simultaneous decrease in F-shunt and improved alveolar ventilation.

A Comparative Review of Equine SIRS, Sepsis, and Neutrophils

The most recent definition of sepsis in human medicine can be summarized as organ dysfunction caused by a dysregulated host response to infection. In equine medicine, although no consensus definition is available, sepsis is commonly described as a dysregulated host systemic inflammatory response to infection. Defense against host infection is the primary role of innate immune cells known as neutrophils. Neutrophils also contribute to host injury during sepsis, making them important potential targets for sepsis prevention, diagnosis, and treatment. This review will present both historical and updated perspectives on the systemic inflammatory response (SIRS) and sepsis; it will also discuss the impact of sepsis on neutrophils, and the impact of neutrophils during sepsis. Future identification of clinically relevant sepsis diagnosis and therapy depends on a more thorough understanding of disease pathogenesis across species. To gain this understanding, there is a critical need for research that utilizes a clearly defined, and consistently applied, classification system for patients diagnosed with, and at risk of developing, sepsis.

Comparison of Enterprise Point-of-Care and Nova Biomedical Critical Care Xpress analyzers for determination of arterial pH, blood gas, and electrolyte values in canine and equine blood

BACKGROUND

Point-of-care analyzers can provide a rapid turnaround time for critical blood test results. Agreement between the Enterprise Point-of-Care (EPOC) and bench-top laboratory analyzers is important to determine the clinical reliability of the EPOC.

OBJECTIVES

The aim of the study was (1) to evaluate the precision (repeatability) of blood gas values measured by the EPOC and (2) to determine the level of agreement between the EPOC and Nova Critical Care Express (Nova CCX) for the assessment of arterial pH, blood gases, and electrolyte variables in canine and equine blood.

METHODS

Arterial blood samples from dogs were analyzed on the EPOC and Nova CCX analyzers to determine precision and agreement of pH, PaCO₂ , PaO₂ , and HCT. The same analytes plus Na⁺ , K^- , and Cl^- were analyzed for agreement using equine blood. Statistical analyses included assessment of precision using the coefficient of variation (CV%), and agreement using the Deming regression, Pearson correlation, and Bland-Altman plots.

RESULTS

Both analyzers provided precise results of pH, PaCO₂ , PaO_2, and HCT, meeting CV% quality requirement values. In both species, Deming regression results were acceptable and correlation values were above 0.93 for arterial pH and blood gases, but lower for sodium and chloride. Bland-Altman plots demonstrated varying degrees of bias, but good agreement between the 2 analyzers was seen when arterial blood gases and electrolytes were measured, except for PaCO₂ and Cl^-. CONCLUSION: The EPOC analyzer provides consistent, reliable results for canine arterial blood gas values and for equine arterial blood gas and electrolyte values. Cl^- results could be acceptable with the application of a correction factor, but the PaCO₂ results were more variable.

Cardiovascular, respiratory and metabolic responses to apnea induced by atlanto-occipital intrathecal lidocaine injection in anesthetized horses

OBJECTIVE

To determine physiologic responses to apnea-induced severe hypoxemia in anesthetized horses.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Six university-owned horses with a median (range) body weight of 500 (220-510) kg and aged 13.5 (0.8-24.0) years scheduled for euthanasia.

METHODS

Xylazine-midazolam-ketamine-anesthetized horses breathing room air spontaneously were instrumented with a facial artery catheter for pressure measurement and blood sampling, and were made apneic with atlanto-occipital intrathecal lidocaine (4 mg kg^-1 ). Cardiopulmonary, biochemical and hematologic variables were recorded before (baseline) and every minute for 10 minutes after lidocaine injection.

RESULTS

PaO₂ values were: baseline, 55 mmHg (7.3 kPa); 1 minute, 28 mmHg (3.8 kPa); 2 minutes, 18 mmHg (2.4 kPa); 3 minutes, 15 mmHg (2.0 kPa), and 4-10 minutes, (8-11 mmHg (1.1-1.5 kPa). PaCO₂ values were: baseline, 50 mmHg (6.7 kPa); 1 minute, 61 mmHg (8.1 kPa), and 2-10 minutes, 64-66 mmHg (8.5-8.8 kPa). Base excess values at baseline, 1 minute and 2-10 minutes were 5.3 mmol L^-1 , 6.5 mmol L^-1 and 7.0-8.1 mmol L^-1 , respectively. Pulse rates at baseline, 1 minute and 2-7 minutes were 36, 53 and 54-85 beats minute^-1 , respectively. Asystole occurred at 8 minutes. Pulse pressures were 50 mmHg at baseline and 1 minute, and 39 mmHg, 31 mmHg, 22 mmHg, 17 mmHg and 1-9 mmHg at 2, 3, 4, 5 and 6-10 minutes, respectively. Lactate was 0.9 mmol L^-1 at baseline, progressively increasing to 1.7-2.4 mmol L^-1 at 7-10 minutes. Packed cell volume increased after 7 minutes of apnea. There were no other major changes.

CONCLUSIONS AND CLINICAL RELEVANCE

Apnea immediately exacerbated hypoxemia and hypercapnia and rapidly caused hemodynamic instability. Apnea in hypoxemic anesthetized horses is associated with a serious risk for progress to cardiovascular collapse.