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.

Ultrasonographic evaluation of the width, thickness, and length of the normal linea alba in standing and dorsal recumbent horses

OBJECTIVE

To determine the variability in length, width, and thickness of the equine linea alba (LA) and the effect of a standing vs dorsal recumbent position on these measurements.

STUDY DESIGN

Descriptive anatomical comparative study.

ANIMALS

Standing horses (N = 75; in 30 horses, measurements were obtained in dorsal recumbency first and repeated after horses were standing).

METHODS

Linea alba length was measured in standing position from xiphoid to umbilicus, and transverse ultrasonographic images were obtained at five reference points to measure LA width and thickness. In 30 horses, measurements were obtained in dorsal recumbency first and repeated after horses were standing.

RESULTS

There was wide variation in LA width and thickness between standing horses, with gradual increase from xiphoid (range, 0.14-0.64 cm) to umbilicus (range, 0.2-2.97 cm). Linea alba length in standing position was 51.09 ± 6.219 cm. Width was independent of the size of the horse; thickness and length were correlated at some reference points to height (r = 0.346-585, P < .05) and weight (r = 0.324-0.642, P < .05). Different LA shapes could be identified. In dorsal recumbency, the LA was smaller in width at all reference points (15%-23%, P < .05) and shorter (20%, P < .001) compared with standing.

CONCLUSION

In addition to the wide variability in LA measurements and shapes between horses, there was a significant decrease in LA width and length when horses changed from standing to dorsal recumbency.

CLINICAL SIGNIFICANCE

The difference in LA length and width between dorsal recumbency and when standing could increase tension on sutures after laparotomy and should be taken into account when surgeons are closing the abdomen.

Prevalence of intra-abdominal hypertension in horses with colic

OBJECTIVE

To determine an abdominal pressure cutoff value for intra-abdominal hypertension (IAH) in the horse and characterize IAH in horses with acute colic.

DESIGN

Prospective clinical cohort.

SETTING

University teaching hospital.

ANIMALS

Nine healthy adult horses and 56 horses with acute colic.

INTERVENTIONS

Ventral intra-abdominal pressure (IAP) was measured in triplicate at end expiration and averaged. Each colic case was classified as medical or surgical and large intestine (LI) or small intestine (SI). Management and final outcome (alive, euthanized, or died) were recorded. IAH was defined as ≥32 mm Hg (mean + 2 SDs of ventral IAP in control horses). Proportions of horses with and without IAH for different lesions were expressed as fractions and percentages. Differences in IAP between groups were determined using ANOVA with post-testing or t-tests. Odds ratio for management strategy (ie, need for medical or surgical) in horses with IAH was performed using Fisher's exact test. P < 0.05 was considered significant.

MEASUREMENTS AND MAIN RESULTS

IAP was higher in horses with colic compared to controls (P = 0.025). Over 18 months, 30.4% of horses with colic had IAH (n = 10 LI lesions, n = 7 SI lesions). Horses with LI medical lesions had the highest IAP of all lesions (mean 36.5 mm Hg). IAH horses with medical lesions were 15 times more likely to survive than IAH horses requiring surgery (P = 0.03).

CONCLUSIONS

Acute colic in horses is associated with an increased ventral IAP compared with healthy controls. IAH does exist in horses with colic, notably LI medical lesions, and is associated with nonsurvival in horses that require surgery.

Comparison of right lateral versus left lateral versus sternal recumbence intra-abdominal pressure measurements in normal dogs

OBJECTIVE

To determine if there is a difference in intra-abdominal pressure (IAP) among 3 anatomic body positions (right lateral, left lateral, and sternal recumbence) in apparently healthy dogs.

DESIGN

Prospective, observational study.

SETTING

University Veterinary Medical Teaching Hospital.

ANIMALS

Fourteen apparently healthy male dogs.

MEASUREMENTS AND MAIN RESULTS

After mild sedation with dexmedetomidine, a water manometer attached to a Foley urinary catheter was used to measure IAP in 3 different body positions in each dog. There was no significant difference in IAP between right lateral (3.87± 3.16 cm H₂ O), left lateral (4.45 ± 3.22 cm H₂ O), and sternal recumbence (4.04 ± 3.57 cm H₂ O).

CONCLUSIONS

Based on data from these 14 apparently healthy dogs, these 3 body positions can be used interchangeably for monitoring an individual dog. However, more research in dogs is needed to see if this conclusion holds true when IAP is abnormal.

Intra-abdominal Pressure Screening of Horses With Colic

The objective of this study was to evaluate changes in intra-abdominal pressure (IAP) in horses with colic by associating the underlying etiologies with directly acquired IAP values and survival rate. This is a 2-year cohort study (2014-2016). Horses with clinical signs of colic were admitted to the veterinary teaching hospital during the period 2014-2016. Twenty-eight horses, of different breeds, males (stallions and geldings) and females, aged between 2 and 20 years, and weighing from 300 to 450 kg presenting with clinical signs of colic, were included in the study. IAP was directly acquired at the right flank (standing under sedation) and at the linea alba (supine position under general anesthesia). Twenty IAP measurements were recorded at end expiration for each recording site. IAP values >0.0 mmHg, obtained at the upper right flank in the standing position, were associated with surgical treatment (P < .05). In these cases, signs of colic were associated with strangulated obstructions of the large colon, and a greater likelihood of death as a result of colic (P < .001). Intra-abdominal pressure varied considerably in horses with colic, even for the same underlying etiologies. Horses with colic related to strangulating obstructions of the large intestine had IAP >.0 mmHg, at the upper right flank. These horses were also considerably more likely to require surgical intervention (P < .05) and death/euthanasia was more likely in this group of horses (P < .001).

Experimentally Induced Open Pneumothorax in Horses

The objective of this study was to evaluate clinical and cardiovascular parameters and pleural and intra-abdominal pressures of horses after experimentally induced unilateral open pneumothorax. Prospective, experimental study-animals: seven healthy adult horses, 4 females and 3 males. Left hemithorax thoracotomy was carried out to create an open pneumothorax for 60 minutes. Pleural pressure (P_pl) was directly obtained at the midpoint of the left eighth intercostal space before thoracotomy. Esophageal pressure (P_es), arterial blood gas analysis, left ventricular function, and ultrasonographic assessment of pneumothorax extent/resolution were performed at the baseline, and 5, 10, 15, 30, 45, and 60 minutes after thoracotomy, and on the first, second, third, fifth, and seventh days postoperatively. Intra-abdominal pressure was only recorded while the pneumothorax was present. There was moderate correlation (Spearman's rs = 0.404; R² = 0.8; P < .00001; Bland-Altman bias = -2.59; s.d. = 2.11) between P_es and P_pl. Esophageal pressure increased (P < .05) after open pneumothorax until the fifth day postoperatively. Partial pressure of oxygen in arterial blood reduced (P < .05) until the third day postoperatively when it returned to the baseline. No significant variations in P_aCO₂, pH in arterial blood, and in left ventricular function were appreciated. The extent of the pneumothorax was assessed by thoracic ultrasonography. Esophageal pressure, in association with blood gas analysis and thoracic ultrasonography, could be used to aid diagnosis of pneumothorax in horses. Horses tolerate open pneumothorax, with minimum cardiovascular impairment, even without aspiration of free air from within pleural space to restore thoracic wall integrity.

Effect of intra-abdominal hypertension on plasma exogenous creatinine clearance in conscious and anesthetized dogs

OBJECTIVE

To evaluate the effect of intra-abdominal pressure (IAP) on plasma exogenous creatinine clearance in both conscious and anesthetized dog models using a balloon technique to generate intra-abdominal hypertension.

DESIGN

Prospective, cross-over, experimental study.

SETTING

University-based small animal research facility.

ANIMALS

Six healthy male Beagle dogs.

INTERVENTIONS

A balloon device comprising a Foley urinary catheter and latex balloon was placed in the intra-abdominal cavity. Plasma exogenous creatinine clearance was compared after intravenous administration of exogenous creatinine solution at 80 mg/kg under 4 different treatment conditions as follows: control and IAP levels of 25 mm Hg in conscious dogs and control and IAP levels of 25 mm Hg in anesthetized dogs (CC, C25, AC, and A25, respectively). Samples were obtained before (T0) and 10, 20, 30, 60, 90, 120, 240, 360, 480, and 600 min after administration of creatinine in all treatment groups.

MEASUREMENTS AND MAIN RESULTS

There were no significant differences in plasma creatinine concentration for CC, AC, and C25 during the treatment period. However, in the A25 treatment condition, the plasma creatinine concentration increased significantly at 10, 20, 30, 60, 90, and 120 min after administration of creatinine (P < 0.05). Plasma creatinine clearances were 5.0 ± 0.5, 4.7 ± 1.2, 5.5 ± 0.9, and 2.5 ± 0.5 mL/kg/min for 600 min (CC, AC, C25, and A25, respectively). In the A25 treatment condition, the plasma exogenous creatinine clearance decreased significantly to 50%, 47%, and 55% of that under control conditions (CC, AC, and C25, respectively). After decompression of the abdomen, plasma creatinine concentrations declined rapidly and returned to basal concentrations.

CONCLUSIONS

Intra-abdominal hypertension under general anesthesia could cause renal hypoperfusion. Timely decompression may improve the outcome of acutely increased IAP when surgery and/or general anesthesia is required in canine patients.