This case was presented in our monthly virtual Abdominal Rounds with Donald E. Thrall DVM, PhD, DACVR in June 2021. If you’d like to join us in these sessions. please click this link.
History: The owner reported that the dog experienced difficulty in urinating. In a standard physical examination, the prostate gland was palpated as enlarged. In survey radiographs, the prostate gland was visible at approximately 2.0 cm. in diameter. For further assessment, a negative contrast retrograde urethrocystogram was performed (Figure 1). Results from hematologic testing and a urinalysis were not available.
- The disc space at L1-L2 is narrow with ventral spondylosis; this is not significant.
- A mildly enlarged prostate gland is visible caudal to the urinary bladder, at the entrance of the pelvic canal.
- The enlarged prostate gland is of homogeneous soft tissue opacity.
- The vesicourethral junction is positioned slightly dorsal of center in the enlarged prostate gland.
- Air is present in the urethra surrounding the catheter, and filling the lumen of the urinary bladder, which is mildly distended.
- The wall of the urinary bladder is of a normal thickness and contour.
- There is retrograde reflux of air into the ureters and both renal pelves and pelvic diverticula.
- The primary consideration for the prostate gland enlargement is benign hyperplasia. Other causes of prostate gland enlargement including prostatitis/abscess, and a prostatic neoplasm, cannot be ruled out from survey radiographs.
- There is no evidence the enlarged prostate gland is impinging on the prostatic urethra, descending colon or rectum. It is questionable whether the prostate gland enlargement is related to the dysuris, however this cannot be ruled out.
- The asymmetric position of the vesicourethral junction may occur more commonly with inflammatory or neoplastic prostate gland disease, but this is not definitive.
- There is no evidence of reflux of air from the urethra into the prostate gland. This lessens the chance of a prostatic abscess as irregular cavities often communicate with the urethra.
- Reflux of air into the ureters and kidneys is of no inherent significance but this likely increases the chance of fatal air embolism.
- The cause of the clinical signs has not been determined by this imaging study.
- If there is suspicion of prostate gland neoplasia, a urinary BRAF test would be justified as most prostate gland tumors are of urothelial origin.
- Ultrasound evaluation of the prostate gland would also be justified to assess the parenchymal characteristics.
- If the prostate gland enlargement is due to benign hypertrophy, neutering would be appropriate.
 CADET® BRAF and CADET® BRAF-PLUS. Antech Diagnostics.
A major point of discussion regarding this case is the use of a negative contrast study of the lower urinary system. Contrast studies of the lower urinary tract have largely been replaced by ultrasound imaging but negative contrast studies are sometimes still performed, as evidenced in this case.
Contrast studies of the lower urinary tract can employ water soluble iodinated contrast medium, negative contrast medium, such as room air or carbon dioxide, or a combination. In general, these studies are safe, but a major and life-threatening complication associated with the use of room air for negative contrast studies of the lower urinary tract is venous air embolism. This patient did not experience venous air embolism, but the ureteropelvic reflux of air provides an opportunity for discussion of the syndrome. A review of various causes of arterial and venous air embolism in human patients, along with a discussion of the pathophysiology, is available (Malik, et al., 2017).
With regard to pneumocystography in canine and feline patients when room air is used, air can enter the systemic venous system via communications of the vascular system with the lumen of the urinary tract, such as hematuria. Hematuria is, of course, very common in patients who are candidates for a contrast study of the lower urinary tract. The fatal event is an air lock occurring in the right ventricle that reduces or occludes pulmonary outflow (Figure 2). Fatal venous embolism following pneumocystography has been documented in both dogs and cats, but there are more reports in cats (Zontine & Andrews, 1978) (Schaer M. , 2018) (Thayer, Carrig, & Evans, 1980) (Ackerman, Wingfield, & Corley, 1972).
At this time, there are no justifiable indications for performing a negative contrast study of the lower urinary tract with room air, especially in a patient with evidence of hematuria. If this study must absolutely be performed, there are several precautions that should be followed (Zontine & Andrews, 1978).
- The patient should be placed in left rather than right lateral recumbency. This may reduce the accumulation of a small amount of air in the pulmonary outflow tract. If the amount of embolized air is large, the side of recumbency is likely not going to be important.
- Carbon dioxide or nitrous oxide should be used instead of room air. Both gases are approximately 20-times more soluble in blood than air or oxygen. Carbon dioxide was actually used safely as a contrast agent in the venous circulation in the 1950s and 1960s (Cho, 2021).
- Regardless of the type of gas used, the urinary bladder should be distended slowly.
- Overdistension of the urinary bladder should be avoided.
Although the focus of this report is venous air embolism occurring as a complication of pneumocystography using room air, there are several other procedures in small animals that have resulted in fatal venous air embolism. These include…
- cryosurgery (Harvey, 1978)
- total hip replacement (Liska & Poteet, 2003)
- laparoscopy (Gilroy & Anson, 1987)
- dental extractions (Gunew, Marshall, Lui, & Astley, 2008)
- endoscopy (Ober, Spotswood, & Hancock, 2006)
- hemilaminectomy (Mortera-Balsa, et al., 2017).
Air can also be introduced iatrogenically, primarily with injections or infusions. Large amounts of injected air are potentially fatal (Pacifico, Weishaar, Boozer, & Nakamura, 2010) (Boitout & Mahler, 2013) (Walsh, Machon, Munday, & Broome, 2005). A small volume of iatrogenically injected air is commonly found in the axillary vein of patients undergoing contrast enhanced computed tomography without complication (Heng, Ruth, & Lee, 2014).
In summary, fatal air embolism is a rare but obviously serious complication of negative contrast cystography where room air is used at the contrast medium, especially in patients with hematuria. The likelihood of this complication can be eliminated by choosing other methods to examine the urinary bladder.
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