Hypoxemia with Hyperventilation in an 8-Month-0ld Samoyed

History. An 8-month-o!d Samoyed is brought to you because it is reluctant to exercise. Ever since the owner obtained the puppy, she has noticed that its behavior is not “puppy-like"; it tires easily and prefers to sleep rather than play.

Clinical Examination. The puppy is not well grown. Even though the owner thinks it has been growing, for a Samoyed of 8 months of age the dog is rather small. When standing quietly in the examination room, the dog breathes normally, but when you call and it runs toward you, its respiratory rate increases and the dog begins to pant. At this point, you notice that the dog’s tongue and gums have a distinct bluish tinge.

Before examining the dog further, you are already sus­picious of a congenital cardiac anomaly. You suspect this because of the dog’s age and history, and because it became cyanotic with minimal exercise.

Palpation of the dog shows that even though it is small, it is not thin. The major abnormalities are the cyanosis of the mucous membranes, an extremely elevated heart rate, and loud, abnormal cardiac sounds. A murmur is audible over the tricuspid valve area during systole. The murmur (grade 4/5) is loud enough to produce a palpable vibration on the chest wall. You explain your suspicions of a cardiac defect to the owner, and together you decide to request some studies to determine the nature of the defect.

An echocardiogram, which is performed to determine the type of cardiac anomaly, demonstrates a large opening between the left and right atria, caused by a patent foramen ovale. Before angiographic studies, an arterial blood gas sample is taken to determine the suitability of the dog for anesthesia; Pao₂ is 61 mm Hg and Paco₂ 23 mm Hg.

Angiography is performed successfully. A catheter is floated into the right atrium of the dog, and dye is injected at this site. Some of the dye passes into the right ventricle, but a large portion passes from the right atrium into the left atrium and out into the systemic circulation.

Comment. The blood gas results are fairly typical for an animal with a major oxygen-exchange problem; this dog has a right-to-left vascular shunt through a cardiac defect. A large amount of the mixed venous blood returning to the heart is bypassing the lungs, resulting in the low Pao₂. The Pao₂ is low enough to cause a major increase in ventilation by stimulating the peripheral chemoreceptors. This increase in ventilation causes excessive elimination of carbon dioxide, resulting in the reduced Paco₂. Perhaps ventilation could have increased further, but the low Paco₂ acting on the central chemoreceptor slows down the increase.

The echocardiogram and angiographic studies are indic­ative of a patentforanιen ovale. Normally this would not result in right-to-left shunting of blood because the pressure in the left atrium is usually higher than that in the right atrium. However, this dog probably also has abnormalities of the tricuspid valve that cause a partial obstruction. This is suf­ficient to increase the pressure in the right atrium and cause blood to flow from right to left through the foramen ovale.

Treatment. The cardiac defect will need to be corrected surgically if the dog is to have any chance of life. Echocardiog­raphy is generally performed before angiography because it can better determine the extent of the abnormalities and is safer. An angiogram could then be performed, if necessary. With some limited cardiac anomalies, a transcatheter device can accomplish closure of the foramen ovale. However, if there are other tricuspid valve abnormalities, the method of repair may be altered.