Critical decisions: when to ventilate and when to dialyze (Proceedings)
The decision to mechanically ventilate or dialyze a dog or cat is a difficult one. It is challenging from an expertise, staffing, emotional, ethical, and monetary perspective. The purpose of these proceedings is not to teach one how to mechanically ventilate or dialyze an animal, but rather to teach one when to recommend these procedures, appropriately choose cases that might be successful, and know when to refer these cases for life-saving treatment.
When to ventilate
Long-term mechanical ventilation in veterinary medicine is very different than human medicine. "Long-term" to veterinarians is typically 5-7 days, whereas it can be months to years in human medicine. Mechanical ventilation is a cumbersome process that requires one-on-one, minute-to-minute care and monitoring. It is true life support; however, it is necessary to recognize that only certain animals fit the criteria for mechanical ventilation. Although veterinary ethics frequently are riddled with personal biases, it is generally accepted that mechanical ventilation in veterinary medicine is not utilized for animals that have end-stage lung disease. Additionally, owners need to be informed about the realistic outcome of cases that are ventilated and the financial commitment. The survival rate for dogs being ventilated secondary to primary lung disease (i.e. pneumonia, pulmonary contusions) is only about 30%, whereas the survival rate of dogs being weaned from ventilation secondary to hypoventilation from cervical surgery is generally 70%. It is possible the success rate may be higher if veterinarians ventilated animals sooner in the disease process, as they do in human medicine, but this is unlikely due to financial constraints in veterinary medicine. The cost at most institutions is approximately $1500-2000 for the first 24 hours and $1200-1500 per day afterwards. Taking these cases day-by-day without a long-term commitment from the owner is frustrating and decreases the morale of the team, as even cases that are only ventilated for 1-2 days need many days of post-ventilatory hospitalization at high cost. Most owners that have animals with successful outcomes have bills in the $15,000-25,000 range and many that have unsuccessful outcomes have bills in the $5,000-10,000 range.To determine if an animal needs mechanical ventilation, an arterial blood gas is necessary. Arterial blood gases are the only way to determine the oxygenation status of an animal and the most accurate way to determine the ventilatory status of an animal. Venous blood gases can be used to determine ventilation if the animal is adequately perfused, however the PCO2 of a venous blood gas is typically 5-10 mmHg higher than an arterial blood gas. In the underperfused patient, a venous CO2 can be artificially elevated. The blood gas also gives insight into the acid-base status of the patient, which is one of the determinates of whether or not an animal would benefit from mechanical ventilation.
When assessing a blood gas, one needs to look for evidence of hypoxemia (low PaO2), evidence of hyper or hypoventilation (low or high PaCO2), evidence of pH abnormalities, and evidence of responsiveness to oxygen. If these are abnormal, then further assessment is necessary.