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Critical decisions: when to ventilate and when to dialyze (Proceedings)

Article

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 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.

Oxygenation analysis needs to be assessed in several different lights to determine if ventilation is necessary. First, the PaO2 needs to be assessed in light of the fraction of inspired oxygen. Absoulute numbers on 21% oxygen are useful. Normal PaO2 at sea level is 85-100 mmHg. However, this should be looked at in conjunction with PaCO2 because hyperventilation can artificially drive up the PaO2. The Alveolar-arterial gradient is especially useful to determine if low oxygen tension is a function of low fraction of inspired oxygen, low partial pressure of oxygen, or alveolar hypoventilation versus true lung dysfunction (i.e. V/Q mismatch, diffusion impairment, or shunt). The A-a gradient should only be performed on arterial blood and on room air (21% oxygen). The Alveolar Gas Equation is:

The normal A-a gradient is 5-15 mmHg. Mild pulmonary dysfunction is 20-30 mmHg, moderate is 30-40 mmHg, severe is 40-50+. Most animals that need mechanically ventilated have an A-a gradient > 50 mmHg.

There are many ways to deliver oxygen, such as flow-by oxygen, mask oxygen, nasal oxygen, oxygen cage, and intubation. However, there are problems with oxygen therapy. If animals are exposed to 100% oxygen for > 12-24 hours, oxygen toxicity can occur. Therefore, the goal is to keep the FIO2<60%. Other problems include absorption atelectasis and reduction of the hypoxic ventilatory drive, which can worsen respiratory acidosis. Mechanical ventilation can reduce the FIO2 in severely hypoxemic animals by applying positive end expiratory pressure (PEEP), which helps prevent atelectasis and improves the functional residual capacity. PEEP also decreases the work of breathing by minimizing the alveolar opening pressure, improves pulmonary compliance, decreases interstitial lung water, and can reduce ventilatory induced lung injury by preventing alveolar collapse.

Another reason to ventilate a patient is due to issues with carbon dioxide. Hypoventilation (hypercarbia) or elevated PaCO2 can lead to respiratory acidosis. Looking at absolute PaCO2 numbers is helpful in determining ventilation status. Normal PaCO2 at sea level is 35-45 mmHg. The next question to ask is the acid-base status being affected? If the pH is < 7.2, then it is cause for concern. Ventilatory failure is considered if the PaCO2 is >60mmHg.

Other parameters to consider ventilating would be the 60/60 rule. This is when the PaO2 is < 60 mmHg and the PaCO2 is > 60 mmHg. Additionally, if the animal has severe hypoxemia that is not oxygen responsive, such as a PaO2 < 60 mmHg on 80-100% oxygen, then ventilation should be considered.

Subjective parameters or specific circumstances should also be taken into consideration when determining if an animal needs ventilated. Animals with excessive work of breathing or who are heading towards respiratory fatigue, regardless of what their blood gas indicates, should be potentially ventilated. Animals that are post-cardiopulmonary arrest, post-operative cardiopulmonary bypass, or those that have invasive monitoring that would be dangerous for them to move around, frequently need mechanical ventilation.

Overall, ventilation is a labor intensive and expensive endeavor, although it can be successful. Knowledge of the indications and case selection of animals that would benefit from mechanical ventilation is extremely useful. Although early ventilation may lead to better success rates in veterinary medicine, it may remain cost prohibitive to many clients.

When to dialyze

Acute renal failure (ARF) is a common disease process in small animal veterinary medicine. Many causes are reversible if treated early or given enough time for the kidneys to heal, assuming the tubular basement membrane has not been damaged. By definition, acute renal failure is an abrupt and sustained decreased in glomerular filtration rate (GFR). This leads to retention of nitrogenous waste and toxins, as well as fluid and electrolyte imbalances. Pre-renal and post-renal causes must be ruled out prior to determining an animal to be in ARF. Most ARF is caused by ischemic events or exposure to nephrotoxins, leading to acute tubular necrosis. The low partial pressure of oxygen and the high metabolic activity of the thick ascending loop of Henle and the pars recta of the proximal tubule make it at high risk of ischemia.

There are many negative side effects of uremia. These include depressed leukocyte function and cellular immunity, decreased clearance of gastrin, stimulation of the chemoreceptor trigger zone, uremic pneumonitis, pericarditis, and encephalitis, as well as altered platelet function and adhesion.

Detection of ARF depends on various physical exam and laboratory parameters. These may include hydration status, body weight, pulse quality/character, urine output, kidney pain/swelling, blood pressure, PCV/TS, BUN/Creatinine, urine specific gravity, urine dipstick, urine sediment, acid/base status, electrolytes, and urine/protein/creatinine ratio.

Medical management of ARF is typically adequate. Dialysis is typically used for oliguric/anuric patients that need help with overhydration, severe acidosis, electrolyte issues (esp. hyperkalemia), severe uremia, or those that have no response to medical management for 24 hours. Early dialysis may help patients feel better and "recover" faster. However, the goal is not focused on fixing the azotemia, but rather to fix the side-effects of the consequences of decreased kidney clearance. Dialysis is important to start early in conditions that are known to have a long course of recovery or in toxicities that have dialyzable toxins that are small and non-protein bound (i.e. ethylene glycol).

The definition of anuria is essentially no urine, of oliguria is <0.25-0.5 ml/kg/hr, and relative oliguria < 1 ml/kg/hr on IV fluids. Candidates for dialysis are dogs and cats that are >2.5 kg, blood pressure > 80 mmHg, adequate PCV, and tractable demeanor.

The ARF patient must be volume loaded in order to determine if true oliguria is present. Volume loading the patient will treat any underlying pre-renal disease. However, it is imperative that the patient is not volume overloaded if it is truly oliguric. To prevent overhydration, monitoring of PCV/TP, body weight, physical examination, CVP, and urine output is necessary. Once volume resuscitation occurs, then fluid therapy should be altered to meet insensible losses and outs to avoid overhydration. Insensible losses are typically 1/3 of maintenance needs. If an oliguric/anuric patient is iatrogenically volume overloaded and is not responsive to diuretics, then dialysis it the ONLY way the overhydration can be resolved.

Prior to committing to dialysis, medical management for oliguria/anuria should be attempted. Loop diuretics are frequently the first line of defense in an attempt to treat volume overload. Lasix may ease the management by converting to a nonoliguric state, but will not increase GFR. It also may not be helpful once acute tubular necrosis is established because delivery of the drug is GFR dependent. Secondly, mannitol is frequently used to treat volume overload since it is an osmotic diuretic. However it may lead to volume overload if it is unsuccessful. High-dose glucose solutions (10-20%) have been used to improve urine output due to its osmotic effects and can be metabolized if it is unsuccessful.

Dopamine has been advocated in ARF because it may increase GFR by direct vasodilation of the afferent and efferent arterioles via dopamine receptors, which tends to cause more of a rise in renal blood flow than GFR. However, of 30 human studies using low-dose dopamine, only 3 had positive effects. The veterinary literature is indeterminate regarding dopamine and ARF. Regarding cats, there is a lower dopamine receptor density in the feline renal cortices. Therefore, fenoldopam is recommended in lieu of dopamine.

Calcium antagonists, such as Diltiazem, are suggested in ARF by inducing preglomerular arteriolar dilation and natriuresis. This appears to be a promising treatment in patients suffering from ARF associated with Leptospirosis.

Medical management of hyperkalemia should be instituted using traditional methods, including potassium free fluids, calcium gluconate to protect the heart, dextrose, regular insulin if needed, and potentially sodium bicarbonate.

Acidosis can be medically managed with sodium bicarbonate, although this is controversial, as it can increase intracellular acidosis despite improving extracellular acidosis.

The types of dialysis that are typically used in veterinary medicine are hemodialysis (usually intermittent (IHD) usually for chronic kidney disease), continuous renal replacement therapy (CRRT) (usually short-term and for ARF), and peritoneal dialysis (usually short-term, for ARF in places where CRRT and IHD are unavailable).

Regarding CRRT, one study by Diehl and Seshadri showed that the survival to discharge was 41% in dogs and 44% in cats. The median duration of CRRT was 16.3 hours in dogs and 11.5 hours in cats. The median drop in BUN was 115 to 33 mg/dL in dogs and 130 to 39 mg/dL in cats. The median drop in Creatinine in was 7.2 to 3.1 in dogs and 12.1 to 3.3 mg/dL in cats. The metabolic acidosis and hyperkalemia resolved in most cases. The complications seen were iatrogenic hypokalemia and metabolic alkalosis, clinical hypocalcemia, total hypercalcemia, filter clotting, anemia, hypothermia, and neurologic complications.

With Intermittent Hemodialysis (IHD), the literature indicates a 41-52% survival. The average duration of therapy is 29 days for survivors. 50% of deaths are from extrarenal causes (pancreatitis and respiratory), and 33% from failure of renal recovery.

Peritoneal dialysis is still an option when CRRT and IHD are unavailable and can be life-saving. It is very useful for stabilizing uroabdomens prior to surgery and for patients that are hypotensive and cannot handle CRRT or IHD. Complications of peritoneal dialysis include septic and sterile peritonitis, leakage, retaining dialysate, dialysate going into the pleural cavity, abdominal pain, and the need for surgery and omentectomy to place the dialysis catheter.

Overall, dialysis can be very useful in ARF and lead to an animal feeling better much faster than with traditional medical management of ARF. This can lead to improved immune function, decreased systemic effects from uremia, improved nutrition, and improved morale. The cost is an issue to many clients, with anywhere from $2000-3000 for the first 24 hours and $1000-2000 for each day thereafter. As with ventilation, the cases need to be chosen correctly and the owners need to be informed of the cost, outcome potentials, and complications.

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