What Are The Native Reservoir And The Ready Reservoir?
Yes, it is possible to to ‘dry out’ a patient during cardiopulmonary bypass (CPB). The article (referenced below) is about pediatric patients, but I believe there is an adult correlation as well. What I learned is that patients coming off CPB with a positive fluid balance have about twice the mortality as those patients with negative or zero balances. There are several reasons for this. But it is well known that the mortality of patients undergoing any kind of major surgery is directly related to the need for fluid administration during surgery and in the post-operative period. The study referenced below is retrospective, but I feel that any prospective study randomizing CPB patients to negative and positive fluid balance groups would be unethical, knowing what we already know.
In terms of drying out a patient, some patients can arrive in the OR with excess fluid on-board due to congestive heart failure (CHF). A 70 kg patient can easily carry around 5-10% (3.5-7L) of his body weight in extra fluid. Similar circumstances occur in pediatric patients as well. Patients are then given anesthesia fluid to compensate for fasting and, of course, they may receive additional fluids from the perfusionist during CPB. These fluids make up a ‘ready reservoir’ of fluid that can be safely and easily removed by ultrafiltration (UF) and other techniques during CPB. However, it is possible to remove all of this ready reservoir fluid and encroach upon the ‘native reservoir’.
The native reservoir is the normal amount of extravascular fluid. This volume is large in children and somewhat smaller in adults. The native reservoir has many purposes, one of which is to prevent shock if there is blood or fluid loss. It is possible for the perfusionist to remove all of the ready reservoir fluids and encroach on the native reservoir without causing an obvious circulating volume loss during surgery. Immediately after CPB, the hemodynamics may be perfectly normal. However, several hours after surgery, as the fluid compartments readjust in the patient, there may be an overt fall in hemodynamic measurements (blood pressure and central venous pressure) caused by movement of fluid out of the vascular compartment and into the depleted extravascular ‘native reservoir’ compartment. Diuresis can often set this off as well. This hypotension often requires treatment with copious amounts of crystalloids to maintain hemodynamics.
In most cases it is safe to remove up to 40 mls/kg during CPB. In a 70 kg man this would be 2.8 L fluid removed before encroaching on the native reservoir (2.8L is probably twice the volume of most adult CPB circuits). Of course, if the patient has CHF and the surgery doesn’t fix it, the perfusionist might re-infuse 3.0 L of fluid just to get the patient off CPB, leaving him with a positive fluid balance and a higher risk of mortality.
On the other hand, even if 60 mls/kg is removed during CPB the patient may wean without the need for any extra fluid if there is good cardiac function. Several hours later, after the fluid compartments redistribute or diuresis starts, the patient may need lots of fluid to maintain his blood pressure. The process I am describing is very complex, but it is thoroughly explained in the article referenced below.