Better Brains

From the very beginning of my career I always worried about patient brain damage after cardiopulmonary bypass (CPB).  Back then we called it “pump head” . “Pump lung” (now called adult respiratory distress syndrome or ARDS) was a common concern as well. In my early experience they were the two most common CPB complications, second only to bleeding. “Pump head “ brain damage ranged from temporary delirium, to neurocognitive loss, to stroke, to brain death.  Sometimes there was a problem during the procedure that could explain the cause of the brain damage, but most times there was no clear definitive cause. Even today, during their preoperative teaching, patients are warned that brain damage from CPB is a distinct possibility. If they are not warned of such, then their consent is not fully informed (1).

In my opinion and experience, the advent of the disposable arterial filter (AF) was a tremendous leap in reducing brain damage (2). Most are depth filters that remove debris effectively and utilize a vortex flow pattern to channel gas bubbles safely away from the patient. I worry today because some younger perfusionists seem to see the AF as a waste of prime volume rather than a brain saver and want to eliminate its use.  Others are happy using oxygenators with a built-in AF which I feel are less than adequate because they may not use depth filters or vortexing (3,4).

Perfusionists have always focused on protecting three organs; the heart, the kidneys and the brain.  The heart is easy to protect because we have ample means of ongoing assessment.  The ECG is an excellent indicator of the electrophysiological health of the heart.  And the surgeon can see and feel the heart directly during most of the case.  The kidneys are not visible but they can nonetheless be adequately assessed by ongoing urine output and electrolyte balance.   But when I think of protecting the brain during CPB, the cliché’ “out of sight, out of mind” seems to explain the stagnation of our brain protection strategy since the advent of the AF. It is not until the procedure is completed and the patient begins to recover that injury to the brain is first detected.

I was very happy to see that AmSECT now calls for the use of cerebral oximetry during CPB, but only as a Guideline, not as a Standard. At least that is a step in the right direction. AmSECT also calls for other safety features such as level detectors and arterial blood line bubble detectors to prevent gas emboli from entering the patient (5).  However in recent years these features have proven themselves inadequate to protect patients from gaseous microemboli (GME). Even the newest oxygenator design allows for some GME passage (4).

As I see it, there are several additional monitors that should be utilized to reduce the risk of brain damage. The use of cerebral oximetry has already been mentioned, but it should be a Standard and not a Guideline  (6).

The use of a redundant blood flow meter could eliminate any aberration in total blood flow due to human error or equipment failure.  Blood flow is arguably the most important parameter measured and yet most CPB systems have only a single means of making this assessment. Blood flow assessments on roller pumps are indirect measurements and subject to human error due to inadequate occlusion, inadequate raceway placement, incorrect raceway tubing selection and incorrect calibration. Blood flow assessment on centrifugal pumps is a direct measurement but can be in error due to incorrect tubing size, incorrect probe selection and/or calibration, incorrect probe placement and electronic malfunction.  In addition, blood flow can be inaccurate due to inadvertent shunting within the circuit on both pump types. Without a redundant flow meter, these errors may not be detected quickly enough to prevent brain damage.

Even with accurate blood flow, anatomic abnormalities such as aorta-to-pulmonary artery collateral vessels can siphon blood flow away from the brain. Bilateral acoustic Doppler monitors should be used to qualitatively assess bilateral carotid artery blood flow prior to CPB.  During CPB these monitors can detect an imbalance in the carotid blood flow. They can also distinguish hyperperfusion or hypoperfusion that might otherwise be missed. Furthermore, these Dopplers can identify GME passing through the carotid arteries that might otherwise be undetected.

Transcranial Dopplers (TCD) are now inexpensive and convenient to use in a variety of clinical and surgical settings (7).  In cardiac surgery specifically, TCD allow assessment of blood flow within the brain and can detect gas emboli (1).

This group of monitors would allow the surgeon to adjust the aortic cannula quickly if the brain was being exposed to hyperperfusion or some other blood flow abnormality. In addition, steps could be taken to quickly deal with GME including stopping the manipulation that is causing them.  Off-gassing nitrogen bubbles that have already entered the brain could be quickly initiated by the perfusionist to remove them from the body.

I believe that all these monitors could reduce the risk of damage to the brains that are in our care during CPB. Unfortunately there are probably perfusionists who do not feel that the benefit is justified by the time and expense needed to implement this monitoring. After all, just reducing risk does not eliminate it altogether.  On the other hand, if this additional brain monitoring was to detect a problem such as hyperperfusion up a carotid artery or gas emboli in the middle cerebral artery that would otherwise remain undetected, then steps could be taken immediately to mitigate the situation and prevent brain damage.

An exciting new concept has come to the forefront of ECLS: hypobaric oxygenation.  This could be used with any ECLS circuit containing an oxygenator.  The principal is to pass the sweep gas of 100% oxygen through the oxygenator at sub-atmospheric pressure.  This removes most of the nitrogen from the blood and reduces the number of gaseous microemboli by 80-100% (8). This could be the key to solving the GME and give us better brains.

 

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Perfusion Theory is an educational platform for the Oxygen Pressure Field Theory (OPFT). August Krogh’s theoretical concept of the oxygen pressure field is explained and then applied to clinical applications in perfusion practice.

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