CPB FMEA: an unusual oxygenator changeout
This is the fourth in a new series of postings from the Safety Committee that looks at dangerous perfusion incidents from the standpoint of failure modes and effects analyses (FMEAs). Some conceptual incidents that could potentially occur may be discussed. But, for the most part, these incidents are genuine like the one described below. This incident will be examined in the light of an existing FMEA to determine why the incident occurred. Was the FMEA deficient in management of the incident or did ignorance of the FMEA management actions result in the incident? You will be the judge. Comments are welcome.
Gary Grist RN CCP Emeritus <[login to unmask email]>
I read an interesting PIRS II report that describes a quality gas exchange oxygenator test that failed (see below*). In all my years the only safety quality oxygenator tests I ever performed pre-CPB were looking for leaks in heat exchangers and cracks in the casing or blood ports and high flow and pressure test to check for leaks and flow path abnormalities. When the PIRS II reporter had an O2 gas exchange test fail, he replaced the oxygenator before CPB. The test is simple enough that I think it should become a standard quality control test. Then, should the oxygenator fail during CPB, there is evidence that a prudent effort was made to determine if the oxygenator was functioning properly before use.
To summarize the PIRS II report: An oxygenator was setup and clear primed. This program’s practice is to do a blood gas analysis on the prime prior to bypass to observe if all CO2 flush from the setup up of circuit has been removed and that the oxygenator is transferring oxygen. They used a sweep gas flow of 80% oxygen and 3L/min for 5min. A prime sample was drawn and analyzed with an IStat machine. The result was a pCO2 = 0mmHg but a pO2 of only 197mmHg. They expected it to be above 400mmHg. The test was repeated with the result being 210mmHg. Repeating once again, the gas flow was increased to 8L/min and FiO2 100%. The pO2 on the IStat analysis still read 210mmhg. The IStat was checked for accuracy and eliminated as a source of the error. They changed out the oxygenator prior to bypass. (Discretion is the better part of valor!) This was the first time they had such a low O2 reading during a priming. Even if the fiber bundle was not defective why take a chance?
Many perfusionists I contacted for comment run the sweep gas during clear priming to remove any excess CO2 particularly when a CO2 flush is used. Sometimes when there is a CDI in line the FiO2 is changed to see if the CDI sensor is responsive. Only one responder uses a CDI goal of 250 mmHg, but that is mainly to assess that the sweep gas system is working. She speculated, “I suppose it could put into question if the oxygenators’ transfer was insufficient if the PO2 was not being reached even with an increase in the FIO2 ?”
Even if the oxygenator had a partially obstructed fiber bundle or if there was significant channeling of prime flow, CO2 is 30 times more soluble than O2. So, a fiber bundle could be significantly impaired and still remove excess CO2 from the prime. But nobody I contacted uses the FiO2 to assess oxygen exchange formally. I am not suggesting that the ability of the fiber bundle to transfer the large volume of oxygen needed when on CPB is measured by this quick test. It does not. But if the fiber bundle does not meet a certain minimal pO2 goal within a short period of time, there could be a defect which could cause a problem during CPB. This quick test can be easily repeated by moving between room air and an elevated FiO2 to see if the fiber bundle responds appropriately.
Suppose the FiO2 is set to 80% and the prime pO2 normally goes to 400+ mmHg within 60 seconds. And this successful test is performed on 1000 oxygenators. Then on the 1001st oxygenator it takes 180 seconds to reach 400+ or it does not reach that goal at all. Would that result suggest a problem with gas transfer on the oxygenator and should it be changed before use? The PIRS II reporter does not use a time limit other than waiting 5 minutes before checking the pO2. But I think a shorter time limit should be used (60 seconds) because if the fiber bundle is only partially defective, over time, the pO2 goal would be attained anyway as it recirculates. In vivo the O2 transfer must occur minute by minute without fail. So, a one-minute test seems reasonable.
This PIRS II report prompted me to re-write my FMEA on oxygenator failure and include the PIRS II quality test for oxygenator O2 transfer. This FMEA was specifically written for my particular perfusion (peds) program in 2011. Many things have changed since then. So, use this FMEA as a guide and template to custom write one of your own. The gas exchange test is added under PRE-EMPITVE MANAGEMENT (see below). The risk priority number (RPN) for the detectability of an oxygenator failure was changed from ‘undetectable;5’ to ‘moderately detectable;3’. Until I learned about this simple test, I thought there was absolutely no way to detect a possible oxygenator gas exchange failure before CPB. I know that many things can cause an oxygenator to fail (defective fiber bundle, channeling, clotting, platelet/fibrin deposition, etc.) This test might not detect all possible failures before they occur, but it might detect some.
FAILURE: Oxygenator gas exchange failure
- Failure to oxygenate the blood effectively.
- Blood exiting the oxygenator will appear dark red or black.
- SAO2 and SVO2 will decrease.
- Failure to remove CO2.
- Blood pH will decrease due to increasing pCO2.
- Patient hypoxia
- Patient hypercapnea
- Sweep gas system connections loose.
- Defect, crack or leak in the oxygenator or sweep gas system
- Failure of oxygen gas supply.
- Condensation within the hollow fibers of the oxygenator
- Clotted oxygenator.
- Gas exchange failure in oxygenator due to fibrous, platelet and cellular accumulations on the gas exchange surface which can increase resistance to blood flow and abrogate the blood path
- Fiber bundle defect or channeling of blood flow within the fiber bundle.
- Pressurize the sweep gas circuit prior to CPB to test for leaks.
- Maintain an emergency O2 source (E tank or O2 outlet from the anesthesia machine) with a line if the sweep gas system fails.
- The sweep gas is scavenged from oxygenator to the OR vacuum gas vent. This allows for visual assessment of the oxygenator exhalation port for a blood leak. It also removes any volatile anesthetics that may be used during CPB.
- Visually confirm gas flow through flow air/O2 mixer flow meter.
- Look for cracks in the casing or blood ports before priming.
- Use high flow and elevated pressure test to check for leaks and flow path abnormalities pre-CPB.
- With clear prime, turn on sweep gas at anticipated flow and with 80% FiO2. Recirculate prime for 60 seconds. Draw clear prime sample for blood gas analysis and test for O2 and CO2 if CO2 flush was used. In line CDI monitor responsiveness could be used in place of a blood gas test. O2 should read 400+ mmHg and CO2 should be 0mmHg. If result is unsatisfactory, repeat the procedure. If still unsatisfactory consider replacing oxygenator before use. (*Increase detectability score to 5 if this clear prime gas test is not used.)
- Use continual monitoring of oxygenator gas exhaust w/ CO2 sensor to confirm adequate sweep gas flow.
- Pre-coat membrane with albumin during priming to prevent excessive platelet adhesion and membrane clogging at initiation of bypass.
- With blood prime visually confirm oxygenation of post-oxygenator blood by color change and blood gas analysis.
- Maintain an activated clotting time of at least 3X baseline to prevent oxygenator clotting.
- Maximize sweep gas FiO2
- Stop any CO2 titration
- Increase sweep gas flow rate.
- Initiate use of emergency O2 from the anesthesia machine or an O2 E cylinder before trouble shooting the sweep gas circuit.
- Pressure check sweep gas circuit for leaks at connections, blender, flow meter & along length of tubing.
- Tap on oxygenator to displace condensation that might be plugging the gas channel.
- Maintain or initiate hypothermia if possible.
- Splice in new oxygenator in parallel using a PRONTO line (Parallel Replacement of the Oxygenator that is Not Transferring Oxygen)
- Without a PRONTO line, perform series change out of the oxygenator. (*Increase Harmfulness RPN to 5.)
- Risk Priority Number (RPN): (select the number from each category that you feel best categorizes the risk).
A. Severity (Harmfulness) Rating Scale: how detrimental can the failure be:
1) Slight, 2) Low, 3) Moderate, 4)
High, 5) Critical
(The harmfulness that this failure causes is 3) moderate if a PRONTO line is used. The harmfulness should be increased to 5 if no PRONTO line is used.)
B. Occurrence Rating Scale: how frequently does the failure occur:
1) Remote, 2) Low, 3) Moderate, 4) Frequent, 5) Very High
(Occurrence is usually 2) low.)
C. Detection Rating Scale: how easily the potential failure can be detected before it occurs:
1) Very High, 2) High, 3) Moderate, 4) Low, 5) Uncertain
(If a clear prime gas test is used, a possible oxygenator gas exchange failure may become apparent before use, giving it a 3 for moderate detectability. Without the test, this problem may not become obvious until late in CPB. So the ability to detect it is 5)
D. Patient Frequency Scale:
1) Only a small number of patients would be susceptible to this failure, 2) Many patients but not all would be susceptible to this failure, 3) All patients would be susceptible to this failure.
(This could happen to any patient. So the Patient Frequency RPN should be a 3.)
Multiply A*B*C*D = RPN. The higher the RPN the more dangerous the Failure Mode.
The lowest risk would be 1*1*1*1* = 1. The highest risk would be 5*5*5*3 = 375. RPNs allow the perfusionist to prioritize the risk. Resources should be used to reduce the RPNs of higher risk failures first, if possible.
(The total RPN for this failure is 5*2*5*3 = 150 if the PRONTO line and clear prime gas test are not used. If a PRONTO line and clear prime gas test are used the score would be lower: 3*2*3*3 = 54 RPN.)