perfusion of obese patients

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    Ya Wen ZhangYa Wen Zhang

    Good morning perfusion community,

    My name is Yawen Zhang and I am a perfusion student from UNMC. I would like to get your opinion on the topic of managing obese patients on CPB.

    Background information:
    According to WHO, body size is defined by BMI calculated based on Quetelet’s formula (weight/height2). Accordingly, obesity is defined as BMI >30 kg/m2.

    According to the National Institute of Health, 1/3 of the US population is obese, and 6% of the population is extremely obese (BMI> 40 kg/m2). In our everyday adult perfusion practice, we are encountering more obese patients. Studies have found that obesity in cardiac surgerical is an independent predictor of in-hospital morbidity & mortality, therefore, this population may deserve special attention from a CPB perspective.

    Below are some of the perfusion practice questions that myself and my fellow colleagues have specifically on CPB management in obese patients. We would appreciate if you could help us to investigate the answers to these questions based on your institutional protocols and experiences.

    1.Does your perfusion department have different perfusion protocol for patients based on different body sizes? If so, what is it for obese patients and how to you define obese patients?

    2.Is circuit modification warranted obese patients from a safety perspective?

    3.How do you calculate pump flow required for the metabolic needs of obese patients on CPB?

    4.How do you dose heparin & protamine obese patient population?

    Yawen Zhang
    Candidate in the MPS program at UNMC

    Kayleigh KrajcikKayleigh Krajcik

    Obese patients should get some room and think about what are they doing to their life. Most of the time it’s not the custom writings but hate fat content that causes health issues for them. This brings their survival index low.

    Amber LickertAmber Lickert

    Hi Yawen,

    Great topic! Here is some information in regards to your questions 2 and 4.

    2. Is circuit modification warranted for obese patients from a safety perspective?

    Longsky et al. (2005) has outlined three major concerns of CPB circuit design to be used on obese patients. Specifically, one should be concerned about the oxygenator’s oxygenation capacity, the high transmembrane pressure that would potentially compromise circuit integrity, and the heat exchanger capability. In this case report, the perfusion team elected to place 2 oxygenators in parallel for a type A dissection repair on a 43 year old male whose BSA= 2.76 m2. The distribution of the body weight was not specifically mentioned, however, the patient is 197cm and 142 kg. In this case, the oxygenator used had a recommended flow rate of 6 LPM with an average oxygen transfer capacity of 392 mL/min. The team elected to place two oxygenators in parallel in anticipation of the inability of one oxygenator meeting the oxygen demand of the patient, but the team did not have a protocol for this decision. In addition, with the type of case and anticipating deep hypothermic circulatory arrest, it was thought to be an appropriate decision placing two oxygenators in parallel for the purpose of heating and cooling the patient efficiently. The oxygenators in parallel were used immediately upon initiation of CPB. Another design would be to initiate CPB with 1 oxygenator with the other oxygenator in-line in parallel position, and only utilize it when indicated. In their discussion, the authors have recommended placing two arterial filters and joining the oxygenators distal to the arterial filters, presumably for the purpose of reducing system’s line pressures. The case study did not have a systematic method or criteria for cutting in a second oxygenator; it was decided as no one has done a case using the current circuitry on such a large BSA patients. Exposing patient to a second oxygenator is not in itself without risks. The more foreign surface area exposed to the patient, the larger overall systemic inflammatory response that will be generated.

    On the other hand, other studies have shown that even a MECC circuit can be used on obese patients. Sartinia et al. (2012) successfully used a MECC circuit on a 40 year old male with a BSA = 2.46m2 with a small oxygenator consisting of membrane surface area of 1.35m2. Only mild hypothermia was utilized during CPB. Instead of targeting a calculated flow based on BSA and CI as the case described by Longsky et al. (2005), Sartinia and their team calculated the oxygen delivery index (DO2i) and ensured the delivery of sufficient flow to keep DO2i> 272mL/min/m2, as described to be the critical value of DO2i (Ranucci). In addition, the team measured eCO2 and calculated VCO2, and kept DO2/VCO2 ratio>5. Moreover, lactate level was monitored to assess the match between O2 delivery and O2 demand. These 3 parameters are monitored and the flow rate was adjusted accordingly without targeting a specific fixed flow number. This is known as goal-directed perfusion. For precaution, the team also modified the circuitry and cut in a venous drainage bag so that one would be able to use to switch from MECC to a conventional, closed siphon drainage system if necessary.

    Blessings et al. (2017) had devised an algorithm for choosing the right size of circuit based on lean body mass (LBM) and combining this with goal directed perfusion. First patient’s BMI is calculated, if the BMI > 30 kg/m2, the patient would be classified as obese and this would initiate the algorithm. Based on ideal body weight adjusted for BMI=25, lean BSA is then calculated. The lean BSA is then used to determine flow at cardiac index of 2.4L/m2. Based on this lean BSA targeted flow, the appropriate circuit size is selected based on surface area of oxygenator, reservoir size and size of AV loop.

    In addition to the consideration of oxygenation, fat microemboli from obese patients can frequently be associated with neurologic complications. Kaza et al. (2002) had looked at inserting an additional 21 micron filter distal to the cardiotomy reservoir. It was found that the additional 21 micron filter nearly completely eliminated all the small and large fat emboli. This is also something the perfusionist can adapt to their own circuit when managing an obese patient on CPB.

    In conclusion, there is no research evidence on the universal circuit modification when managing obese patients on CPB. The practice at its current state relies on anecdotal evidence, case reports and center-specific experience.

    4. How do you dose heparin & protamine in the obese patient population?

    Several researchers have shown the safe use of lean body mass calculating loading heparin dose in patient’s undergoing CPB (Baker, et al., 2005; Haas, et al., 2016). Baker, et al. (2005), compared two groups of patients of BMI> 27; one receiving 300IU/kg of heparin of total body weight, and the other group receiving 300IU/kg of lean body weight. Protamine was administered at 1mg/100IU of heparin administered. It was found that there was on average 25% of dose reduction in both heparin and protamine administration in the lean body weight group. However, in this study, only ACT was monitored. In contrast, Haas et al. (2016) monitored both ACT and plasma heparin concentration. Haas et al. compared the same bolus calculation of heparin based on total body weight between obese and non-obese groups. It was found that the obese group had a significantly higher plasma heparin concentration. It was concluded that based on total body weight, they overdosed the obese group on heparin. The recommendation for heparin management during CPB for BMI>30, loading dose is 340 IU/kg of ideal body weight while targeting plasma heparin concentration for 4.5 IU/mL.

    Vienne et al. (2018) conducted a randomized controlled trial on patients whose BMI >30. One group received 300IU/kg of heparin based on total body weight, and the second group received 340IU/kg of heparin based on ideal body weight. During CPB, target ACT> 400 and heparin concentration of 4.5 IU/mL was maintained. A poor correlation was found between ACT and heparin concentration, the ACT lagged behind the rising plasma heparin concentration. In conclusion, it was determined that overdose of heparin cannot be assessed by ACT alone.

    Baker, M., et al. (2005). Can lean body mass be used to reduce the dose of heparin and protamine for
    obese patients undergoing cardiopulmonary bypass? The Journal of the American Society of Extra
    Corporeal Technology. 37:153-156.
    Blessings, J. & Riley, J (2017). Lean flow: optimizing cardiopulmonary bypass equipment and flow for
    obese patients – a technique article. The Journal of Extracorporeal Technology. 49: 30-35.
    Haas, E., et al. (2016). Identifying optimal heparin management during cardiopulmonary bypass in
    obese patients, a prospective observational comparative study. European Journal of Anaethesiology.
    Kaza, A., et al. (2003). Elimination of fat microemboli during cardiopulmonary bypass. The Annals of
    Thoracic Surgery. 75:555-559.
    Longsky, V., et al. (2005). Use of two parallel oxygenators in a very large patient (2.76m2) for an acute
    “A” dissecting aortic aneurysm repair. ACTA MEDICA. 48(2):95-98.
    Sartini, P, Winfield, A. & Bizzarri, F. (2012). The successful introduction of an adapted form of the mini
    extra corporeal circulation used for cardiac surgery in an obese patient. Journal of Cardiothoracic
    Surgery. 7:20.
    Vienne, M., et al. (2018). Adjusted calculation model of heparin management during cardiopulmonary
    bypass in obese patients: a randomized controlled trial. European Journal of Anaethesiology. 35: 613

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