Literature Review of Large Patients Undergoing Cardio­ pulmonary Bypass: Concerns, Management and Future Considerations

C. Hamilton1, B. Engelhardt2, F. Weinbrenner2, D. Marin2

1 HPS Consulting
2Cardiac Surgery Department, Schoen Clinic Vogtareuth, Germany


KARDIOTECHNIK 2020; 029(4):124-130
doi: 10.47624/kt.029.124


Abstract des Artikels

PDF des Artikels


Schlüsselworte
Kardiopulmonaler Bypass, Membranoxygenator, Sauerstoffverbrauch, Adipositas, parallele Oxygenatoren

Abstract
Patients with large body surface areas (BSA) and excessive body mass indices often arrive in the operating theatre on short notice and a prompt decision has to be made in preparation for cardiopulmonary bypass (CPB), which may include the addition of an extra oxygenator, along with blood volume management and optimizing cannulation.

The goal of this review was to analyze the publications on large patients requiring CPB in terms of the type of perfusion systems, concerns regarding management and problems encountered. In the literature search, a total of eight publications were found in which seven were in the form of case reports and one was part of an original paper. Also, the last case report will entail an extensive description on perfusion management.

All publications describe patients with BSA values ranging from 2.68 m2 to 3.2 m2. The main concerns were the capability of the oxygenator to adequately oxygenate the patient of such a large size and the adequacy of venous drainage given the higher blood flows. Excessive blood volumes were managed with either an extra reservoir or blood bags to remove volume. Heat exchanger efficiency was mentioned but did not seem to be problematic in any case. Some difficulties reaching full flows were due to high arterial line pressures but the main problems encountered were blood flow limitations due to poor venous drainage.

Keywords
Cardiopulmonary bypass, membrane oxygenator, oxygen consumption, obesity, parallel oxygenators

Zusammenfassung
Patienten mit einer großen Körperoberfläche (KÖF) und einem hohen Body-Mass-Index (BMI) müssen häufig kurzfristig operiert werden. Als Vorbereitung für einen kardiopulmonalen Bypass (CPB) müssen dann unverzüglich Entscheidungen getroffen werden, unter anderem bezüglich der Hinzunahme eines zusätzlichen Oxygenators sowie des Blutvolumen-Managements und optimaler Kanülierung.

Das Ziel dieses Literaturreview war es, die Publikationen zu großen Patienten, die einen CPB benötigen, im Hinblick auf Perfusionssysteme und Bedenken hinsichtlich der Behandlung und auftretender Probleme zu analysieren. Bei der Literatursuche wurden insgesamt 8 Publikationen gefunden, von denen 7 Fallberichte waren und eine Teil einer größeren wissenschaftlichen Arbeit. Der letzte Fallbericht enthält außerdem eine umfassende Beschreibung des Perfusionsmanagements.

Alle Publikationen beschreiben Patienten mit KÖF-Werten zwischen 2,68 m2 und
3,2 m2. Die größten Bedenken betrafen die Fähigkeit des Oxygenators, den so großen Patienten adäquat mit Sauerstoff zu versorgen und angesichts des stärkeren Blutflusses einen angemessenen venösen Rückfluss zu gewährleisten. Bei überhöhtem Blutvolumen wurden entweder zusätzliche Behälter oder Blutbeutel eingesetzt, um das Volumen zurückzutransportieren. Die Effizienz von Wärmetauschern wird erwähnt, scheint aber in keinem Fall problematisch gewesen zu sein. Manche Schwierigkeiten, den vollen Fluss zu erreichen, werden mit hohem arteriellem Blutdruck begründet, doch die größten Probleme betrafen Einschränkungen des Blutflusses aufgrund von unzureichendem venösem Rückfluss.

Introduction

It is well known that CPB in morbidly obese patients is associated with a significantly increased risk of postoperative morbidity and mortality. The increase in total blood volume, cardiac output, oxygen consumption (VO2), and arterial pressure is a result of the metabolic demands of the excess adipose tissue [1]. The majority of adult patients who approach larger than normal BSA values are obese. Body Mass Index (BMI) is a guide to classify obesity in adults and is very easy to calculate as the formula uses only weight and height (Appendix 1). Because of the simplicity of calculating BMI, the degree of adipose tissue may be overestimated or in some cases underestimated depending on age, physical statue, activity levels and gender. The Classification of obesity according to BMI are shown in table one.

Case Reports
The following case reports pertain specifically to the information provided by the author merely with regard to perfusion issues and systems. They are divided into reports that used parallel oxygenators and those that used single oxygenators.

Summary of case reports describing the use of two oxygenators in parallel [2, 3, 4, 5]
1989, Cleland A, et al, described the use of parallel oxygenators (Bentley BCM-7) in a 27-year-old male (183 cm, 141 kg, 2.68 m2, BMI 42 kg/m2) who underwent normothermic arrythmia surgery. The decision to use parallel oxygenators was based on the increased oxygen demands (former pro-football player) and the ability of the oxygenator to adequately transfer enough oxygen. Also, the technique for this particular surgery required lifting the heart and they anticipated a poor venous return which would further reduce the oxygen saturations. Heparin: 40,000 IU (284 IU/kg) with a resultant ACT of 632 sec. Cannulation: Aorta 24 Fr. and bicaval venous SVC 30 Fr., IVC 32 Fr. Conduct of CPB: The calculated blood flow using 2.4 mlO2/min/m2 cardiac index (CI) was 6.7 l/min. Prime volume was 3500 ml. Within 20 minutes on CPB, upon lifting of the heart, reduction of blood flow as a result of poor venous return caused the venous saturation (SvO2) to decrease, which increased the demands on the oxygenator reducing the arterial saturation (SaO2) to 90%. The incorporation of a second oxygenator successfully improved SaO2 to 98%. The mean blood flows averaged 5.5 l/min.

Table 1: Classification of BMI (WHO international, 2000)


1993, Hamilton C, described the use of parallel oxygenators (Cobe CML) in a 33-year-old male (189 cm, 159 kg, 2.88 m2, BMI 45 kg/m2) who underwent an emergency Bentall procedure under normothermia for repair of an acute dissecting Type A ascending aortic aneurysm following a motorcycle accident. The calculated blood flow, using a 2.4 CI was 6.9 l/min and the theoretical VO2 was calculated as 375 ml/min (130mlO2/min/m2). Since the oxygen transfer (O2T) of the CML oxygenator was rated at 400 mlO2/min at a maximum blood flow of 8 l/min, to deal with the possibility of inadequate oxygenation the decision was made to use a second oxygenator in parallel. Heparin: 45,000 IU (284 IU/kg) with a resultant ACT of 467 sec. Cannulation: Femoral artery: 22 Fr. and right atrial 51 Fr. two-stage venous. Conduct of CPB: The total prime volume was 2800 ml. Within three minutes on CPB, the SvO2 dropped to 38% and the second membrane was opened resulting in the recovering of the SvO2 to 60%. Although the blood flows were adequate, higher flows of over 6.4 l/min could not be achieved due to limitations of venous return and high line pressures inherent to femoral cannulation. The average Hemoglobin (Hgb) was 90 gms/l and the indexed VO2 (VO2i) ranged from 90 to 103 mlO2/min/m².

2001, Gygax E, et al, described the use of parallel oxygenators (Medtronic Affinity) in a 34-year-old male patient (190 kg) for the repair of an acute Type B aortic dis­section using deep hypothermia circulatory arrest (DHCA). Concerns were the potential difficulties with perfusion and oxygenation. Conduct of CPB: The perfusion was unproblematic during the entire operation with a flow of 6 l/min. The lowest SvO2 was 62%. The use of parallel oxygenators allowed an adequate oxygen supply during all phases of the intervention and furthermore had the beneficial effect of accelerating the estimated rewarming time.

2005, Lonsky V, et al, described the use of parallel oxygenators (Oxim II-34: ­Edwards Lifesciences) in a 43-year-old male (197 cm,142 kg, BSA 2.76 m2, BMI 37 kg/m2) who underwent a Bentall procedure for an acute Type A dissecting aortic aneurysm repair under hypothermic temperatures of 26°C. Cannulation: Femoral artery: 22 Fr. and right atrial 51 Fr. two-stage. Conduct of CPB: The calculated blood flow was 6.63 l/min. The first concern was the capability of the oxygenator to adequately oxygenate such a large patient, the second was the transmembrane pressure gradient and the third was the oxygenator heat-exchanger efficiency given the high calculated perfusion flow rates. The highest oxygenator gas flow was 2.6 l/min with a maximum FiO2 of 0.42. The case was uneventful concluding that the use of two parallel oxygenators was very effective, easy and safe.

Summary of case reports describing the use of a single oxygenator [6, 7, 8, 9].

1984, Hill A, et al, described the use of a single oxygenator (5.4 m2 Terumo Capiox) in a 59-year-old male (201 cm, 165 kg, 2.99 m2, BMI 41 kg/m2) who underwent coronary artery bypass surgery. The perfusion set-up included parallel oxygenators as a safety aspect although it was not used. Their concern was oxygenation and blood flow requirements since the calculated blood flow using a CI of 2.5 l/min/m2 was 7.5 l/min. Their perfusion system consisted of a ½“ arterial and ½“ venous line. Prime volume totaled 3500 ml. Cannulation: Aorta 24 Fr. and bicaval venous SVC 42 Fr., IVC 42 Fr. Heparin: 52,000 IU (340 IU/kg). Conduct of CPB: After 3 minutes, the bypass surgery was terminated due to inadequate venous drainage. The IVC cannula was removed and replaced with a 52 Fr. 2-stage venous cannula positioned in the right atrium. Venous return was much improved but a LV vent was later inserted due to distension of the heart with cooling to 30°C. Blood flows mentioned ranged from 6.5 up to 9.5 l/min. Rewarming from 30 to 35°C took 15 min at flows of 6.8 l/min. Venous return was the greatest problem and they suggested for later use a single Terumo Capiox employing two separate ½“ venous lines.

An added addendum described the next patient (196 cm, 156 kg, 2.82 m2, BMI 41 kg/m2) in which they used a single 5.4 m2 Terumo and employed two separate ½“ venous lines with excellent venous return.

2008, Molnar J, et al, described the use of a single oxygenator (Cobe CML Duo) in a 19-year-old male (203 cm, 187 kg, 3.2 m2, BMI 45 kg/m2) with Marfan’s syndrome who underwent an arch replacement under DHCA. Their main concern was to accommodate large circulating volumes and permit high CPB flow rates. Blood flows of 9.2 l/min were calculated using a 2.4 CI plus a 20% margin for safety.

The blood volume of 13 liters was calculated using 70 ml/kg. Total prime volume: 2000 ml. An extra venous reservoir was added along with two 1000 ml infusion bags attached to the venous and arterial recirculation lines. A centrifugal pump was used to minimize potential higher hemolysis. Heparin: 56,000 IU (300 IU/kg). Cannulation: Aorta was cannulated directly with the 3/8″ connector of the 3/8″ arterial line. (24 Fr. was deemed too small). Bicaval venous cannulation: SVC 34 Fr. single-stage, IVC 36/46 Fr. two-stage. Conduct of CPB: There was no further mention of the actual blood volume nor any CPB parameters.

2014, Hunter K, et al, described the use of a single oxygenator (Trillium coated Affinity NT) in a 49-year-old male (172 m, 180 kg, BSA 2.73 m2, BMI 61 kg/m2) who underwent an emergent aortic valve replacement secondary to infective endocarditis under mild hypothermia of 34°C. Thought was given to use of parallel oxygenators but with emergent arrival of the patient and the necessity to commence CPB quickly, a single oxygenator was selected. Heparin: 54,000 IU (300 IU/kg). Cannulation: Aorta 24 Fr. and right atrial 36/46 Fr. venous cannula. Conduct of CPB: Immediately after initiation of CPB, vacuum-assisted venous drainage (VAVD) was used to decompress the right-side of the heart. However, decompression of the heart was achieved only after a 16 Fr. pulmonary artery cannula was inserted. The author also provided valuable perfusion data in tabular form, allowing the calculation of DaO2 and VO2. With the FiO2 set
at 1 throughout CPB, the paO2 ranged from 335-521 mmHg, the VO2 ranged from 173-318 mlO2/,min and the DaO2 ranged from 288-753 mlO2/min/m2.

2016, Hamilton C, et al, described in a previous publication, the use of a single oxygenator (Medtronic Fusion) in a 48-year-old male (179 cm, 170 kg, BSA 2.74 m2, BMI 53 kg/m2) who underwent a Bentall procedure at moderate hypothermia to 34°C after being diagnosed with an aortic aneurysm as a result of many years of hypertension [9]. Since the Medtronic Fusion O2T was validated up to 419 mlO2/min and the maximum predicted VO2 of the patient was 356 mlO2/min, (using a VO2i of 130 mlO2/min/m2), the use of a single oxygenator was justified.

A summary of the case reports is given in table 2.

Management of CPB
Oxygenator performance

One of the main issues are the limits of the oxygenator performance that must be sufficient to adequately oxygenate the patient. Along with maximum rated blood flow, one must know the maximum O2T of the oxygenator and the estimated patient oxygen requirements in terms of VO2.

Expected VO2
Even though the VO2i varies during CPB, mean values of a range may be used as a preparation pre-CPB. Ranges of VO2i values are given in the literature for hypothermic and normothermic temperatures. For adult patients, the theoretical VO2i at 37°C most commonly used is 100 mlO2/min/m2. At 34°C, the metabolism decreases by approximately 20% and 80 mlO2/min/m2 is used to calculate the theoretical VO2 [10].

Higher levels of VO2i ranging from 130 to 150 mlO2/min/m2 should also be calculated when the limits of an oxygenator comes into question, such as in the extremely large patients or in patients that one anticipates a higher metabolic rate, such as young age, high body muscle mass, and general conditions such as sepsis [11]. This does not mean that patients will reach these values, but they serve as a useful guideline and are essential to aid in clinical judgement decisions such as the use of parallel oxygenators.

Venous return
The major concerns and difficulties regarding large patients was the adequacy of venous return. Therefore, considerations should be made for the largest possible venous cannulae and the possible use of VAVD. The dynamics of optimal venous return depends on several factors. The type, size and position of the venous cannula, the size and length of the venous line, the distance between the level of the heart and the level in the cardiotomy venous reservoir (CVR) (hydrostatic pressure) and the type of cardiotomy venous reservoir. The CVR plays an important factor in the resistance to venous return not only in the distance that the CVR is placed in relation to the patient and the level of blood in the reservoir but also consideration should be made to the configuration and structure of the CVR. Innovations implemented in some CVRs have led to a significant reduction in resistance to venous flow so that there has been a marked improvement in venous return without the addition of VAVD [12].

Calculations for blood flow
Two methods can be used to calculate the required blood flow.

First: The conventional CI of 2.4 l/m/m2 often termed the “full flow”. This simply gives a “perfusion flow” that does not take into account the hemoglobin level.

Second: Lessons learned from Goal Directed Perfusion (GDP) confirm that blood flow should be guided according to the Hgb level to meet a “minimum” Delivery of Oxygen index (DaO2i) of 272 mlO2/min/m² in adults [13]. The DaO2i is calculated by multiplying the CI by the arterial oxygen content (CaO2) Appendix 2. The short form formula: DaO2i=Hgb*1.36*CI can be used to calculate the CI for each level of Hgb (or vice-versa) given a DaO2i of 272 mlO2/min/m2. For example: rearranging the formula using a Hgb of 100 gms/l, the CI would be 2 l/min/m2.

DaO2i/(1.36*Hgb) = CI (272/1.36*100 = 2)

Table 3 shows the Hgb for each CI to attain the minimum DaO2i of 272 mlO2/min/m2. (Note that a CI of 2.4 l/min/m2 would be the minimum flow required when the hemoglobin is 83 gms/l).

Blood volume
Unfortunately, there is no easy way to measure blood volume. The mean value for blood volume of normal weight adults is around 70 ml/kg. Although it is documented that blood volume decreases with increasing weight and can be as low as 50 ml/kg, the absolute blood volume is higher [14]. Therefore, a good approximation of blood volume may be calculated using a range from 50 to 70 ml/kg pre-CPB. Once on CPB, the blood volume can also be estimated by using the dilutional hematocrit formula [15] (Appendix 3).


Table 2: Summary of case reports
Table 3: CI at varying levels of Hgb to reach a DaO2i of 272 mlO2/min/m2



Detailed case report [9]

Patient
A 48-year-old male (179 cm, 170 kg, BSA 2.74 m2, BMI 53 kg/m2) diagnosed with an aortic aneurysm as a result of many years of hypertension underwent a Bentall procedure at moderate hypothermia to 34°C.

CPB systemOxygenator: Medtronic Fusion (integrated depth filter; prime volume 260 ml, rated blood flow up to 7 l/min, maximum O2T 419 mlO2/min) [9, 16]. An open system CVR; Stockert S5 heart-lung machine (HLM) with an external (mast) arterial pump; Stockert Data Management System (DMS) for electronic charting; Blood cardioplegia with infusion pump (Fresenius)
for infusion of Potassium/Magnesium; Spectrum Medical M2 arterial/venous saturation monitor. The arterial line was 3/8″ and the venous line ½“. The total priming volume was 1200 ml (700 ml Ringers Lactate, 500 ml Volulyte, 10,000 units of heparin). Extra additions: To prepare for higher blood volumes, two 1000ml infusion bags were connected to a stopcock on the arterial line.

Pre-CPB calculation of blood flow
Since this patient’s Hgb pre-CPB was 132 gms/l, one could safely say that the Hgb would be ≥ 100 gms/l on CPB. Therefore, using a CI of 2 l/min/m2 as a starting point, the minimum blood flow calculated was 5.5 l/min. As a reference point, a CI of 2.4 l/min/m2 was calculated to be 6.6 l/min.

Pre-CPB calculation of VO2
Multiplying the BSA of 2.74 m2, by the VO2i of 80, 100 and 130 mlO2/min/m2, resulted in VO2 values of 219, 274 and 356 mlO2/min/m2 respectively (Graph 1).

Predicting FiO2
As part of our protocol, the values of the estimated VO2 may also be used to predict the FiO2 required to attain a paO2 of 150 mmHg [9]. (Appendix 4)

Using the formula: Predicted FiO2 = (VO2+43)/4.62), the predicted FiO2 would be 0.57, 0.69 and 0.86 (Graph 1).

Pre-CPB calculation of blood volume
The estimated blood volume for this patient using a range from 50–70 ml/kg would be in the order of 8500 to 11,900.

Lean Body Weight adjusted-BSA
Using a BMI of 25 kg/m2, the LBW would be 80 kg and the LBW adjusted-BSA would be 2 m2. Making all the new calculations for blood flow:

a) 2.4 CI=4.9 l/min and
b) 2 CI=4 l/min.

This would dramatically reduce the necessary blood flows for CPB and is therefore not used as part of our protocol.

Heparin (Ratiopharm)
was administered using 350 ml/kg (60,000 I.U.) resulting in a pre-CPB ACT of 900 sec.

Table 4: Patient parameters during CPB including the calculations of the oxygenation indices


















Graph 1: Pre-CPB protocol used as a guideline to estimate the required FiO2 for the predicted
VO2of the patient (Medtronic Fusion Membrane Oxygenator)



Cannulation
The aorta was cannulated using a 24 Fr. arterial cannula and the right atrium was cannulated with a 36/48 Fr. two-stage venous cannula.

Initiation of CPB
A constant blood flow maintained at
5 l/min, resulted in a MAP 70 mmHg, CVR
2 mmHg and SvO2 75%. The surgeon confirmed that the heart was empty and cooling to 34°C was continued. The blood volume was excessive so that 2000 ml of blood was immediately removed, leaving 2700 ml in the CVR. The FiO2 on CPB was set at 0.65, where it remained throughout the case, even for rewarming. While on CPB, four arterial and venous blood gas readings were taken at 15, 60, 90 and 110 minutes (table 4). 

Total CPB time was 141 minutes, cross-clamp time 125 minutes and 12 minutes of reperfusion time. The patient was extubated the same day and sent to the step-down unit the next day.

As shown, the VO2 values correspond to the estimated pre-CPB values and the DaO2i was above 272 mlO2/min/m2.

The VO2 (y-axis) was calculated to be 219 mlO2/min, 274 mlO2/min and 356 mlO2/min with the respective FiO2s (x-axis) of 0.57, 0.69, and 0.86 to attain a
paO2 of 150 mmHg. Added to the graph 1 are the actual results of the VO2 (219 to 295 mlO2/min) during CPB.

Indication of adequate blood flows
All electrolytes, including lactate, were within the normal range. The paCO2 values ranged from 37 to 40 mmHg (mean 39) with gas flows of 4 l/min. Because both arterial and venous blood gases were taken, the calculated paCO2 to pvCO2 difference were in the normal range (4 to 7 mmHg) indicating sufficient blood flows [17].

With blood flows ranging from 4.9 to 5.6 l/min, the DaO2i was more than adequately met since the Hgb remained around 120 gms/l. The SvO2s were within the normal range for the duration of CPB, so in this respect, it was not necessary to have excessive blood flows as high as 6.6 l/min (Graph 2).

Blood volume
The first blood gas on CPB revealed an HCT of 35%. This was an absolute drop of 4% compared to the pre-CPB value of 39%. The estimated blood volume was calculated to be approximately 10,500 liters (62 ml/kg).

Venous return
The modifications made to the Medtronic Fusion CVR (curved venous inlet, inner venous tube widens from ½“ to 5/8″, venous filter separate from the suction filter) has a low resistance to venous blood flow and therefore the flow dynamics are significantly improved, allowing the use of 3/8″ venous lines up to 100 kg [12]. Therefore, in this case using a ½“ venous line, gravity drainage was used throughout the whole procedure as there were no problems encountered with venous return regardless of blood flow.

Heat exchanger efficiency
Although we did not measure the heat exchanger efficiency, the patient was rewarmed from 34 ºC to 36.2 ºC within 20 minutes. Therefore, the heat exchanger was not a limitation.

Future considerations

BSA Calculation and Lean Body Weight (LBW)

The BSA is the most commonly used measure for determination of perfusion parameters such as systemic blood flow and calculation of oxygen indices for CPB. For many clinical purposes, the BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass [18]. There has ­been, and still is, much controversy on using LBW adjusted-BSA to “downsize” obese patients to allow for lower blood flows [19, 20]. Some centers may use this to some extent but there is not enough clinical evidence and the usefulness or efficacy is less well established [21].

Graph 2 shows the blood flows during CPB, in reference to the DaO2i and to a CI of 2.4



One area where LBW seems to be proven clinically is the regimen based on administering drugs such as heparin where the dosage is given according to weight (kg). The current LBW regimen of heparin administration may be used efficiently in obese CPB patients, thereby avoiding overdose which cannot be accurately assessed by ACT monitoring alone [22].

Evaluation of blood flow
Even if the minimum DaO2i of 272 mlO2/min/m2 is met or even if the blood flows are set at 2.4 CI, perfusion flows may not be adequate and should be assessed utilizing not only SvO2 but additional blood parameters such as pvCO2, lactate, and acid-base status.

Sampling both arterial and venous blood samples simultaneously can be used to measure the pCO2 gradient (p(v-a)CO2) which should not be greater than 7 mmHg [17]. A higher than normal pvCO2 is indicative of poor blood flow and is also followed by ensuing acidosis. Along with the development of acidosis, rising lactates are indicative of altered perfusion.

Another method that shows real promise but is not discussed in this paper is the use of VCO2 in which the CO2 elimination rate may be used for an indirect assessment of the metabolic state of the patient [23].

Oxygenator performance
Each oxygenator must meet certain minimum standards as recommended by the American Association of Medical Instrumentation (AAMI). “A standard or recommended practice is an important reference in responsible decision-making, but it should never replace responsible decision-making” [24]. The manufacturer, by law, includes with each oxygenator, their IFUs in which the maximum blood flow and maximum O2T are specified. Some adult oxygenators may show the O2T graphs that go up to 8 l/min, but in the IFUs, it may be stated that the maximum blood flow should not exceed 7 l/min. Professional experience and judgement play an important role and it is important to have the tools to test the results in a clinical setting such as that validated for the Medtronic Fusion [9].

Conclusion
In comparison to over 20 years ago, there have been significant developments which have certainly optimized perfusion systems. Modifications of perfusion systems in the form of external pumps, oxygenators with integrated filters, and MiECC circuits all result in the reduction of prime volume and hemodilution. With less hemodilution and higher hemoglobins, DaO2 levels are optimized and extreme blood flows do not have to compensate for low hemoglobin ­levels.

Also, the improvement in the cardiotomy reservoir dynamics lowers the resistance to venous drainage reducing potential complications of poor venous return.

In the few reported cases, or on profes­sional forums, O2T in reference to the oxygenator and oxygen consumption according to patient requirements are rarely mentioned.

Future publications following a general format to share information and address these issues will support professional judgement and optimize treatment of this patient group.

It seems generally acceptable to use one oxygenator for adult patients up to approximately 2.9 m2 depending on the metabolic status of the patient and of course the oxygenator specifications.

If at all in doubt, it is good practice to setup parallel oxygenators and keep it clamped out until required or cut in two Y-connectors so that a second oxygenator can be easily set up.

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Conflicts of interest

The authors assert that no conflicts of interest exist.

Appendix 1:
Body Mass Index formula:

BMI (kg/m2) = weight (kg) / Height (m*m)
Where kg is kilograms and m is meters.

Appendix 2:
CaO2: CaO2 mlO2/L = Hgb*1.36*SaO2

Delivery of Oxygen Index formula:
DaO2i = Hgb(gms/l) * 1.36(mlO2/gmHgb) * CI (l/min/m2) * SaO2 + paO2 * 0.03(mlO₂/mmHg/l) * CI

Where DaO2i is the delivery of oxygen index in mlO2/min/m2, Hgb is the hemoglobin in gms/l, the Hüfner factor is 1.36 mlO2/gmHgb, CI is the cardiac index in l/min/m2, the SaO2 is the arterial saturation as a fraction, and paO2 is the pressure of oxygen in mmHg.

For ease of calculation, the modified short form may be used during CPB: DaO2i = (Hgb*1.36* CI).

*Since this short form does not include the pO2s, the SaO2 is considered to be 1.

Appendix 3:
Dilutional hematocrit formula

TBV = dilutional HCT*(Volume preCPB+ Prime volume)/(drop in HCT)

TBV is Total Blood Volume in ml, dilutional HCT is the first hematocrit taken on CPB, volume pre-CPB is the volume given and volume lost pre-CPB, plus the prime volume in ml, and the drop in hematocrit is the difference between the HCT pre-CPB and the HCT on CPB.

Appendix 4:
Calculating cFiO2:150 mmHg:

Step 1) Convert PaO2 into a fraction (FaO2) by ­taking the PaO2 and dividing this by the ­barometric pressure (Pb) minus the water vapor pressure of 47 mmHg (713).

*Pb = Barometric pressure (taken as 760 mmHg).

PaO2 /(Pb–PH2O)

Step 2) The FiO2 minus the FaO2 gives the anoxic fractional difference (AFD). The AFD is the FiO2 at which the PaO2 is zero.

FiO2-FaO2 =AFD

Step 3) Add 0.21 to the AFD to obtain the cFiO2

Final formula: cFiO2:150 mmHg = FiO₂-(PaO2 /(713)+0.21

*The cFiO2 is referred to as the “cFiO2:150mmHg” so the standard of using a specific PaO2 is clearly stated.