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http://prf.sagepub.com/content/early/2014/03/04/0267659114525986The online version of this article can be found at:

 DOI: 10.1177/0267659114525986

published online 4 March 2014PerfusionM Park, ELV Costa, AT Maciel, EVS Barbosa, AS Hirota, GdeP Schettino and LCP Azevedo

artificial lungEffect of flow rate and temperature on transmembrane blood pressure drop in an extracorporeal

  

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Perfusion 1 –9

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Introduction

The use of extracorporeal membrane oxygenation (ECMO) has been encouraged in recent years to support acute respiratory distress syndrome (ARDS) patients with severe respiratory failure.1–5 The modern poly-methylpentene membrane shows a low pressure drop (≤15 mmHg) during transmembrane blood passage, enabling low rates of haemolysis and thrombocytopenia when this high gas-exchange efficiency membrane is used.6

During conventional respiratory ECMO support, blood flow is the primary determinant of oxygen trans-ference and blood flow is inversely proportional to the drop in transmembrane pressure.6 The transmem-brane pressure denotes the resistance to the passage of the blood. Inside the membrane, blood characteristics, such as density and viscosity, and the diameter and

length of the hollow fibres and flow velocity are the theoretical determinants of blood friction and, thus,

Effect of flow rate and temperature on transmembrane blood pressure drop in an extracorporeal artificial lung

M Park,1,2 ELV Costa,1,2 AT Maciel,1,2 EVS Barbosa1,2, AS Hirota,1,2

GdeP Schettino1,2 and LCP Azevedo1,2

AbstractIntroduction: Transmembrane pressure drop reflects the resistance of an artificial lung system to blood transit. Decreased resistance (low transmembrane pressure drop) enhances blood flow through the oxygenator, thereby, enhancing gas exchange efficiency. This study is part of a previous one where we observed the behaviour and the modulation of blood pressure drop during the passage of blood through artificial lung membranes.Methods: Before and after the induction of multi-organ dysfunction, the animals were instrumented and analysed for venous-venous extracorporeal membrane oxygenation, using a pre-defined sequence of blood flows.Results: Blood flow and revolutions per minute (RPM) of the centrifugal pump varied in a linear fashion. At a blood flow of 5.5 L/min, pre- and post-pump blood pressures reached -120 and 450 mmHg, respectively. Transmembrane pressures showed a significant spread, particularly at blood flows above 2 L/min; over the entire range of blood flow rates, there was a positive association of pressure drop with blood flow (0.005 mmHg/mL/minute of blood flow ) and a negative association of pressure drop with temperature (-4.828 mmHg/oCelsius). These associations were similar when blood flows of below and above 2000 mL/minute were examined.Conclusions: During its passage through the extracorporeal system, blood is exposed to pressure variations from -120 to 450 mmHg. At high blood flows (above 2 L/min), the drop in transmembrane pressure becomes unpredictable and highly variable. Over the entire range of blood flows investigated (0 – 5500 mL/min), the drop in transmembrane pressure was positively associated with blood flow and negatively associated with body temperature.

Keywordsacute respiratory distress syndrome; mechanical ventilation; swine; extracorporeal membrane oxygenation and multiple-organ failure

1Research and Education Institute, Sírio Libanês Hospital, São Paulo, Brazil2 Intensive Care Unit, Emergency Department, Hospital das Clínicas, São Paulo, Brazil.

Corresponding author:Marcelo ParkResearch and Education InstituteSírio Libanês HospitalRua Francisco Preto 46São Paulo, SPZIP 05623 - 010Brazil.Email: marcelo.park@hc.fm.usp.br

525986 PRF0010.1177/0267659114525986PerfusionPark et al.research-article2014

Original Paper

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2 Perfusion

the resistance to blood flow and the drop in transmem-brane pressure.7

Using this rationale, we hypothesised that blood flow, haemoglobin, pH, core temperature, central venous pressure, positive end-expiratory pressure (PEEP) and sodium concentration may determine the drop in trans-membrane pressure. To test this hypothesis, in this study, which is a part of a previous one, pre- and post-membrane blood pressures were measured at varying blood flows, generated using a centrifugal pump. A sim-ilar procedure was also performed after the induction of lung injury and sepsis, with the objective of observing the behaviour of blood pressure during the passage of blood through the extracorporeal membrane.

Methods

This study was approved by the Institutional Animal Research Ethics Committee and was performed accord-ing to the National Institutes of Health guidelines for the use of experimental animals. Instrumentation, surgical preparation, pulmonary injury and the induction of sepsis was performed as described previously.8 The

timeline used for procedures in this study is shown in Figure 1.

Instrumentation and surgical preparation

The room temperature was set to 240Celsius. Five domestic female Agroceres pigs (80 kg, range 79,81) were anesthetised using thionembutal (10 mg/kg, Tiopental, Abbott Laboratories, São Paulo, Brazil) and pancuronium bromide (0.1 mg/kg, Pavulon, AKZO Nobel, São Paulo, Brazil) and were connected to a mechanical ventilator (Evita XL Dräger, Dräger, Luebeck, Germany) at the following parameters: tidal volume 8 mL/kg, end-expiratory pressure 5 cmH2O, FIO2 initially set to 100% and subsequently adjusted to maintain the arterial saturation between 94 - 96% and the respiratory rate was titrated to maintain a PaCO2 between 35 and 45 mmHg or end-tidal CO2 (NICO, Dixtal Biomedica Ind. Com, Sao Paulo, Brazil) between 30 and 40 mmHg. The electrocardiograms, heart rates, oxygen saturations and pressures of the animals were monitored using a multi-parametric monitor (Infinity Delta XL, Dräger). Anaesthesia was maintained during

anesthesia

instrumentation stabilization

1 hour

First baseline

30 minutes

Second baseline

ECMO startSweep f low = 0 L/minBlood f low = 1500 mL/min

Sweep f low = 0 L/minBlood f low = 0 to 5500 mL/min in steps of 500 mL/min every minute

11 minutes

Period ofdata collection

Lung lavage CESAR trialsimilar ventilation

Open lungapproach ventilation

6 hours 6 hours

Lung recruitment maneuverand PEEP titration

Pre-multiple organdysfunction baseline

Multiple organdysfunction baseline

New data collection

Peritonitis induction

11 minutes

A

B

Figure 1. Timeline of the study procedures. Grey backgrounds denote the periods of data collection used for this report. A denotes the data collection period without multi-organ dysfunction and B denotes the data collection period after inducing multi-organ dysfunction.

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Park et al. 3

the study with midazolam (1 - 5 mg/kg/h) and fentanyl (5 – 10 mcg/kg/h, Fentanyl; Janssen-Cilag, São Paulo, Brazil) and muscular relaxation was achieved using pancuronium bromide (0.2 mg/kg/h). The adequacy of the depth of anaesthesia during the surgical period was evaluated based on the maintenance of physiological variables (heart rate and arterial pressure), the absence of reflexes (corneal and hind limb flexion response) and the absence of responses to stimuli during manipula-tion. Supplementary boluses of 3 - 5 mcg/kg fentanyl and 0.1 – 0.5 mg/kg midazolam were administered as necessary.

The left external jugular vein was cannulated (guided by ultrasonography) to introduce a pulmonary artery catheter which was used to monitor cardiac output, core temperature, cardiac filling pressures and venous mixed oxygen saturation. The right external jugular vein was punctured to introduce a 25-cm ECMO atrial cannula (Edwards LifeSciences, Irvine, CA, USA). The right femoral vein was punctured to insert a 55-cm ECMO drainage cannula (Edwards LifeSciences) which was positioned close to the right atrium, guided by trans-hepatic ultrasonographic visualisation. The guidewires were kept in place only until after the first baseline measurements after the stabilisation period; the guide-wires were then replaced by cannulas. After the inser-tion of the guidewires, an infusion of 1000 IU per hour of heparin was initiated. A central venous catheter and an invasive arterial blood pressure catheter were placed in the left femoral vein and artery, respectively.

Using a midline laparotomy, a cystostomy was per-formed and a bladder catheter inserted. A 2-cm incision was made into the descending colon and 1.0 g/kg of fae-cal content was removed and stored. The intestinal inci-sion was sutured; two large-bore catheters were inserted into each flank of the animals and the laparotomy was closed. During surgical interventions, 15 mL/kg/h of lactated Ringer’s solution was infused continuously and boluses of 250 mL were used to maintain a systemic mean arterial blood pressure (ABPm) of 65 mmHg or greater, a central venous pressure (CVP) of 8 mmHg or greater and an SvO2 of greater than 65% until the end of instrumentation.

Stabilisation and support of the animals

The animals were allowed to stabilise for 1 h after the end of instrumentation. From the beginning of the sta-bilisation period, a continuous infusion of 3 mL/kg/h of lactated Ringer’s solution was initiated and maintained throughout the entire experiment. After the end of the stabilisation period, the first baseline measurements were collected. Blood glucose was monitored at least hourly. If the blood glucose was lower than 60 mg/dL, 40 mL of 50% glucose was infused.

When the animals became hypotensive (ABPm <65 mmHg), a bolus of 500 mL of lactated Ringer’s was infused. If the ABPm failed to rise above 65 mmHg after the bolus, an infusion of norepinephrine 0.1 mcg/kg/min (Norepine, Opem Pharmaceuticals, São Paulo, Brazil) was initiated and titrated to an ABPm ≥65 and <80 mmHg.

The core temperature of the animals was not con-trolled actively if ≥35 and ≤410Celsius. If the tempera-ture varied from this range, the Quadrox-D membrane system was used to control body temperature using cir-culating water at a predetermined temperature to return the temperatures to within the range cited.

ECMO priming, initiation, and maintenance

The ECMO system (Permanent Life Support System - PLS, Jostra – Quadrox D, Maquet Cardiopulmonary, Hirrlingen, Germany) was primed using a 370Celsius normal saline solution and then connected to a centrif-ugal pump (Rotaflow, Jostra, Maquet Cardiopulmonary). With the circuit filled, 1000 IU of heparin was injected into the circulating fluid. Anticoagulation was moni-tored by measuring the activated coagulation time (ACT) at baseline and every 6 hours. The infusion of heparin was titrated to maintain the ACT at 1.5 – 2.5-fold the first baseline ACT value.

The PLS uses a polymethylpentene membrane; the tubes are coated with a bioactive and a biopassive sys-tem (Bioline, Maquet Cardiopulmonary).9 Two Luer-locks are connected in the pre- and post-membrane ports to enable the measurement of pressures and the collection of blood samples. Pressure lines are con-nected to ports in the drainage tube (before the centrifu-gal pump) before and after the ECMO membrane; pressure measurements are performed in real time using a multi-parametric monitor (Dx 2020, Dixtal Biomedica Ind. Com).

The extracorporeal circulation was initiated at a blood flow of 1.5 L/minute and with the gas flow turned off. After a stabilisation period of 30 min, the second baseline was collected, with blood, but no gas flow through the membrane.

During extracorporeal circulation, if the system began to shake or cavitate or if the blood flow decreased despite unchanging RPM in the pump, a standardised sequence of procedures was performed: first, the position of the animal was gently modified and lateralised and, if necessary, the animal was then placed in a semi-recumbent position; second, the PEEP level was raised by one or two cmH2O. If the PEEP was greater than 6 cmH2O, an attempt to lower the PEEP by 1 or 2 cmH2O was also performed; third, boluses of 250 mL of lactated Ringer’s solution were administered.

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Varying blood and sweeper flow

After the second baseline measurement, the blood flow was set to zero and was increased in steps of 0.5 L/min up to a maximum of 5.5 L/min. At the end of each min-ute, the number of RPM of the pump was registered, together with the pressures in the drainage system and the pressures pre- and post-membrane.

Induction of multi-organ dysfunction (MOF)

After this first sequence of data collection, we induced lung injury by surfactant depletion (lung lavage with ali-quots of 1 litre of normal saline at 370Celsius until the P/F ratio was <50); we then induced sepsis via faecal peritonitis (1 g/kg of faeces was administered into the peritoneum followed by an infusion of amikacin, 1000 mg, and metronidazole, 500 mg, at 4 hours after induc-tion of peritonitis; metronidazole, 500 mg, was infused again every 8 hours). At 12 hours after inducing MOF, the same sequence of data collection as described above was repeated (except for the baseline with and without gas flow). For a characterisation of the induction of MOF, in Table 1, we show the physiological data col-lected from the animals at the worst organ function measurement and at 12 hours after the induction of peritonitis. Data were collected during this interval and are published elsewhere.10,11

Statistical analysis

Normality was tested using the Shapiro-Wilk goodness-of-fit-model. All data, unless otherwise specified, are presented as medians [percentile 25th, percentile 75th]. Data pre- and post-MOF were tested using the Wilcoxon’s test. Data were plotted in scatter plots, and analyses of correlations were performed using Spearman’s method. Multivariate analyses were per-formed using a backward elimination mixed generalised model with the animal used as a fixed factor (to avoid the amplification of an effect due to the individual varia-tions). The Markov chain Monte Carlo procedure was used (with 10,000 simulations to reach the equilibrium of distributions) to retrieve a fixed probability of each resultant-dependent variable from the mixed gener-alised model after the backward elimination. The single co-linearity among the dependent variables was tested using Spearman correlations in a matrix including all variables tested and taking into account only variables with an r coefficient <0.85. Multi-co-linearity was tested, using the variance inflation factor (VIF). Variables with a VIF <2.5 were considered appropriate for analysis. The pseudo-R2 was calculated for each model to show goodness-of-fit: the calculation was per-formed using the squared ratio between the Spearman correlation, the fitted values of the model and the origi-nal values retrieved from the experiment. The R free

Table 1. Animal multi-organ dysfunction characterization.

Cardiovascular function Time to the maximum organ dysfunction (hours)a

Maximum organ dysfunctionb Value at the end of 12 hours of observationc

ABPm – mmHg, mean [range] 7 [6,9] 60 [47,98] 62 [54,68]CO – L/minute 9 [4,11] 3.9 [3.7,5.5] 4.9 [3.7,5.5]Heart rate – beats/minute 6 [5,10] 185 [180,188] 170 [138,174]LVSW – mL/mmHg/kg/beat 7 [7,12] 18 [15,21]] 18 [15,24]Norepinephrine dosage – mcg/kg/min

7 [3.9] 1.0 [0.6,1.1] 0.4 [0.2,1.3]

Lactate – mEq/L 8 [7.10] 8.0 [3.0,10.8] 7.6 [1.8,10.1]Renal functionSBE – mEq/L 8 [3,12] −8.8 [−9.6,-8.8] −8.8 [−8.8,-5.3]Cumulative fluid balance - mL 12 [11,12] 14 [−3,53] 14 [−8,53]Urinary flow - mL/kg/h 9 [6,10] 0.13 [0.13,0.27] 0.13 [0.13,0.27]Respiratory functionRespiratory static compliance –mL/cmH2O

4 [2,5] 9 [5,10] 17 [14,29]

Pulmonary Shunt – % 4 [2,7] 100 [85,100] 45 [38,69]

aTime between peritonitis induction and maximum organ dysfunction.bValue of the analyzed variable compatible with the maximum organ dysfunction.cThese values represent the organ’s dysfunction at the time of the second sequence of data collection beginning.ABPm: mean arterial blood pressure, CO: cardiac output, LVSW: left ventricle stroke work,SBE: standard base excess.

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Park et al. 5

source statistical package and comprehensive-R archive network (CRAN)-specific libraries were used to build the graphics and to perform all statistical analyses.12

Results

A 20-French cannula was used in four animals and a 21-French cannula was used in one animal to drain the blood to the ECMO device. A 21-French return cannula from the ECMO system was used in three animals and, in two, 20-French cannulas were used. The baseline (second baseline) data are shown in Table 1. The fluid

balance between the periods of data collection was pos-itive (8386 [6734 – 14534] mL of lactated Ringer’s). The ACT controls at 6, 12 and 18 h after the first baseline were 2.0-fold [2.0,2.6], 2.2-fold [1.7,2.6] and 1.8-fold [1.7,2.0] the control (first baseline), respectively. Table 2 shows the baseline systemic characteristics of the ani-mals prior to data collection in the pre- and post-MOF context. None of the animals required active core tem-perature control as described in the methods section.

Figure 2A shows the correlation between the RPM of the centrifugal pump and blood flow, the correlation between blood flow and the pressures before the

Table 2. Haemodynamic, respiratory and support measures in the animals pre-multi-organ failure (MOF) and after MOF induction.

Second baseline MOF baseline P-value

Haemodynamicb

Heart rate – beats/minute 130 [129,135] 170 [ 138,174] 0.314ABPm – mmHg 140 [123,146] 62 [54,78] 0.038PAPm – mmHg 49 [36,50] 44 [34,54] 0.784CVP – mmHg 7 [5,8] 16 [10,19] 0.063PAOP – mmHg 13 [10,13] 14 [7,18] 1.000CO – L/min 5.9 [5.8,6.3] 4.9 [3.7,5.5] 0.058RVSW – (mL.mmHg)/beat 27 [19,29] 11 [7,11] 0.036LVSW – (mL.mmHg)/beat 78 [71,87] 18 [15,24] 0.026PVR – dyn.sec−1.(cm5)−1 379 [353,508] 653 [233,757] 0.313SVR – dyn.sec−1.(cm5)−1 1765 [1310,1871] 647 [553,849] 0.043Respiratory Ventilatory mode Volume controlled Pressure controlled PaO2 – mmHg 82 [64,94] 77 [69,93] 0.814Sat O2 – % 94 [88,96] 92 [86,99] 1.000PaCO2 – mmHg 39 [32,40] 42 [35,47] 0.813FiO 0.30 [0.30,0.40] 0.30 [0.30,0.45] 0.813Tidal volume – mL 600 [580,625] 180 [145,294] 0.063Resp. rate – breaths/min 20 [18,30] 15 [10,25] 0.588Ppeak – cmH2O 31[28,39] 30 [27,33] 0.313Pplateau – cmH2O 17 [17,22] 30 [27,33] 0.100PEEP – cmH2O 5 [5,5] 18 [18,20] 0.030Cst – mL/mmHg 50 [7,54] 17 [14,29] 0.053MetabolicHaemoglobin – g/dL 11 [10,12] 9.0 [8.1,9.3] < 0.001pH 7.48 [7.44,7.55] 7.19 [6.92,7.32] < 0.001SBE – mEq/L 3.2 [2.3,5.8] −9.4 [−13.1,0.4] < 0.001Sodium – mEq/L 139 [138,141] 140 [134,148] 0.892Temperature – OC 37.5 [37.0,38.4] 38.5 [38.2,38.8] 0.686SupportNorepinephrine – mcg/kg/min 0 0.4 [0.2,1.3] ECMO - blood flow – mL/min 1500 2520 [1750,3690] ECMO - sweeper flow – L/min 0 8 [5,12]

All the present data were collected just before the beginning of the standardized blood and sweeper flow sequences and gas transference data collection.P-value of the comparison between pre- and after MOF.ABPm: mean arterial blood pressure, PAPm: mean pulmonary artery pressure, CVP: central venous pressure, PAOP: pulmonary artery occlusion pressure, CO: cardiac output, RVSW and LVSW: right and left ventricle stroke work, respectively, PVR and SVR: pulmonary and systemic vascular resistance, respectively, Cst: respiratory static compliance.

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centrifugal pump and right before the membrane and across the membrane (Figures 2 B, C and D, respectively). Figure 2D shows an increased scatter of the transmem-brane pressures when the blood flow was greater than 2.0 L/minute. To explore this scatter, a multivariate explanatory model was built, categorising blood flow as lower than 2000 mL/minute or equal to or higher than 2000 mL/minute. These analyses are shown in Table 3 and, of note, the temperature and blood flow were the only variables associated with modulating transmem-brane pressure drop with an inversely proportional effect of temperature. This association between temper-ature and transmembrane pressure was greater when the blood flow was ≥2000 mL/minute.

Discussion

In this study, during the transit through the ECMO cir-cuit at a blood flow of 5500 mL/minute, blood was exposed to low pressures, such as -120 mmHg in the pre-pump loop and high pressures, such as 450 mmHg in the post-pump and pre-membrane loop (Figure 2). At high blood flows (above 2 L/min), transmembrane pressures became highly variable and depended propor-tionally on blood temperature and blood flow (Table 3).

Our results showed a linear association between the centrifugal pump rotations per minute and increased blood flow (Figure 2). This association was similar at baseline and during multi-organ failure despite the

Figure 2. A shows the correlation between blood flow and the rotations of the centrifugal pump per minute. B shows the correlation between blood flow and the drainage system pressure. C shows the correlation between blood flow and the pre-membrane pressure and D shows the correlation between blood flow and transmembrane pressure.

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Park et al. 7

increase in temperature, falling haemoglobin levels and severe reductions in pH (Table 2). Anaemia and slight hyperthermia reduce blood viscosity,13–15 which may result in low resistance to the passage of blood through the ECMO circuit. pH modulates erythrocyte morphol-ogy; however, its effect on blood viscosity is negligible, even during severe acidosis.16 Thus, one may expect dif-ferences in the relationship between RPM and blood flow between these models. However, it appears that blood viscosity did not induce increased ECMO-system impedance. In contrast, only slight transmembrane pressure scatter occurred, which was primarily associ-ated with increased blood flow through the system (Figure 2) and this scatter was positively associated with blood flow and negatively associated with blood tem-perature (Table 2). These findings suggest a minor impact of blood viscosity, primarily on complex micro-structures of the system.

The reduced transmembrane pressure drop at higher temperatures may be due to a reduction in total blood viscosity and, consequently, a reduction in transmem-brane resistance.15,17 Paradoxically, higher temperatures are associated with an increase in plasma viscosity due to alterations in the structure of the plasma proteins.18 Haemoglobin has been traditionally considered an important determinant of blood viscosity.18 In this study, haemoglobin fell from 11.0 [10.0,12.0] to 9.0 [8.1,9.3] g/dL and it was not independently associated with the drop in transmembrane pressure. This finding

is probably associated with other physiological altera-tions observed in the animals studied, such as the fall in pH, temperature, ECMO blood flow and PEEP, and CVP elevations. These complex alterations result in unpredictable changes in total blood viscosity, as already described. For example, patients with acute myocardial infarction show high total blood viscosity upon admis-sion that is associated with increased haemoglobin. Within the first few days after admission, the haemoglo-bin levels fall, although viscosity remains high, probably due to increases in plasma proteins and enhanced eryth-rocyte aggregation.19 Thus, it is difficult to ascribe the association of temperature with transmembrane pres-sure to a single factor. Furthermore, in confirmatory experiments, it will be imperative to include an anaemia control.

The fall of 4.828 mmHg of transmembrane pressure for each 10C increase in temperature is clinically impor-tant, particularly for a system that normally works at very low pressure drops (~15 mmHg). Despite the higher dispersion of transmembrane pressures at high blood flows (>2000 mL/minute), the blood flow and core temperature remained correlated with the drop in transmembrane pressure at blood flows <2000 mL/min-ute (Table 2). This former finding could be important when using interventional lung assist (iLa) devices, where a vascular pressure gradient drives the blood through the membrane. Because the normal mean arte-rial blood and central venous pressures are approximately

Table 3. Backward elimination multivariate analysis exploring variables associated with the transmembrane pressure.

Analysis with all blood flows (R2 = 0.72)

Variable Beta-unstandardized coefficienta P value VIFb

Blood flow (mL/min) 0.005 < 0.001 1.000Core temperature(oCelsius)

− 4.828 < 0.001 1.175

Analysis with blood flow <2000 mL/min (R2 = 0.77)Variable Beta-unstandardized coef-

ficientP value VIF

Blood flow (mL/min) 0.005 < 0.001 1.000Core temperature(oCelsius)

− 2.113 < 0.001 1.175

Analysis with blood flow ≥ 2000 mL/min (R2 = 0.85)Variable Beta-unstandardized coef-

ficientP value VIF

Blood flow (mL/min) 0.005 < 0.001 1.000Core temperature(oCelsius)

− 6.149 < 0.001 1.175

This multivariate analysis was performed using a generalized linear mixed model adjusted to each animal with a backward elimination.The initial dependent variables in the transmembrane pressure analysis were blood flow, haemoglobin, pH, core temperature, central venous pressure, PEEP and sodium. pH was eliminated from the analysis in order to reduce the VIF bellow the threshold of 2.5. Haemoglobin, central venous pressure, PEEP and sodium dropped out during the backward elimination of the multivariate analysis.The blood samples were acquired from the pre-membrane port.aBeta-unstandardized coefficient denotes the estimated variation of the oxygen transference in mL/min for each unit (the units are cited in the table) variation of the independent variables, bVIF denotes variance inflation factor.

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8 Perfusion

70 and 8 mmHg, respectively, the membrane blood resistance, expressed as the drop in transmembrane pressure, is an important modulator of the iLa blood flow; thus, the elevation in arterial blood pressure and patient warming are used as measures to increase blood flow through the membrane and to enhance the efficacy of the system.

It is interesting to note that our pre-membrane pres-sures reached values as high as 400 mmHg at a blood flow rate of 5500 mL/minute. These values are not typi-cally used during patient support. However, the trans-membrane pressure remained at an acceptable value (<50-60 mmHg), suggesting that the post-membrane system resistance was the primary cause of the high pres-sure observed. We have used cannulas of 20-21 French for blood return, which enhance considerably the resist-ance of the blood return when using a high blood flow.

These data should be taken into account when a renal replacement therapy (RRT) system is due to be installed in the ECMO circuit to avoid interruptions of the RRT due to pressure-limit alarms. The low pressure inside the ECMO system is associated with haemolysis.20 The venous loop of the RRT system, when connected to the ECMO pre-pump loop, further reduces blood osmolar-ity to an extremely low pressure, thereby, enhancing the possibility of haemolysis.21 Therefore, the blood flow in the ECMO is a variable that should be lowered when significant intravascular haemolysis is detected at the site of blood flow drainage to the RRT device at the ECMO pre-pump loop. Other configurations should be used to install the RRT device, such as placing the arte-rial line in the pre-membrane port and the venous line in the pre-pump port or vice-versa or using an inde-pendent catheter inserted in an additional vein.

This study has several limitations. First, the small number of animals may enhance the individual charac-teristics of each animal. Second, it is difficult to extrapo-late animal data to humans. Third, this analysis is explanatory, therefore, it is necessary to confirm our findings in future dedicated experiments.

Conclusions

The pressure oscillation through the venous-venous ECMO system was wide and varied from -120 mmHg in the pre-pump loop to 400 mmHg in the post-pump and pre-membrane loop. In this explanatory analysis, the observed drop in transmembrane pressure was posi-tively associated with blood flow and negatively associ-ated with core temperature.

Declaration of Conflicting Interest

All the authors received a donation of PLS systems from Maquet Cardiopulmonary in Brazil to do the research and to support patients.

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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