Sequential Organ Failure Assessment Score at Presentation Predicts Survival in Patients Treated With Percutaneous Veno-Arterial Extracorporeal Membrane Oxygenation

Abstract: Background and Purpose. This study sought to investigate demographic, clinical, and procedural determinants of outcomes in patients treated with percutaneous veno-arterial (VA) extracorporeal membrane oxygenation (ECMO) initiated in the cardiac catheterization laboratory with a portable system. Methods. We performed a retrospective review of patients treated with percutaneous VA-ECMO during the study period at our institution. A logistic regression model was applied to investigate the association between sequential organ failure assessment (SOFA) score and survivor status. Fisher’s exact test was used to examine the association between survivor status and cannula size (15 Fr vs >15 Fr). Results. Percutaneous VA-ECMO was initiated in 25 patients. At 30 days, 10 patients were alive (40%). Fifteen patients had cardiac arrest (CA) prior to ECMO initiation, of which 5 were alive at 30 days (33%). Survivors had a lower baseline median SOFA score (9 vs 16; P=.02; odds ratio, 0.577). Use of a smaller cannula was associated with survival (P=.01). There was an association between the size of the arterial cannula and increased blood transfusions (P<.01). Conclusions. Lower presenting SOFA score and smaller cannula size were associated with increased survival in patients with cardiogenic shock (CS) or CA who underwent percutaneous VA-ECMO placed in the cardiac catheterization laboratory using a portable system. Calculation of the SOFA score at presentation may help physicians determine which patients may derive benefit from ECMO. Smaller cannula size, while decreasing the amount of flow, may result in decreased bleeding and increased survival.

J INVASIVE CARDIOL 2016;28(4):133-138. Epub 2016 February 15. 

Key words: cardiogenic shock, extracorporeal membrane oxygenation, mechanical circulatory support

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Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) is a form of mechanical circulatory support employed for the treatment of patients presenting with cardiogenic shock (CS) or cardiac arrest (CA). While ECMO has been available for decades, the requirements for an operating room and specialized staff have possibly limited widespread adoption. The development of portable miniaturized ECMO has made it possible to initiate ECMO at bedside or in the cardiac catheterization laboratory in a timely manner. 

Despite substantial advances in medical care, outcomes for patients with CS or CA remain poor.^1-4 The most common form of circulatory support is the concomitant use of intraaortic balloon pump (IABP) counterpulsation and vasopressors. However, results of randomized trials have failed to show benefit from the routine use of IABPs, potentially creating an opening for a greater role for advanced forms of circulatory support.^5,6 ECMO has been shown to be associated with acceptable outcomes in selected critically ill populations and has been used as a bridge to recovery or transplant. However, overall mortality remains high and it is not clear which patients derive benefit from ECMO.^7-9

The Sequential Organ Failure Assessment (SOFA) score is a scoring system designed to describe the extent of a patient’s organ failure. The score is based on six individual scores, each describing the function of a different organ system: respiratory, cardiovascular, hepatic, coagulation, renal, and neurologic.¹⁰ The SOFA score has been validated to predict mortality in the setting of critical illness due to various etiologies.^11-13 We examined the role of the SOFA score in predicting outcomes in patients treated with percutaneous portable VA-ECMO initiated in the cardiac catheterization laboratory. The size of the arterial cannula in the VA-ECMO system determines the degree of circulatory support delivered to the patient, with larger-bore cannulae achieving larger flows. However, larger-bore cannulae may be associated with a higher degree of vascular complications.^14,15 Our study sought to investigate technical predictors of outcomes in patients treated with VA-ECMO, particularly the impact of arterial cannula size. 

Methods

Study design. We performed a retrospective review of all patients treated with percutaneous VA-ECMO at the University of Chicago Medical Center from January 2012 to December 2014 with the portable miniaturized CardioHelp system (Maquet Cardiovascular). Patients with either profound CS or CA were initiated on ECMO at the discretion of the interventional cardiologist. 

Inclusion criteria. The decision to initiate ECMO was made at the discretion of the referring physician. Patients in CS, defined as hypotension (systolic blood pressure <90 mm Hg) for >15 minutes with presumed cardiogenic etiology, that was not responsive to inotropes, vasopressors, and/or IABP, were considered. Furthermore, if the patient had poor oxygenation (SaO₂ <90% on FIO₂ of 100%) in the setting of CS, consideration was given to initiate ECMO. Patients with CA were considered if it was thought to be secondary to a primary cardiac cause. Finally, ECMO was only considered if the common femoral artery was at least 5 mm in diameter and had no significant iliac lesions or significant calcification, since smaller arterial size would preclude safe insertion and increase the risk of significant limb ischemia

Exclusion criteria. Patients with incurable disease (such as advanced cancer), prolonged resuscitation prior to consideration for ECMO, active or recent hemorrhage, or in whom non-cardiac or metabolic conditions such as sepsis were felt to be the primary cause of CS or CA were not considered for VA-ECMO initiation. 

ECMO implantation and weaning procedure. VA-ECMO was initiated at presentation via cannulation of the femoral vein and femoral artery using Seldinger’s technique. In order to optimize distal limb perfusion, anterograde cannulation of the superficial femoral artery with either a 5 Fr or 6 Fr sheath (Cordis Corporation) was also performed at the time of ECMO initiation to preserve distal limb perfusion. Cannulation with Biomedicus arterial and venous cannulae (Medtronic) was performed in the cardiac catheterization laboratory or at bedside. Four sizes of retrograde arterial cannulae were used, including 15 Fr, 17 Fr, 19 Fr, and 21 Fr. The sizes of the arterial cannulae were chosen by the initiating physician based on the body mass index (BMI) of the patient, with patients of higher BMI receiving larger cannulae due to larger flow requirements. The ECMO circuit consisted of a centrifugal pump in conjunction with a membrane oxygenator with integrated heat exchanger (Maquet AG). A representative photograph of the percutaneous ECMO setup is presented in Figure 1. When started on ECMO, pump flow rates were initiated at up to 4.5 L/min and gas supplied to the oxygenator was initially 100% oxygen. Subsequent flow rate and gas supply were adjusted according to the patient’s need. Patients were heparinized while on ECMO with goal activated clotting times maintained at a target range of 180-220 seconds. Weaning was started from full flow to approximately 1 L/minute while observing hemodynamic status. 

Variables. Baseline demographic and clinical characteristics, size of arterial cannulae, blood transfusions, vascular complications, and clinical outcomes of survivors and non-survivors were recorded. Independent variables in the statistical analyses included baseline demographic and clinical characteristics, cannula size, and blood transfusion requirements. Survival status was the dependent variable. ECMO-related complications were also recorded, including limb ischemia, arterial laceration, significant hemorrhage at site of cannulation, and ischemic or hemorrhagic stroke. Limb ischemia was defined as acute loss of limb perfusion that threatened limb viability and resulted in a change in peripheral pulses, including any surgical repair. Hemorrhage was defined according to the Thrombolysis in Myocardial Infarction (TIMI) major definition, including clinically significant bleeding associated with a drop in hemoglobin of >5 g/dL, any intracranial bleeding, or fatal bleeding.¹⁶Pulmonary hemorrhage was defined as massive hematemesis or new pulmonary opacity on chest radiograph clinically felt to be hemorrhage by the treating physician. SOFA score at presentation was calculated using available clinical data. Laboratory data, including pH and lactic acid, were measured at the time of ECMO initiation. When available, pulmonary artery pressure measurements were also obtained at the time of initiation.

Statistical analyses. For categorical variables, Fisher’s exact test was used to compare the two outcome groups. Student’s t-test was used for continuous variables. We applied an age-adjusted logistic regression model to investigate the association between SOFA score and survivor status. Fisher’s exact test was used to examine the association between survivor status and cannula size – specifically, the patients receiving >15 Fr cannulae were compared vs those with cannulae ≤15 Fr. The association between cannula size and transfusion of packed red blood cells over the first 24 hours of treatment was investigated using a quadratic regression model. A P-value of <.05 was considered significant. Statistical analysis was performed using STATA (StataCorp). 

Results

Demographic and clinical characteristics. Percutaneous VA-ECMO was initiated in 25 patients with CS or CA. Demographic and clinical characteristics at baseline are presented in Table 1. There was a significant positive relationship between female gender and survivor status. Other baseline demographic characteristics, including age, coronary artery disease risk factors, prior diagnosis of heart failure, and baseline left ventricular ejection fraction, were not significantly different between the two outcome groups. Of the laboratory and clinical characteristics evaluated, only SOFA score was found to be significantly different between the outcome groups, with survivors having lower baseline median SOFA score (9 vs 16) (Figure 2). The association reached statistical significance (P=.02) in the logistic regression model, with an odds ratio of 0.577 (95% confidence interval, 0.353-0.944) for higher SOFA score resulting in lower likelihood of \survival.

Vascular complications. Four patients experienced significant hemorrhage related to cannulation. Other major complications included limb ischemia (n = 1), heparin-induced thrombocytopenia (n = 1), and disseminated intravascular coagulation (n = 1). Neurologic deficits due to ischemic stroke or intracranial hemorrhage were not observed during the study period. Observed complications of ECMO are presented in Table 2. The use of a smaller cannula was significantly associated with survival (P<.01) (Figure 3). There was an association between the size of the arterial cannula and blood transfusion needs (P<.01), suggesting that increasingly large cannula size may be associated with disproportionately higher transfusion requirements. Although not statistically significant, there was a signal toward higher risk for TIMI major bleeding in patients receiving arterial cannulae that were larger than 15 Fr. 

Discussion

In this study, we present a retrospective review of 25 patients presenting with CS or CA treated with percutaneous, portable VA-ECMO that was initiated in the cardiac catheterization laboratory. Our findings suggest that lower SOFA score at presentation is associated with increased survival in patients with CS or CA treated with percutaneous VA-ECMO. Additionally, among our patients, the use of a smaller arterial cannula was associated with survival. This survival benefit was independent of indices of patient size, such as BMI. These findings are relevant in light of growing adaption of mechanical circulatory support and a recent investigation that has described the limitations of IABP support in CS.⁵ 

Despite the technological advances of the past several decades, CS and CA have continued to be associated with high mortality.^1-4,17,18 The survival rate in our study is comparable with published data for CS with or without mechanical support. Rapid reversal of CS is occasionally possible in specific disease states, such as acute myocardial infarction that is rapidly reperfused. However, even when reperfusion is achieved, myocardial stunning may still lead to shock, multiorgan failure, and death.¹⁹ CS of other etiologies – such as valvular dysfunction – is often more refractory to immediate management. A study of patients from the SHOCK trial registry demonstrated that cardiac power is an important hemodynamic predictor of mortality, suggesting that mechanical support may be a reasonable therapy for patients with profound CS.²⁰ Mechanical support may be initiated as a temporizing measure to prevent the onset of multiorgan failure while definitive therapy is prepared or the myocardium recovers. The most common form of mechanical support currently in use is the IABP. While used for over 40 years, recent studies have questioned the survival benefit of IABP therapy in CS.^5,6 This has resulted in increased adoption of advanced forms of circulatory support, specifically peripherally implanted ventricular assist devices such as the Tandemheart (CardiacAssist), Impella, and ECMO. Limited side-by-side comparison of the various mechanical support devices including the Impella and the TandemHeart have not demonstrated any one of the assist devices to be superior to the others.^21,22 The amount of circulatory support needed to improve outcomes in CS has not been established. The most commonly used form of circulatory support, the IABP, is able to augment cardiac output by less than 1 L/minute, which has not been shown to consistently result in improved clinical outcomes.^5,23 The Impella devices offer a range of reported cardiac output augmentation depending on the size of the device, with some suggestion that the Impella 5 may be associated with improved outcomes over the Impella 2.5.²⁴ However, the biventricular mechanical support and oxygen replacement of ECMO make it more adaptable to a range of clinical scenarios and may make it a more attractive assist device.^25-28

ECMO can rapidly restore circulation and improve end-organ perfusion; nevertheless, mortality after ECMO initiation remains high. Studies have shown that the initiation of VA-ECMO has increased survival in patients with acute respiratory distress syndrome after H1NI.^29,30 Recently, rapid initiation of VA-ECMO in CA survivors has also shown clinical benefit.³¹

For ECMO to become an established therapeutic alternative, optimal patient selection and timing of intervention must be established. Previously reported demographic predictors for in-hospital mortality of patients treated with ECMO have included age,^19,25 BMI,²⁸ diabetes,²⁵ coronary artery disease,²⁵ and a cardiac procedure prior to ECMO.²⁵ Clinical predictors of in-hospital mortality include elevated lactate,²⁵ prior use of IABP,⁷ APACHE score,^28,32 and baseline left ventricular ejection fraction.²⁸ Our study adds to these findings and suggests that the SOFA score at presentation is associated with 30-day survival in patients treated with VA-ECMO. These findings support the hypothesis that improving cardiovascular hemodynamics may not be sufficient to prevent death if multiorgan failure has preceded treatment initiation. The SOFA score of patients under consideration for treatment with VA-ECMO may help determine which patients are likely to derive benefit. 

Rapid initiation of ECMO, especially in critically ill patients, may afford sufficient hemodynamic support for survival. Instituting ECMO in the cardiac catheterization laboratory may allow for greater and more rapid initiation of ECMO. Interventional cardiologists in the catheterization laboratory have developed protocols to manage acute ST-segment elevation myocardial infarction patients in under 60-90 minutes, and similar protocols may be developed for patients who are candidates for ECMO. Furthermore, increasingly widespread treatment of peripheral and aortic disease has increased the comfort of interventional cardiologists with antegrade access and management of large-bore sheaths. 

The amount of circulatory support provided by VA-ECMO is determined by the size of the arterial cannula. In our patient population, survival was associated with the use of smaller arterial cannulae. Smaller cannulae may improve survival by preventing bleeding complications. Although not statistically significant in a population with limited events, our analysis identified a signal toward an increased risk of TIMI major bleeding in patients receiving arterial cannulae that were larger than 15 Fr. Larger cannulae may increase the risk for bleeding with larger sequential vessel dilation and increased risk for arterial laceration. In addition, larger cannulae may create more tension on the vessel wall, which could increase the risk for traumatic bleeding with patient or cannula movement. Recent published studies reporting bleeding with transcatheter aortic valve implantation (TAVI) have shown that a reduction in diameter of catheter delivery systems with the newer TAVI valves is associated with a reduction of bleeding and major vascular complications, lending some credence to the notion that smaller catheters and cannulae may be associated with decreased bleeding.³³ 

In addition to bleeding, ECMO is associated with a variety of vascular complications with an incidence of up to 20%,^14,15,34,35 but the clinical impact of these complications on survival is not clear. One study of 176 patients treated with VA-ECMO did not show an association between vascular complications and worse outcomes.¹⁵ Overall, our patients had a low rate of vascular complications. This may be partially due to the efficacy of anterograde cannulation of the superficial femoral artery in maintaining distal limb perfusion. 

In addition to vascular complications, ECMO is associated with a number of non-procedural complications related to flow rate, including left ventricular distension in the setting of increased afterload, the systemic inflammatory response syndrome, and hemolysis due to contact between red blood cells and hardware in the ECMO circuit. During the study period, we did not experience difficulty with maintaining adequate flow rates in patients treated with smaller cannulae, and were consistently able to maintain flow rates above 4 L/minute with a 15 Fr arterial cannula. Overall, our surviving patients tended to be less critically ill at presentation than the non-survivors as determined by SOFA score. This finding is consistent with a previously described association between SOFA score and mortality in critical illness of other etiologies.^11-13 Nevertheless, the association between use of smaller cannulae and survival suggests that there may be a threshold of effective circulatory support that was reached in our patients, above which increased flow rate may not lead to increases in survival but may be associated with more complications. 

Study limitations. Our study was a single-center, retrospective review and patients were selected for treatment based solely at the discretion of the treating physician. Thus, there is the possibility of referral bias and blinding was not possible. We found SOFA score at presentation and smaller arterial cannula size to be associated with better outcomes. The small sample size made it impossible to determine whether these predictors were additive or interactive. The sample size also limited our statistical analysis and power. Therefore, these findings need to be replicated with larger studies. We were limited by the infrequent occurrence of patients qualifying for ECMO during the study period. In this study, we reported results at 30 days from initiation of treatment. Our follow-up at 30 days was 100%, so it may be possible to reevaluate the study population at later times to determine if predictors of survival at 30 days remain valid at later follow-up. Finally, there was a large amount of variability between patients with regard to time of presentation to initiation of ECMO, which may have also influenced outcomes.

Conclusion

Lower presenting SOFA score was associated with increased survival in patients with CS or CA undergoing percutaneous VA-ECMO. Calculation of the SOFA score may help determine which patients will derive benefit from ECMO support. Smaller arterial cannulae result in lower flow rates, but may also be associated with reduced bleeding and vascular complications. Further study is warranted to determine the balance between providing circulatory support and reducing complications in patients treated with percutaneous, portable VA-ECMO. 

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From the ¹Section of Cardiology, Department of Medicine, University of Chicago Medical Center, Chicago, Illinois; ²Duke University Medical Center, Durham, North Carolina; ³Department of Surgery, University of Chicago Medical Center, Chicago, Illinois; and ⁴Barnabas Medical Center, Newark, New Jersey.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.

Manuscript submitted September 18, 2015, provisional acceptance given October 5, 2015, final version accepted November 2, 2015.

Address for correspondence: Atman P. Shah, MD, The University of Chicago Medicine, 5841 S. Maryland Ave, MC 6080, Chicago, IL 60637. Email: ashah@bsd.uchicago.edu