The physiology of extracorporeal membrane oxygenation: The Fick principle

Extracorporeal membrane oxygenation (ECMO) can be delivered in veno-arterial (VA) and veno-venous (VV) configurations based on the cannulation strategy. VA and VV ECMO are delivered primarily for haemodynamic and respiratory support in patients with severe heart and lung failure, respectively. The Fick principle describes the relationship between blood flow and oxygen consumption - key parameters in the physiological management of extracorporeal support. This review will discuss the application of the Fick principle in: (i) recirculation in VV ECMO; (ii) the quantification of oxygen delivery (DO2) in VV ECMO and (iii) the quantification of transpulmonary blood flow and systemic arterial oxygen saturation in VA ECMO.

The Impact of Recirculation on Extracorporeal Gas Exchange and Patient Oxygenation during Veno-Venous Extracorporeal Membrane Oxygenation-Results of an Observational Clinical Trial

Background: Recirculation during veno-venous extracorporeal membrane oxygenation reduces extracorporeal oxygen exchange and patient oxygenation. To minimize recirculation and maximize oxygen delivery (DO2) the interaction of cannulation, ECMO flow and cardiac output requires careful consideration. We investigated this interaction in an observational trial. Methods: In 19 patients with acute respiratory distress syndrome and ECMO, we measured recirculation with the ultrasound dilution technique and calculated extracorporeal oxygen transfer (VO2), extracorporeal oxygen delivery (DO2) and patient oxygenation. To assess the impact of cardiac output (CO), we included CO measurement through pulse contour analysis. Results: In all patients, there was a median recirculation rate of approximately 14−16%, with a maximum rate of 58%. Recirculation rates >35% occurred in 13−14% of all cases. In contrast to decreasing extracorporeal gas exchange with increasing ECMO flow and recirculation, patient oxygenation increased with greater ECMO flows. High CO diminished recirculation by between 5−20%. Conclusions: Extracorporeal gas exchange masks the importance of DO2 and its effects on patients. We assume that increasing DO2 is more important than reduced VO2. A negative correlation of recirculation to CO adds to the complexity of this phenomenon. Patient oxygenation may be optimized with the direct measurement of recirculation.

Structural recirculation and refractory hypoxemia under femoro-jugular veno-venous extracorporeal membrane oxygenation

The performance of each veno-venous extracorporeal membrane oxygenation (vv-ECMO) configuration is determined by the anatomic context and cannula position. A mathematical model was built considering bicaval specificities to simulate femoro-jugular configuration. The main parameters to define were cardiac output (Q_C ), blood flow in the superior vena cava (Q_SVC ), extracorporeal pump flow (Q_EC ), and pulmonary shunt (k_S-PULM ). The obtained variables were extracorporeal flow ratio in the superior vena cava (EFR_SVC  = Q_EC /[Q_EC  + Q_SVC ]), recirculation coefficient (R), effective extracorporeal pump flow (Q_eff-EC  = [1 - R] × Q_EC ), Q_eff-EC /Q_C ratio, and arterial blood oxygen saturation (SaO₂ ). EFR_SVC increased logarithmically when Q_EC increased. High Q_C or high Q_SVC /Q_C decreased EFR_SVC (range, 68%-85% for Q_EC of 5 L/min). R also increased following a logarithmic shape when Q_EC increased. The R rise was earlier and higher for low Q_C and high Q_SVC /Q_C (range, 12%-49% for Q_EC of 5 L/min). The Q_eff-EC /Q_C ratio (between 0 and 1) was equal to EFR_SVC for moderate and high Q_EC . The Q_eff-EC /Q_C ratio presented the same logarithmic profile when Q_EC increased, reaching a plateau (range, 0.67-0.91 for Q_EC /Q_C  = 1; range, 0.75-0.94 for Q_EC /Q_C  = 1.5). The Q_eff-EC /Q_C ratio was linearly associated with SaO₂ for a given pulmonary shunt. SaO₂  < 90% was observed when the pulmonary shunt was high (Q_eff-EC /Q_C  ≤ 0.7 with k_S-PULM  = 0.7 or Q_eff-EC /Q_C  ≤ 0.8 with k_S-PULM  = 0.8). Femoro-jugular vv-ECMO generates a systematic structural recirculation that gradually increases with Q_EC . EFR_SVC determines the Q_eff-EC /Q_C ratio, and thereby oxygen delivery and the superior cava shunt. EFR_SVC cannot exceed a limit value, explaining refractory hypoxemia in extreme situations.

An overview of medical ECMO for neonates

Extracorporeal membrane oxygenation (ECMO), a life-saving therapy for respiratory and cardiac failure, was first used in neonates in the 1970s. The indications and criteria for ECMO have changed over the years, but it continues to be an important option for those who have failed other medical therapies. Since the Extracorporeal Life Support Organization (ELSO) Registry was established in 1989, more than 29,900 neonates have been placed on ECMO for respiratory failure, with 84% surviving their ECMO course, and 73% surviving to discharge or transfer. In this chapter, we will review the basics of ECMO, patient characteristics and criteria, patient management, ECMO complications, special uses of neonatal ECMO, and patient outcomes.

Quantifying Recirculation in Extracorporeal Membrane Oxygenation: A New Technique Validated

Rationale

The efficacy of veno-venous extracorporeal membrane oxygenation is limited by the phenomenon of recirculation, which is difficult to quantify. Existing measurement techniques using readily available equipment are unsatisfactory.

Objectives

1) To compare the accuracy of measurements of recirculation made using equations comparing blood oxygen content or saturation alone at different points in an ex vivo circuit; 2) to validate a new step-change technique for quantifying recirculation in vivo.

Methods

Anesthetized greyhound dogs cannulated for veno-arterial support were connected to a circuit that allowed the creation of a known level of recirculation ex vivo and blood oxygen content/saturation monitoring. In two dogs, the accuracy of measurements derived from oxygen content and oxygen saturation were compared. The potential of a new technique for measuring recirculation in vivo by comparing the oxygen content of blood sampled during oxygenator bypass to that following a step-change in circuit oxygenation was demonstrated in a veno-venous pilot study and validated in a three-dog veno-arterial study.

Results

Measurements made using oxygen content versus oxygen saturation showed superior correlation with true recirculation (r2=0.87 vs. 0.64, p<0.0001) and less proportional measurement bias (10.3% vs. 49.8%, p=0.0045). Measurements of recirculation made using a step-change in circuit oxygenation and comparing oxygen content as is required for measuring in vivo recirculation overestimated by only 18.6% (95% CI: 3.9–33.2%) and had excellent correlation with true values (r2=0.89).

Conclusions

1) Measurement of recirculation using oxygen content is superior to that using oxygen saturation alone, which demonstrates significant measurement bias; 2) the novel step-change technique is a sufficiently accurate technique for the measurement of recirculation in animal models.

Cannula Design and Recirculation During Venovenous Extracorporeal Membrane Oxygenation

Extracorporeal membrane oxygenation (ECMO) is used as a lifesaving rescue treatment in refractory respiratory or cardiac failure. During venovenous (VV) ECMO, the presence of recirculation is known, but quantification and actions to minimize recirculation after measurement are to date not routinely practiced. In the current study, we investigated the effect of draining cannula design on recirculation fraction (R_f) during VV ECMO; conventional mesh cannula was compared with a multistage cannula. The effect of adjusting cannula position was also studied. Recirculation was measured with ultrasound dilution technique at different ECMO flows and after cannula repositioning. All patients who were admitted to our unit between October 2014 and July 2015 catheterized by the atrio-femoral single lumen method were included. A total of 108 measurements were conducted in 14 patients. The multistage cannula showed significantly less recirculation (19.0 ± 12.2%) compared with the conventional design (38.0 ± 13.7). Pooled data in cases improved from adjustment showing reduced R_f by 7%. In conclusion, the choice of cannula matters, as does adjustment of the draining cannula position during atrio-femoral VV ECMO. By utilizing the ultrasound dilution technique to measure R_f before and after repositioning, effective ECMO flow can be improved for a more effective ECMO treatment.

Quantification of recirculation as an adjuvant to transthoracic echocardiography for optimization of dual-lumen extracorporeal life support

PURPOSE

Proper cannula positioning in single site veno-venous extracorporeal life support (vv-ELS) is cumbersome and necessitates image guidance to obtain a safe and stable position within the heart and the caval veins. Importantly, image-guided cannula positioning alone is not sufficient, as possible recirculation cannot be quantified.

METHODS AND RESULTS

We present an ultrasound dilution technique allowing quantification of recirculation for optimizing vv-ELS.

CONCLUSION

We suggest quantification of recirculation in addition to image guidance to provide optimal vv-ELS.

Calculating mixed venous saturation during veno-venous extracorporeal membrane oxygenation

INTRODUCTION

Recirculation (R), the shunting of arterial blood back into to the venous lumen, commonly occurs during veno-venous extracorporeal membrane oxygenation (VV-ECMO) and renders the monitoring of the venous line oxygen saturation no longer reflective of patient mixed venous oxygen saturation (S(V)O(2)). Previously, we failed to prove the hypothesis that, once R is known, it is possible to calculate the S(V)O(2) of a patient on VV-ECMO. We hypothesize that we can calculate S(V)O(2) during VV-ECMO if we account for and add an additional correction factor to our model for dissolved oxygen content. Therefore, the purpose of this study is to derive a more accurate model that will allow clinicians to determine S(V)O(2) during VV-ECMO when ultrasound dilution is being used to quantify R.

METHODS

Using an extracorporeal circuit primed with fresh porcine blood, two stocks of blood were produced; (1) arterial blood (AB), and (2) venous blood (VB). To mimic recirculation, the AB and VB were mixed together in precise ratios using syringes and a stopcock manifold. Six paired stock AB/VB sets were prepared. Two sets were mixed at 20% R increments and 4 sets were mixed at 10% R increments. The partial pressure of oxygen (pO(2) ) and oxygen (O(2)) saturation of the stock blood and resultant mixed blood was determined. The original model was modified by modeling the residual errors with linear regression.

RESULTS

When using the original model, as the partial pressure of arterial oxygen (P(a)O( 2)) of the stock AB increased, the calculated S(V)O(2) was higher than actual, especially at higher R levels. An iteration of the original model incorporating the P(a)O(2) level (low, medium, high) and R was derived to fit the data.

CONCLUSIONS

The original model using R and circuit saturations for the calculation of S(V)O( 2) in VV-ECMO patients is an oversimplification that fails to consider the influence of the high pO(2) of arterial blood during therapy. In the future, further improvements in this model will allow clinicians accurately to calculate S(V)O(2) in conjunction with recirculation measurements.