Raymond J. Strobel and Nicholas R. Teman
In patients with respiratory failure or cardiogenic shock refractory to medical management, temporary mechanical circulatory support can be utilized as a bridge to recovery, bridge to decision, or bridge to destination therapy (i.e., transplant, ventricular assist device [VAD]). Options available to the cardiothoracic surgeon include the intra-aortic balloon pump, extracorporeal membrane oxygenation (ECMO), and temporary VADs. Durable VAD therapy is discussed in depth in the Durable Mechanical Circulatory Support chapter.
Intra-Aortic Balloon Pump (IABP)
The IABP remains the most commonly utilized tMCS device for cardiogenic shock. This device consists of two fundamental parts: 1.) a double-lumen 8.0-9.5 French catheter with a 25-50mL balloon attached to the distal end and 2.) a console with a pump to drive this balloon. The balloon is inflated with helium gas (selected for its low density and easy absorption into the blood in case of balloon rupture). Balloon size is determined based on patient height.
Inserted percutaneously via the femoral artery, the balloon is passed retrograde up the aorta. The balloon’s distal tip should be just distal to the left subclavian – positioning the IABP too distally in the patient’s aorta will drastically reduce its clinical effect and can result in malperfusion of abdominal aortic branches. Fluoroscopic or echocardiographic guidance can be utilized to ensure correct positioning.
Once positioned, inflation/deflation of the balloon is triggered via ECG or arterial pressure tracing. When ECG is utilized for timing, the middle of the T wave of the ECG triggers inflation and the peak of the R-wave triggers deflation. When arterial pressure tracing is utilized for timing, inflation occurs at the dicrotic notch (onset of diastole) and deflation occurs at end-diastole (just prior to systole). Inflation results in augmentation of aortic root pressure thereby improving coronary perfusion pressure and cardiac output. Deflation just prior to systole decreases afterload, which in turn reduces myocardial work, and augmenting systolic blood pressure. The balloon can be set to assist every beat (1:1 balloon augmentation to heart beat) or less frequently (1:2, 1:4, 1:8) as indicated by the patient’s hemodynamics.
Correctly recognizing inappropriately timed IABP inflation/deflation is critical (Please see figure in Santa-Cruz RA, Cohen MG, Ohman EM. Aortic counterpulsation: a review of the hemodynamic effects and indications for use. Catheter Cardiovasc Interv. 2006;67[1]68-77). Manually adjustments to timing must be made if aberrant inflation or deflation is present.
Relative contraindications to utilization of an IABP include significant aortic or iliofemoral atherosclerotic disease (rendering insertion of the balloon impossible or the risk of iatrogenic embolic phenomena prohibitive), as well as tachyarrhythmias (which do not allow sufficient time for balloon inflation and deflation). Absolute contraindications include aortic insufficiency, aortic aneurysms, presence of intra-aortic stents, and aortic dissection.
Weaning from IABP is accomplished by gradually reducing the ratio of augmentation over 12 hours. Note that the balloon should never be turned completely off while within the patient unless the patient is systemically anticoagulated (and even then this should be avoided if at all possible) due to risk of thrombus formation.
Extracorporeal membrane oxygenation
Extracorporeal membrane oxygenation, or ECMO, is an established modality for supporting patients with respiratory failure despite optimal mechanical ventilation as well as for patients with cardiogenic shock. A patient’s deoxygenated blood is drained from a vein with the assistance of a pump, passed through a gas exchanger (O2 and CO2 exchange), and is then returned to either another vein (Venous-Venous ECMO) or artery (Venous-Arterial ECMO).
Venous-Venous (VV) ECMO
Indications
Venous-venous ECMO is indicated in general for those patients with respiratory failure whose oxygenation and/or ventilation does not response to optimized mechanical ventilation (as well as adjuncts including proning and chemical paralysis). The Extracorporeal Life Support Organization (ELSO) guidelines include the following criteria as indications for VV ECMO:
1. Hypoxic respiratory failure due to any cause where the risk of mortality is 80% or greater (consider ECMO if mortality is 50-80%)
80% Mortality is associated with:
- PaO2/FiO2 <100
- Oxygen index ([FiO2 x mean airway pressure] / PaO2) >40
- Murray score ≥2
- Components of the Murray score include: Consolidation on CXR, P/F ratio, PEEP, and lung compliance
2. CO2 retention on mechanical ventilation despite high Pplat (>30 cm H2O)
3. Severe air leak syndromes
4. Need for intubation in patient on lung transplant list
5. Immediate respiratory collapse (PE, airway obstruction)
Contraindications
There are no absolute contraindications to ECMO. Many relative contraindications exist, including:
1. Mechanical ventilation for 7 days or more
2. Immunosuppression
3. CNS hemorrhage
4. Irreversible comorbidity (e.g., terminal malignancy)
5. Advanced age
The decision to initiate ECMO in the presence of one or more of these relative contraindications should be considered in an individualized fashion.
A Respiratory ECMO Survival Prediction (RESP) score can be calculated to predict likelihood of survival to discharge for a patient being considered for ECMO in the setting of acute respiratory failure. Components of this score include age, immunosuppressed status, diagnosis, and clinical characteristics related to the severity of a patient’s cardiopulmonary collapse prior to initiating ECMO. Patients are assigned to one of five classes, ranging from Class I (92% survival) to Class V (18% survival).
Access
Several access configurations exist for drainage and reinfusion catheter placement. There are advantages and drawbacks to each. Considerations include surgeon familiarity with the access technique, flows, and relative risk for recirculation of oxygenated blood back into the ECMO circuit.
| Cannulation Strategies | Pros | Cons |
| Femoral-femoral | -Ease of access | – Higher risk of recirculation – Limited flows (3-6L/min) – Harder to mobilize patient with cannulas in bilateral groins – Need for two access sites |
| Femoral-internal jugular | – Highest flows (7L/min) – Easy addition of second femoral drainage catheter if needed – Decreased recirculation (relative to femoral-femoral approach) | – Groin access point (harder to mobilize patient – Risk of infection – Need for two access sites |
| Bicaval, dual lumen IJ | – Great mobilization – One access site – Avoids groin | – Lower flows (5L/min) – Requires fluoroscopy or echocardiography for placement |
Venous drainage cannula sizing is critical. The shortest and widest drainage cannula should be placed to maximize drainage and flow. Vessel diameter can be measured with ultrasound (in mm) and then multiplied by (2/3)*pi to yield the largest French size insertable without obstructing the vessel. Flow charts can be used to assist in this decision-making.
Femoral drainage cannulas should be positioned with their distal tip in the intrahepatic IVC. Return cannulas (femoral or IJ) should terminate at the cavo-atrial junction. Ideally, 8cm of distance should be present between the drainage and reinfusion cannulas to avoid recirculation.
Management and Troubleshooting
Patients should be anticoagulated just prior to cannulation with 50-100u/kg of heparin and maintained on therapeutic anticoagulation. ACT, PTT, or anti-Xa levels can be utilized to monitor anticoagulation. In the presence of contraindications to anticoagulation (e.g., trauma) or the development of bleeding, anticoagulation can be held with vigilance for thrombotic phenomena.
In general, while on VV ECMO the following oxygen saturations should be maintained: SpO2 >80-85%, PaO2 >50-55, and SvO2 >65%. Blood in the reinfusion cannula (SaO2) should be maximally oxygenated (i.e., 100%). However, low systemic PaO2 and oxygen saturations may be tolerated if delivery is sufficient for end-organ function (monitored by lactic acid, UOP).
Poor oxygenation can be addressed by the following maneuvers:
- Increasing pump flow
- Increase RPMs
- Improve venous drainage
- Increase oxygen carrying capacity of blood
- Blood transfusion to increase hematocrit
- Address recirculation (oxygenated blood from reinfusion cannula is returning to drainage cannula without circulating throughout the patient; pre-oxygenator oxygen saturation is higher than the peripheral oxygen saturation)
- Increasing distance between cannula
- Add additional drainage cannula
- Ensure reinfusion cannula is at cavo-atrial junction
- Convert to bicaval, dual lumen IJ cannula
- Address causes of increased oxygen consumption
- Sepsis (broad-spectrum antibiotics)
- Hyperthermia (antipyretics, cooling)
- Seizures (anticonvulsants, paralytics)
- Tachycardia (beta-blockade)
- Agitation/tachypnea (sedatives, paralytics)
- Replace oxygenator
- Improve native pulmonary contribution to oxygenation
Poor ventilation can be addressed by:
- Increasing the sweep (gas flow rate, L/min)
- Increasing the minute ventilation by adjusting ventilator settings
While on VV ECMO, three pressures are monitored: pre-pump pressure (venous pressure), pre-oxygenator (intrinsic pressure), and post-oxygenator (arterial pressure). Pre-pump pressure should remain less negative than -100 mmHg. Pre-oxygenator pressure is not held at an absolute value, but rather should be trended. Increasing pre-oxygenator pressure is a sign of oxygenator thrombosis. Post-oxygenator pressure should be <300 mmHg.
Typical flows on VV ECMO should be 50-80cc/kg/min. Low pump flows with venous cannula “chattering” indicates that the flow of blood into the ECMO circuit is inadequate. This can be addressed by repositioning cannula, lowering RPMs or increasing circuit volume (crystalloid/colloid/RBC).
Low pump flows without venous cannula “chattering” suggests an issue with the pump itself or afterload. Thrombosis within the circuit or kinking of cannula should be assessed. The presence of abdominal compartment syndrome or tension pneumothorax should be ruled out. While on VV ECMO, lung protective ventilation (“rest-settings”) is critical to promote recovery of native lung function.
Plasma free hemoglobin and lactate dehydrogenase should be monitored daily as a marker of hemolysis. Clinically significant hemolysis is diagnosed when plasma free hemoglobin >50 mg/dL or LDH >2000 u/L. Thrombosis within the circuit should be sought if clinically significant hemolysis is present. Post-oxygenator pressures can be reduced as well.
Decannulation
Decannulation may be considered once the patient demonstrates improved oxygenation with stable ECMO settings. CXR may also demonstrate progressive improvement. Weaning entails gradual decreasing of sweep gas and ECMO FiO2, thereby returning the burden of oxygenation and ventilation to the patient’s lungs. Flows are not altered during weaning from VV ECMO. A capping trial with sweep and ECMO FiO2 turned off should be undertaken prior to decannulation. Lastly, a Cilley test can be done – this involves increasing the fraction of inspired oxygen of the ventilator circuit to 1.0 (while the ECMO circuit is capped). If the patient’s SpO2 rapidly increases to 100%, separation from ECMO has good likelihood of success.
Outcomes
As of January 2020, 60% of patients undergoing VV ECMO survive to hospital discharge (ELSO Registry). Randomized studies are limited to two landmark trials analyzing the impact of VV ECMO for respiratory failure. These include:
CESAR Trial
- Published in 2009, the study randomized 180 patients with severe respiratory failure to a care at an ECMO center vs. conventional management at a non-ECMO center
- Significant difference in primary outcome (survival without severe disability) favoring referral to ECMO center (63% vs. 47%, P = 0.03)
- However only 75% patients who were referred to ECMO center received ECMO, limiting inference about the benefits of ECMO therapy itself
EOLIA Trial
- Published in 2018, the study randomized 249 patients with severe respiratory failure to VV ECMO vs. conventional ARDS treatment (control group) across 23 centers
- No significant difference in primary outcome (60-d mortality; 35% vs. 46%, P = 0.09)
- However, large proportion of crossover from control group to ECMO group confounded results
Venous-Arterial (VA) ECMO
Indications
VA ECMO is indicated for cardiogenic shock, postcardiotomy shock, and in patients with acute-on-chronic cardiomyopathy as bridge therapy.
Hypoxia should be addressed prior to making the decision to initiate VA (vs. VV ECMO), as often times this will result in correction of the patient’s hypotension. Patients who are both hypoxic and hypotensive deserve additional consideration. Specifically, hypoxic patients with mild hypotension or low-dose vasopressor requirement in the setting of previously normal heart function will likely require VV rather than VA ECMO. Correction of the hypoxia with VV ECMO will often resolve the hypotension as well.
Contraindications
Absolute contraindications to VA ECMO include:
1. Nonrecoverable cardiac injury in a non-candidate for transplant or VAD
2. Aortic insufficiency
3. Contraindication to anticoagulation
Relative contraindications include:
1. Advanced age
2. Chronic end-organ failure (ESRD on iHD, cirrhosis, COPD)
Access
VA ECMO can be established centrally, via ascending aortic arterial and right atrial venous cannula, or peripherally. Peripheral access can be established via bi-femoral, IJ-axillary, or IJ-common carotid cannulation. If femoral access is utilized for the inflow cannula, a distal limb perfusion cannula can be placed in the superficial femoral artery to reduce the risk of ischemic limb complications.
Management
As with VV ECMO, the patient is heparinized prior to cannulation and maintained on therapeutic anticoagulation during their ECMO run. However, in contrast to VV ECMO, due to the elevated risk of systemic embolism during VA ECMO there should be a very high threshold to stop anticoagulation.
Once on VA ECMO, a cardiac index >2.2 L/min/m2 is targeted. Vasoactive agents are weaned as able. Lactic acid and UOP is trended as markers of end-organ perfusion, and serial echocardiography is utilized to monitor native cardiac recovery.
A number of complications should be anticipated while on VA ECMO. These complications (and potential maneuvers to address them) include:
1. LV distension
– Decrease afterload via inclusion of IABP or percutaneous VAD
– Vent the LV via transeptal atrial cannula or surgical LV drain
2. North-South Syndrome (upper body preferentially receives poorly oxygenated blood from the LV during peripheral VA ECMO)
– Optimize mechanical ventilation
– Optimize venous drainage to reduce LV output by adding an additional drainage cannula (VVA ECMO)
– Return oxygenated blood into the superior vena cava to enhance the oxygenation of blood ejected from the LV (VAV ECMO)
3. Limb ischemia
– Addition of distal limb perfusion cannula to SFA on side of peripheral arterial cannula
Decannulation
Patients who are receiving VA ECMO as a bridge to VAD or transplant will be decannulated when bridging is accomplished. For those bridging to recovery, weaning trials should be attempted when improved aortic pulsatility and contraction on echocardiography is noted. Flow should first be reduced to 50%, then 25%, and finally to minimal support (1L/min). If the patient tolerates this without requiring significant increase in pressor dosing, decannulation should be considered.
Outcomes
According to the ELSO data registry, survival to discharge for patients placed on VA ECMO for cardiogenic shock is 41%.
Temporary Ventricular Assist Devices
Temporary ventricular assist devices are additional options in the armamentarium of temporary mechanical circulatory support.
Indications
As with VA ECMO, these devices are indicated for cardiogenic shock.
Impella
The Impella (Abiomed) comes in several configurations for left heart support, including the 2.5, CP, and 5.0. A right heart configuration is also available (Impella RP).
The Impella 2.5 and CP system is inserted percutaneously via the femoral artery on a 9-Fr catheter and advanced in a retrograde fashion up to and across the aortic valve. The device’s tip sits in the LV where blood is extracted and then returned to the patient through an outflow orifice in the patients ascending aorta. The pump itself is internal, and is microaxial. The 2.5 system achieves flows of up to 2.5 L/min and the CP achieves flows of up to 4.3 L/min.
The 5.0 system is inserted via surgical cut-down into the femoral or axillary artery on a 9-Fr catheter. It is positioned across the aortic valve in a manner similar to the 2.5. As its name implies, this pump achieves flows of 5.0 L/min.
As these devices sit across the aortic valve, they are contraindicated in the setting of moderate or greater AS or AI. The presence of LV thrombus is also a contraindication, as is severe peripheral vascular disease. This device requires systemic anticoagulation.
The Impella is approved for 6 hours of use or less, but in clinical practice is typically used on a timing scale of days to weeks.
TandemHeart
The TandemHeart (LivaNova) is a percutaneous VAD system which extracts blood from the left atrium and returns it to the femoral artery. This is accomplished via 21F transseptal inflow cannula, centrifugal pump, and a 17Fr femoral artery catheter. The pump can deliver up to 4L of flow.
Contraindications for the TandemHeart system include VSD, factors prohibiting systemic anticoagulation, aortic insufficiency, and severe peripheral vascular disease. The device is approved for 6 hours of use or less, however reports describe its use for durations of support of approximately a week.
Centrimag
The CentriMag (Abbott) is a surgically implanted, extracorporeal centrifugal pump which is able to provide up to 10L/min of flow. For RV support, the system utilizes a 22 Fr outflow catheter placed in the PA and a 32 Fr inflow cannula in the RA. For LV support, the outflow catheter is placed in the ascending aorta with inflow established in either the LV or LA. Use of the Centrimag is contraindicated if systemic anticoagulation is not possible.
Suggested Readings
- Krishna M and Zacharowski K. Principles of Intra-Aortic Balloon Pump Counterpulsation. Continuing Education in Anaesthesia Critical Care & Pain 2009;9(1): 24–28.
- ESLO Guidelines for VV and VA ECMO. https://www.elso.org/resources/guidelines.aspx.
- Mark Gdowski, Jonathan D. Wolfe, David L. Brown. 46 – Temporary Mechanical Circulatory Support Devices, Cardiac Intensive Care (Third Edition), 2019, pages 478-492, https://doi.org/10.1016/C2014-0-03291-1.
- Julliard W and Teman N. Extracorporeal Membrane Oxygenation: How I Teach It. Ann Thorac Surg. 2020;109(2):325-328.
- http://www.respscore.com/
