Assess adequacy of ECMO support and setting
For VV ECMO target blood flows must provide adequate arterial oxygenation while allowing non-injurious lung ventilation.
For VA ECMO target blood flows in combination with the native cardiac function must provide adequate systemic cardiac output and oxygen delivery. VA ECMO support must also allow enough pulmonary circulatory blood flow to prevent (1) clot formation within the heart and pulmonary vasculature (2) provide enough pulmonary oxygen delivery particularly in lung transplant patients and (3) ensure adequate decompression of the left and right heart chambers.
Patients on VA ECMO need optimal physiologic support and hemodynamic loading conditions to achieve the following key VA support outcomes
- Ensure adequate end-organ perfusion (brain, kidneys, gut, liver, peripheries, lungs) and oxygen delivery
- Allow myocardial recovery
- Prevent intra-cardiac complications (LV thrombus, pulmonary oedema, excessive LV wall tension/ischemia due to LV dilatation, moderate-severe functional mitral regurgitation)
Clinical parameters to guide assessment
Target parameter of adequate circulatory flow and oxygen content
- ECMO blood flow (+ native cardiac output) > 2.0 l/min/m2
- MAP > 65 mmHg
- SpO2 > 90%, Haemoglobin > 80 g/L
Markers of adequate end-organ perfusion
- Mixed venous saturation > 60%
- Lactate < 2.0 mmol/L
- ETCO2 >20 mmHg (depends on individual desired pulmonary flow)
- Warm peripheries
- Urine output and gastrointestinal function have to be interpreted in the context of the critical illness
- A Faecal Management system (FMS) is often beneficial for these patients. Discussion regarding initial insertion needs to be on the ICU ward round ( or V round) and the contraindications for insertion need to be reviewed prior to insertion. Insertion needs to be by an accredited FMS practitioner.
Consideration for myocardial recovery
- Control of dysrhythmias and heart rate < 110 beats per minute
- Exclusion of coronary flow limitation
- SpO2 > 90% via right radial arterial line representing coronary saturation if pulsatile
- Estimation of LVEDP/ LV filling pressures
- Exclude severely increased LV end-diastolic volume
- Minimise inotropes
Prevention of intra-cardiac complications
- Pulse pressure > 10 mmHg on the arterial line
- No pulmonary oedema on CXR
- Echo features
- LV internal diameter normal size or at most mild to moderately dilated with no progression of dilatation
- MR only mild
- No LV thrombus
- Aortic valve opening with each cardiac cycle
Setting ECMO blood flow
Setting blood flow is determined by systemic oxygenation targets and desired oxygen delivery. The peripheral oxygen saturation is determined by the fraction of cardiac output that is captured by the ECMO and the residual lung function/ ventilator settings.
Minimum blood flow on VA ECMO must allow for adequate support as described in the previous section any change beyond this effects of change and circumstances need to be considered.
Changing the flow rate has the following effects
- Biventricular preload reduces with increasing VA flow rates (and vice versa)
- Effects on afterload with increasing blood pressure as the blood flow is increased – may require intervention
- Ratio of extrinsic versus intrinsic contribution to systemic perfusion increases with higher flow rates
Special circumstances in VA ECMO
- Aortic regurgitation and afterload needs to be observed with higher blood flows
- Differential hypoxia may increase with lower blood flow rates
- Pulse pressure and LV ejection may increase with lower flow rates
- In the absence of aortic regurgitation, LV distension may improve with higher ECMO flow rates due to less cardiopulmonary blood flow
Setting the ventilator
Ventilation in ECMO patients adds to the gas exchange at the oxygenator and should, therefore, be managed in consultation with the ICU consultant.
Lung Ventilation strategies for patients on VA ECMO
Ventilator settings are chosen to provide the following outcomes
- Maintain adequate lung aeration i.e. prevention of atelectasis and maintenance of normal FRC. This is achieved through adequate levels of PEEP and providing some ventilation to the lung.
- Adjust the lung ventilation in relation to the lower native pulmonary artery blood flow generally aiming for ETCO2 levels (depending on dead space and native cardiac function) of 20 to 30mmHg.
- Generally, the ventilation is set to a low level and the fresh gas flow is used to adjust to target CO2 (e.g. Volume controlled ventilation 350ml, frequency 10/min or pressure-controlled ventilation PS 10mmHg over PEEP frequency 10/min).
- In lung transplant patients on VA ECMO support, it is essential to provide maximal blood flow to the lungs. In the absence of blood flow through the bronchial arteries, the lung parenchyma depends on oxygen supply via this route. Aim for ETCO2 >20 mmHg if possible. Loss of pulsatility/ ETCO2 waveform requires immediate assessment. In particular changes in volume state or bleeding are important.
- In predominantly right ventricular dysfunction, minimising RV afterload is desirable and a carefully balanced with mean airway pressures should be maintained.
Effects of changing PEEP
- Reduced LV distension (i.e. APO) with higher PEEP
- Less collapse/atelectasis, and improved oxygenation with higher PEEP
- Higher RV afterload, less RV preload with higher PEEP
- Improved oxygenation and reduced RV native CO will reduced differential hypoxia with higher PEEP
Fresh gas flow (FGF) and minute ventilation
- Minimal flow rate should be at least 50% of blood flow rate
- The primary way to achieve near-normal PaCO2 (PaCO2 35 – 45 mmHg) is the FGF rather than changing both ventilator and FGF
- Note the end-tidal CO2 is a useful indicator for lung perfusion, generally >20 would suggest reasonable lung perfusion
- Also consider the effect of elevated CO2 on pulmonary vasoconstriction and increased RV afterload
- With pulsatility present and native cardiac output reaching the right radial artery, FiO2 can be titrated to right arm SpO2 (aim 94-98%)
- If retrograde flow is perfusing the right arm (minimal pulsatility), it is important to remember that coronary blood flow is still likely to be determined by ventilator FiO2 should be considered whenever manipulating the ventilator.
- Pulmonary and systemic hyperoxia may have adverse pro-inflammatory effects and are currently investigated
Use of inotropes
Commonly used inotropes are referenced in mcg/min in our unit. Inotropes once on VA ECMO should be used with caution and generally be reduced. Their effect will depend on the loading conditions altered by the ECMO support and the myocardial recruitability. Seizing the opportunity to ‘rest’ the heart rather than stimulating intrinsic cardiac output is a common approach early in the ECMO run.
The titration may achieve objectives such as aortic valve opening but have to be balanced with potentially detrimental effects:
- Dysrhythmias in particular ventricular arrhythmias
- Increased myocardial oxygen demand with potentially reduced myocardial recovery
- APO may worsen if RV contraction (recruitment) improves out of proportion to LV contraction
- Differential hypoxia may worsen due to increased cardiac output
Volume status, just like in any other ICU patient, is a key part of daily management. The following implications are relevant in ECMO.
Volume overload (consider prior to volume administration)
- Impaired gas exchange
- Vicious cycle of deteriorating oxygenation in VV ECMO requiring higher blood flow, temptation to administer further volume
- Liver and kidney venous congestion with associated organ impairment
- Myocardial LV distension in VA ECMO with potential disastrous aortic annulus dilation and increasing aortic regurgitation
Volume depleted states
- Impaired oxygen delivery
- Delayed renal recovery
- Hypotension with inappropriate vasopressor use
Comments on assessment
- Presence or absence of access insufficiency is not an accurate reflection of the intravascular fluid state
- CVP measurements need to be interpreted with caution and may not be accurate in the proximity to the access cannula
- Echo can be a useful modality in particular to assess cannula position right heart filling