Types of oxygenators — advantages & disadvantages ECMO

Vortex pumps versus Roller pumps for ECMO

Potential problems of circuitry for long term ECMO & solutions

Emergency ECMO following cardiac surgery

Veno-arterial & veno-venous ECMO

		ECMO
	Total Heart lung bypass	Partial Heart lung bypass
	*Cardiopulmonary bypass*	*Venoarterial*	*Venovenous*	*Arteriovenous*
*Extracorporeal Flow*	100% of total cardiac output	30-80% of total cardiac output	30-90% of total cardiac output	20-30% of total cardiac output
*Cardiac effect*	Full support	Partial support	None	Extra load
*Duration*	< 5 hours	< 21 days	< 21 days	< 5 days
*ACT*	>400 sec	150 - 250 sec
*% O₂ consumption transported extracorporeally*	100%	20 - 90%	20 - 90%	20%
*% CO₂ consumption transported extracorporeally*	100%	20 - 90%	20 - 90%	50%
*Prevalence*		Most common route used in neonates	Most common route used in adults	Most commonly for dialysis; CAVHD
*Blood drainage sites*	IVC + SVC	IVC	IVC	umbilical; radial
*Blood infusion sites*	Aortic root	Axillary; carotid; femoral	SVC	SVC
*Technique of drainage*		Venous blood drained by gravity from a large bore cannula inserted into either the right internal jugular or the femoral vein & advanced so that tip is in RA	Venous blood drained by gravity from a large bore cannula inserted into the femoral vein & advanced so that tip is in IVC
*Technique of reinfusion*		After blood is passed via membrane lung, it is returned: Adult: axillary; femoral artery Neonate: Right common Carotid	After blood is passed via membrane lung, it is returned through a cannula inserted in the right internal jugular vein and advanced so that its tip lies just above the right atrium
*Distribution of oxygen & decarbondioxide blood*		Non uniform ABGs likely. Dependent on: tip of arterial cannula; magnitude of extracorporeal flow	Uniform ABGs. All returned blood enters RA before being mixed & ejected by heart, Efficiency of extracorporeal gas exchange dependent on proximity of the two cannula tips

Cardiopulmonary bypass

Venoarterial

Venovenous

Arteriovenous

Advantage for neonates

Total systemic flow is is a summation of cardiac output & extracorporeal flows; provides partial CPB support by decreasing preload & native cardiac output.

Provides cardiac support in severe pulmonary hypertension; offers a wider safety net for haemodynamically labile patients

Elevated pulmonary artery oxygenation; may aid in lung healing; minimal disturbance of cerebral vessels & circulation but cannot support failing heart

· Percutaneous cannulation obviates the need for extensive surgical procedures and may reduce local bleeding problems so that support can be established rapidly and almost at the bedside

· Adult Veno-veno ;

· percutaneous cannula; both femoral vein - long cannula inserted just below RA (pumping oxygenated blood in) , short cannula inserted into femoral vein (draining blood) - designed to minimise venous blood mixing - returned blood enters right heart to be ejected

· Concern is CO2 removal - still oxygenated by lungs - may use 2 oxygenators in parallel - for maximal CO2 removal - and are able to replace a failing oxygenator

· Are affecting patients filling pressure but minimal effect on cardiac output

· Adult veno-arterial

· Femoral vein (percutaneous) to carotid artery (femoral type cannula cut down)

· Similar circuit to veno-veno

· Danger of draining heart & reducing effective ejection - hypoperfusion of brain & heart (coronary ischaemia in a patient who is not initailly cardiac compromised (only lungs) - if heart is not ejecting properly are not perusing myocardium effectively (even with adequate mean pressures)

Types of oxygenators — advantages & disadvantages ECMO

1. SciMed

a) Description

i) Most widely used lung for ECMO

ii) Only oxygenator specifically designed for long term use

iii) Long, spirally wound envelope of silicone rubber

iv) Gas circulates within the envelope and blood passes lengthwise between the windings of the spiral

v) Available in a variety of sizes up to 4.5 m²

a) 1 m² of oxygenator surface per 10 Kg

vi) Expensive

b) Advantages

i) Exceedingly reliable

ii) Excellent gas exchange; able to maintain stable CO₂ & O₂ for long periods of time (weeks)

iii) Low priming volumes

iv) FDA approved

c) Disadvantages

i) Requires high perfusion pressures to overcome its high resistance

ii) Difficult to prime (vacuum prime - avoid SPPS)

iii) Difficult to modulate oxygen exchange

a) Must choose appropriate size SciMed for patient

(CO₂ controlled by gas flows)

2. Hollow Fibre

a) Description

i) Microporous capillaries, around which the blood flows, conduct the sweep gas

b) Advantages

i) Low resistance

ii) Low priming volumes

c) Disadvantages

i) Leak plasma into gas phase after 18 — 48 hours

ii) Becomes a ultrafiltrater, results in changes in losses sodium, losses in heparin & reduced CO2 removal, & even losses of volume (up to 800 mls lost per hour)

3. Bubbler

a) Not applicable to ECMO

i) High degree of trauma to blood components

ii) High risk or air embolisation

ECMO heat exchangers

1. During rapid warming required for cardiac surgery, it is important to place the heat exchanger proximal to the oxygenator

2. Theoretical problem of generating bubbles by heating saturated blood has proved to be less relevant for the more gradual heat gradients seen during ECMO

3. In long term extracorporeal circulation, normothermia is maintained

4. Best position to place heat exchanger is after the membrane lung

a) Allows blood to be at normothermia just prior to entering body after blood has cooled in oxygenator

i) Membrane lung ventilating gases cool when expanded from high pressures

ii) Significant evaporative heat loss as the dry ventilating gases become saturated with water vapour

iii) Higher gas flows exacerbate cooling effect

b) Especially relevant to infants & neonates; heat exchanger position is less important in older children and adults

5. Countercurrent water flow from the heater must warm the blood to body temperature without excessive localised heating that may cause haemolysis

6. After leaving the oxygenator, blood flows through a separate heat exchanger where it is warmed to 37°C

7. Placing the heat exchanger immediately after the oxygenator will warm the blood before it returns to the patient

8. In adults not so important as may be using an integrated oxygenator-heat exchanger

Vortex pumps versus Roller pumps for ECMO

1) Centrifugal Pump

i) Popular for ECMO

ii) Cannot pump large quantities of air

iii) Allow blood propulsion without concern about the disastrous effects of undetected, inadvertent outflow or inflow occlusion of the pump

(a) Centrifugal pump stalls whenever occlusion occurs without generating high suction or outlet pressure

(b) Does not require a continuous perfusionist presence

iv) Haemolysis & other damage to formed blood elements is similar to that encountered with roller pumps??

v) Decreased entrainment effect

vi) Reduced wear & tear on the system (pump head; high pressures)

2) Roller pump

i) Inexpensive

ii) Predictable pump flow based on pump speed

iii) Can pump large quantities of air

iv) Potential to overpressurise circuit if inadvertently clamp/obstruct

v) Failure of pump rare during prolonged extracorporeal use

(a) Note that maintenance is due on a pump after a prolonged trial of ECMO - a 7 day run = 144 average CABG cases

vi) Cumulative wear on the segments of plastic that lies under the roller head is the most frequent cause of failure

(a) Routinely feed pump head tubing along to distribute wear

(b) Mount 2 pumps in parallel

vii) Some prefer roller for ECMO

(a) Simpler design-less moving parts to malfunction

(b) Better-quicker response to servocontrol

Safety devices used in ECMO

1. Mixed venous saturation monitoring of blood siphoned from RA

a) Venous-arterial

i) True mixed venous saturation

ii) Dependent on

a) Hct

b) SaO2

c) CPB flow rate

d) Patient’s own cardiac output

e) Rate of tissue metabolism

b) Venous-venous

i) Not a true mixed venous saturation due to recirculation of blood

ii) Magnitude of reading less important than trends

2. Servoregulation

a) Reduced venous return collapses bladder thereby turning off pump thereby filling bladder with blood thereby starting pump

b) This system operates better with roller pump than centrifugal due to more rapid response (centrifugal pump continues to spin even when turned off)

3. Pressure monitoring

a) Transducers connected immediately before and after the oxygenator

i) Provide continuous assessment of oxygenator function in terms of blood flow characteristics

ii) A high pressure drop across the oxygenator indicates impending lung failure & need for replacement

iii) High post oxygenator pressures may indicate cannula kinking or use of too small a cannula

4. Heparinisation

a) Loading dose 100U/Kg

b) Infusion of 30 - 60 U/Kg/hr to maintain ACT 180 - 200 secs

c) ACT measured hourly

5. Arterial filter

a) Optional (not required on veno-venous)

b) May exacerbate platelet consumption

c) Traps air, thrombi & other emboli

6. In line bubble detector

a) Detects microbubbles in arterial line and turn off pump

7. In line blood gases

8. Backup battery pack + hand cranks in case of power failure

9. Venous reservoir level detectors

Potential problems of circuitry for long term ECMO & solutions

1. Tubing rupture in roller pump head

a) Walk tubing to distribute wear regularly (eg every 8 hours)

b) Mount 2 pumps in parallel

i) Note that both pumps must be rotating or clot will form in stagnant blood

ii) Therefore alternate high & low flow pumping 537 Hensley

2. Oxygenator failure

a) Eg due to clotting, use of antifibrinolytics

b) Assessed by: transmembrane pressures, ABGs, elevated FSP or decreased platelet in absence of sepsis, visible clots

c) Mount 2 oxygenators in parallel

d) Oxygenator capillary blockage (thrombosis)

e) Plasma leakage

3. Infection

a) All stopcocks attached to the ECC must be rinsed after use with an antibiotic solution

b) Minimise blood sampling

c) Blood cultures at first sign of infection

4. Pump outflow occlusion

a) Malpositioning of cannula when patient is repositioned

b) Secure all positive pressure connectors with nylon ties to prevent blow off of tubing

c) Use centrifugal pump

5. Thrombus formation

a) Illuminate circuitry & components regularly with a torch to identify clots

b) Monitor ACT regularly

c) Increase ACT if clots become visible

d) If extensive clotting; require change of whole circuit

6. Air in circuit

a) Sources include via improperly used stopcock, air in IV solution, rupture in circuitry, occluded gas outflow in oxygenator

7. Cannula problems

a) Venous

i) Impaired venous drainage

b) Arterial

i) Malpositioning may lead to:

a) Vessel damage/dissection

b) Catheter dislodgment

c) High inflow pressures & line resistance

c) Confirm position with X-ray or cardiac echocardiography

8. Routinely change the whole circuit every 100 hours in a staggered technique to minimise haemodilution

9. Pressure area care - log roll - spinal bed

Emergency ECMO following cardiac surgery

1) Essentially maintain full CPB post failure to wean

2) ‘Rest’ left heart & right heart by drainage/decompression

3) Monitor filling pressures

4) Mild cooling eg 35°C to reduce oxygen consumption, pump flow etc

5) Ventilator on at a few breaths/minute to reduce atelectasis

6) Normal ACTs

7) Maintain good perfusion pressures to perfuse heart & lungs

8) Anticipate coagulopathies when weaning

9) Time periods expected?

10) Versus LVAD, RVAD & BVAD?

11) Monitor BSL, adequacy of flows (SvO2, BE)

12) Must have a well vented/decompressed heart

13) Indication/benefits

a) fail to wean from CPB

b) Less stress on opposite ventricle cf VAD

c) Easier circuitry than BVAD

d) Can turn off inotropes & rest heart (unlike LVAD, BVAD, RVAD)

e) IABP - keep on for pulsatile flow

f) Heat exchanger (unlike VAD)

14) Minimum 12 hours; assess at 3 hours - if heart improves - a further 6 hours - assess every 3 hours looking for improvement - maximum 12 - 18 hours

KCPotgerã