Definition and Features
Access insufficiency is a state where the suction pressure at the access cannula is excessive in relation to the venous return. The inflow is interrupted by intermittent or partial occlusion of the inlet ports on the access cannula by the walls of the surrounding collapsible vein. Features depend on the severity, the type of access cannula and the equipment platform.
To diagnose access insufficiency accurately both features will be present:
Variable or falling blood flow with or without visible and palpable access line movements
Immediate improvement in ECMO blood flow with reduced speed (rpm)
Additional features may include
- High variation in the measured blood flow; and therefore falling average flow
- Blood flow will NOT increase with pump speed increases, instead, blood flow will fall further and line “chattering” will worsen. (link video)
- Multi-stage cannulae may demonstrate partial occlusion of the cannulae access holes without any visible or palpable line movement
- In VA ECMO, pulmonary or aortic pulsatility may be lost as the right side of the heart is “sucked-down” and the preload to native heart reduced
- The Cardiohelp/HLS system incorporates negative access pressure (pre-pump head) monitoring. Negative pressures usually increase before access insufficiency is clinically evident. Increasing negative pressures associated with stable or falling blood flow indicates early access insufficiency
Access insufficiency at times will be more subtle e.g in the setting of a thrombosed access cannula, overlapping with high negative access pressure. It can result in haemolysis with a rise in plasma free Hb as the key presenting feature.
Insert video – heart pulsation
Access Insufficiency is caused by inadequate venous return relative to the degree of negative access cannula pressure. Causes range from benign to life-threatening in arterial failure with bleeding or regurgitant blood in the pulmonary circulation. It is also not a sensitive sign and maybe be absent for example in cardiac tamponade or deceivingly maintains some circulation in significant bleeding.
- Hypovolaemia, always consider bleeding
- Poorly sited access cannula (too low)
- Cardiac tamponade (common post sternotomy)
- Excessive RPM setting
- Patient coughing or straining
- Other reasons for increased intrathoracic pressure i.e. tension hydrothorax
- Positional (after turning the patient)
- Acute vasodilatation (sedation bolus)
- New onset of sepsis
- Increased intra-abdominal pressure
- Severe aortic regurgitation / fatal pulmonary haemorrhage (VA ECMO)
Conceptually, the causes can be thought of as related to cannula position, volume status (including vasodilatation), increased pressure in the thorax or abdominal compartment.
The immediate treatment is to reduce the speed settings until the features of access insufficiency disappear whilst attempting to maintain adequate patient support. If the former pump speed settings cannot be re-established, then a bolus of fluid should be administered to allow time to investigate the cause of the access insufficiency in parallel.
Repeated episodes should not be treated with fluid, as volume overload/oedema will arise unless hypovolaemia/ bleeding has been established as the cause. If unclear, cardiac tamponade and bleeding must be excluded. If no specific cause can be found and/or the patient requires deep sedation to maintain blood flow, an additional access cannula may be required to access sufficient venous return.