Case 1

The pre-pump side of the ECMO circuit is under negative pressure. Centrifugal pumps are preload-dependent, meaning ECMO blood flow depends on adequate venous return to the drainage cannula. When venous return lags behind the negative pressure generated by the pump, drainage insufficiency occurs. As the pressure drop along the drainage cannula increases, a higher venous return is required to maintain equilibrium. When this is not achieved, the non-rigid vasculature around the drainage ports collapses, occluding the drainage holes and causing intermittent or sustained loss of blood flow. This is analogous to native cardiac physiology where the heart is preload-dependent.

Answer: B. Pulmonary embolism is NOT associated with drainage insufficiency because it does not decrease venous return to the drainage cannula. PE increases right ventricular afterload and pulmonary vascular resistance, but venous return to the ECMO circuit is preserved. 

In contrast, tension pneumothorax (obstructive shock reducing venous return), hypovolemia (absolute reduction in circulating volume), and excessive pump speed (generating excessively negative drainage pressures) are all recognized causes of drainage insufficiency.

A sequential approach is recommended: 

(1) First, reduce pump speed until blood flow is stable. If the patient remains adequately supported at the lower speed, maintain it. If not, incrementally increase while monitoring for recurrence. 

(2) Evaluate the patient and circuit for clinically evident etiologies—including the recent repositioning, which may have caused cannula malposition. Obtain blood work and diagnostic imaging as indicated. 

(3) Assess fluid responsiveness. In volume-responsive patients, resuscitate guided by clinical response, but cease once volume replete. 

(4) If drainage insufficiency persists despite treating identifiable causes and achieving volume repletion, the blood flow requirement exceeds what the current drainage cannula can provide, and an additional drainage cannula should be considered.

Placement of an additional drainage cannula creates a VV-V ECMO configuration. The second drainage cannula is Y-connected with the first. The physiologic rationale is that an additional drainage cannula allows the same (or greater) blood flow at significantly reduced drainage pressures. For example, whereas a single 25 Fr cannula may provide approximately 4.5 L/min of flow, a 25 Fr plus a 23 Fr cannula together may provide approximately 6.5 L/min for a similar drainage pressure. By distributing venous drainage across two sites, the negative pressure demand at each individual site is reduced, resolving the mismatch between venous return and drainage requirements.

Answer: B. Progressively negative drainage pressure with chattering (kicking) of the drainage line is the hallmark finding. The chattering correlates with intermittent drops or cessation of blood flow as the vasculature collapses and re-opens around the drainage ports. 

Choice A (increased post-membrane pressure with stable flow) suggests return obstruction, not drainage insufficiency. Choice C (both lines bright red) suggests recirculation. Choice D (elevated pfHb with dark urine) suggests hemolysis.