Suspected aortic dissection on CPB: no flow, no pressure, NIRS dropping
The perfusionist reports 'can't get full flow — pump pressure is off.' Arterial line pressure drops. NIRS falls with a left-right asymmetry. Dissection or a cannula problem? Evaluate the visible aorta on TEE while recognizing where you cannot see.
Intraoperative setup
Aortic valve replacement. Shortly after initiation of CPB, the perfusionist reports inability to achieve target flow with unexpectedly high pump line pressure. Patient arterial line pressure is far lower than expected. NIRS falls bilaterally with asymmetry. Venous return is also poor. The surgeon suspects an aortic problem and requests urgent TEE assessment. Aortic injury and malperfusion are not limited to CPB initiation. They may occur during arterial cannulation, initiation of bypass, cross-clamp application, cross-clamp release, or cannula removal.
What this case is really asking
- Is CPB flow going into the true lumen — or is the false lumen being perfused?
- Is there a new intimal flap in the descending aorta or visible arch — the first place to look on TEE
- How to handle the ascending aorta — recognize the TEE blind spot and plan for epi-aortic echo or direct surgical inspection
- Does the NIRS asymmetry suggest branch vessel malperfusion from arch involvement?
- Is there a venous drainage problem — check cannula position simultaneously with the aortic assessment
TEE clues to notice
- Descending aorta (look here first): new intimal flap, true/false lumen identification, false lumen expansion, flow distribution
- Aortic arch (visible range only): flap, atheroma, dilation, possible branch perfusion abnormality
- Ascending aorta (limited): only the proximal portion is reliably assessed — the distal ascending and proximal arch is a TEE blind spot (trachea and right mainstem bronchus), which is exactly where the cannula and cross-clamp sit
- Pericardial effusion/hemopericardium: blood in the pericardium is a direct sign of aortic injury or dissection
- LV function: secondary deterioration from coronary malperfusion or LV distension from inadequate venting?
- Venous cannula position (check simultaneously): is it in the correct RA/SVC/IVC position? RA wall suction or RA collapse visible?
Likely pattern
Four patterns to organize the differential. Low flow on CPB is not caused only by dissection. Aortic Dissection / False Lumen Perfusion: New intimal flap visible in the descending aorta or arch. False lumen perfusion suggested by flap mobility, false lumen expansion, or asymmetric flow distribution. NIRS asymmetry supports branch vessel malperfusion from arch involvement. If this pattern is identified: do not continue escalating flow — immediately discuss cannulation site change or re-cannulation with the surgeon and perfusionist. TEE Negative But High Clinical Suspicion: No flap visible in the TEE-visualized segments, but the clinical picture (low flow, elevated pump pressure, NIRS drop) strongly suggests dissection. 'No flap in visualized segments' does not exclude ascending aortic dissection — the ascending aorta, cannulation site, and cross-clamp location are outside TEE's reliable assessment zone. Request epi-aortic echo or direct surgical inspection. Femoral Perfusion-Related Descending Dissection: When femoral artery cannulation is used, a new flap in the descending aorta is the most accessible diagnostic finding. Caution: temperature gradients and laminar flow patterns immediately after CPB initiation can transiently mimic an intimal flap. Confirm that the flap is persistent, mobile, and associated with flow in the false lumen before committing to the diagnosis. Venous Drainage Problem: Low flow on CPB can result from venous cannula malposition — RA wall suction, excessive SVC/IVC advancement, or RA collapse. This is TEE-visible and must be checked in parallel with the aortic evaluation. Do not attribute all low-flow states to dissection without checking the venous side.
Common pitfall
Thinking CPB initiation is the only dangerous window — aortic injury and malperfusion can occur at cannula insertion, cross-clamp application, cross-clamp release, and cannula removal. Reporting 'no dissection' because TEE appears normal — the correct phrase is 'no flap in visualized segments; the ascending aorta, cannulation site, and clamp site are not fully assessed by TEE.' Attributing all low-flow states to dissection without checking the venous cannula — malposition is a TEE-visible alternative diagnosis. Not integrating NIRS data with TEE findings — asymmetric NIRS drop may indicate branch vessel malperfusion from arch involvement. Failing to capture image documentation during the crisis — emergencies require records more than routine cases do.
Ascending aorta limitation
The ascending aorta, aortic clamp site, and cannulation site are outside TEE's reliable assessment zone. The correct statement is 'no flap in visualized segments' — not 'no dissection.' The distal ascending and proximal arch (where the cannula and cross-clamp sit) are obscured by the trachea and right mainstem bronchus. Epiaortic ultrasound or direct surgical inspection is required for those segments. Absence of TEE findings does not exclude ascending aortic dissection.
Immediate intraoperative takeaway
- 'Low flow, high pump pressure' from the perfusionist is a critical signal — check TEE and NIRS immediately
- Start with the descending aorta — it is the most accessible segment, and with femoral perfusion, often where the first diagnostic clue appears
- 'No flap in visualized segments' is not the same as 'no dissection' — request epi-aortic echo or direct surgical inspection for the ascending aorta
- Always check venous cannula position alongside the aortic assessment — low flow has multiple causes on CPB
- If dissection is likely: do not escalate flow — discuss cannulation site change or re-cannulation with the surgical team and perfusionist immediately