Abdominal counter pressure in CPR: what about the lungs? An in silico study

The external pumping action in CPR should generate sufficient flow and pressure, but the pump must also be 'primed' by ongoing venous return. Different additions to standard CPR are in use just for this purpose. Active decompression of the thorax (ACD-CPR) to 'suck in' venous blood has proven successful, but, theoretically, compression of venous reservoirs in the abdomen should be even more effective. We compared different techniques for improved CPR with specific attention to the pulmonary circulation. We did our comparisons 'in silico' rather than 'in vivo' in a well-evaluated computer model.

We used an adapted version of Babb's computer model for CPR, reprogrammed in Matlab(®). (1) We compared standard chest compression-only CPR (CO-CPR) and ACD-CPR to CPR with interposed abdominal compression (IAC-CPR). (2) Since the thorax/heart configuration differs between patients, and consequently the way blood is propelled by the chest compressions, we checked the influence of the ratio thoracic/cardiac pump effectiveness.

(1) Only IAC-CPR leads to physiological values for mean aortic pressure and cardiac output. (2) However, since the whole heart is in the pressure chamber of the compressed thorax, pulmonary artery pressure rises to about the same level as aortic pressure. In practice, this might lead to pulmonary edema during and after CPR, unless (3) intra-abdominal compression pressure is strictly limited; simulations indicate that intra-abdominal pressure should not exceed 30-40 mmHg.

IAC-CPR outperforms the other techniques in achieving good aortic pressure and cardiac output. However, abdominal pressure should be limited.