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From Brown Paper Bag to CABG Booklet

By Susan Eymann, MS07 Oct 2015

flow-assessmentIt was a treasure trove of raw data. Reams of strip chart recordings from coronary artery bypass graft (CABG) cases filled a brown paper bag to its brim. Chief of Cardiothoracic Surgery at a suburban New Jersey hospital, Dr. Bruce Mindich was handing over for analysis the recordings of 88 representative CABG cases culled from the more than 500 cases he had performed up to that time in early 2002. Ninety-five percent of the cases had been performed off-pump; 5 percent had been performed on-pump.

At Transonic Systems, the 300 chart recordings were first cut into pieces, and were mounted on letter-size sheets. They were labeled as Case No. 1 through Case No. 88. Then the analysis began. In the next several weeks, waveforms of each type of arterial or venous graft to each coronary target were collated and compared. LIMA-LAD and LIMA-Cx graft waveforms abounded. RIMA-RCA and SVG-PDA had fewer representative examples. The flow profiles of each graft were examined. Left heart grafts were compared to right heart grafts. Grafts exhibiting competitive flow were analyzed. The mean flows of all the grafts were scrutinized. Pulsatility indices were calculated. Each case was methodically examined to ascertain its unique condition.

Slowly, a method for interpreting CABG flow waveforms and rules of thumb for successful flow-based graft patency assurance emerged. They are:

Rule No. 1: Measure properly with a correctly sized flowprobe, with adequate ultrasonic couplant to assure good signal quality. Avoid motion artifacts. Occlude the native coronary to dismiss the presence of competitive flow.

Rule No. 2: Assess mean flow to confirm graft patency. Normally, 20 mL/min or higher mean flow indicates a patent graft. Graft flows under 5 mL/min indicate a graft in trouble. Check for kinks or twists in the graft, vasospasm, clotting, MAP or a wayward stitch. Redo anastomosis, if technical error is indicated.

Rule No. 3: Apply waveform analysis to grafts in medium flow range (> 5 mL/min - < 20 or 30 mL/min). Check if the flow exhibits the expected pattern for its location (left heart versus right heart grafts). Assess any other factor that may account for a lowered flow such as small target vessel, small patient, small graft capacity or poor runoff.

The first Flow-based Intraoperative Coronary Graft Patency Assessment handbook began to take shape. Engineer Cor Drost, Transonic founder, wrote a chapter on the physiology of graft flow that included painstakingly rendered schematics of a bypass and coronary flow circuit, graft flow distribution to the left and right ventricles and a schematic of a bypass flow circuit when competitive flow is present. A coronary graft patency assessment protocol was formulated. The rules of thumb were incorporated into the 80-page booklet along with 97 discrete graft waveforms representing 24 distinct types of grafts and three native conduits. Interesting case studies were added, along with an FAQ section and an appendix that included information of other modalities for graft flow assessment, transit-time ultrasound technology and a reference list of off-pump and on-pump publications.

The first edition of the handbook was published in 2002 and was distributed worldwide to cardiothoracic surgeons. It was translated into Japanese and Chinese. Some surgeons referred to it as their bible for flow-based graft assessment. The rules of thumb presented in the booklet have stood the test of time. Mean flow is the key parameter for flow assessment. Waveform analysis is advised when flows are in a “questionable range.” Competitive flow should be checked when flows are lower than expected.

Moreover, the conclusion from Dr. Mindich’s abstract presented to the New York State Thoracic Society in 2001 still is relevant. “The intraoperative use of flow measurements provide invaluable information in a timely, accurate, cost-effective manner allowing for the surgical correction of a surgical problem. This has significantly reduced the complications related to early technically induced graft failure. In an era of rapidly changing surgical technique, this provides documentation of the sine qua non of the operation: patency.”

CABG Flow Interpretation