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Lower Extremity Bypass |
Intraoperative Blood Flow Measurement - PDF |
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Courtesy of Ian Gordon, MD, PhD, Assistant Professor, Department of Surgery, University of California, Irvine
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 Surgical Approach
- After completing a lower extremity arterial bypass, flow is measured before closure of
the wound. Generally, we measure flow immediately after finishing the last anastomosis of the bypass and do not reverse heparinization.
- A 4 or 6 mm flowprobe is employed for saphenous vein, a 6 mm probe is employed
for the popliteal artery, an 8 or 10 mm probe for the common femoral artery, and a 4 mm probe for tibial arteries.
- Dacron grafts, which we very rarely use distal to the common femoral artery, allow direct measurement of flow by transit time
probes. Expanded PTFE grafts cannot be studied immediately by a probe placed on the graft, as air trapped in the graft interstices
interferes with ultrasound transmission, and an accurate measurement is not possible until this gas is expelled.
- Three methods, A, B, C, are employed to measure flow. Method A is suitable for saphenous vein or dacron; and Methods B
and C are useful for PTFE. We frequently employ Method C to measure the distribution of flow beyond the distal
anastomosis in retrograde and antegrade directions. Method B is employed whenever exposure of the distal vessel receiving the bypass is poor, and
placement of the probe on both sides of the distal anastomosis is difficult. Probes with back exiting cables are easier to use when exposure is poor.
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Probe Sizes |
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- saphenous vein HQ 4SB or HQ 6SB
- popliteal artery HQ 6SB
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- common femoral a. HQ 8SB or HQ 10SB
- tibial artery HQ 4SB
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Before: Mean Graft Flow, 45 ml/min
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After: Mean Graft Flow, 90 ml/min
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These waveforms show a case where a previously placed vein graft had a
stenosis near its origin. The waveform on the left shows the flow before connecting the stensosis. The waveform on the right shows the flow pattern after correcting the
stenosis.
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Method A: Saphenous Vein or Dacron |
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To measure flow
with saphenous vein or dacron, the probe is first placed just distal to the proximal anastomosis [position A]. Average flow [0.1 Hz filter] and the flow vs. time waveform
[30 Hz filter] recorded with either an analog chart recorder or microcomputer using the (-P) Option. The presence of a hemodynamically significant stenosis causing
turbulence is easily detected by a characteristic artifact in the flow vs. time waveform [Fig. 1]. Similarly, the distal anastomosis is studied for turbulence by placing
the flowprobe on the target vessel for the bypass just distal to the distal anastomosis [position B].
Assuming no technical problem requiring graft revision is present, we carry out our definitive flow measurement.
The probe is placed on the bypass at any convenient position and flow is measured. We routinely measure flow with the graft temporarily clamped to confirm zero flow is
accurately measured. The flow in the graft [Fgraft] is then measured again. In order to measure resistance of flow through the graft and into the distal run-off vessels,
we measure the pressure drop across the graft. A 26 gauge needle connected to a three-way stopcock and connected by plastic extension tubing to a sterile pressure
transducer [usually the anesthetist's radial artery catheter transducer] is brought onto the surgical field. The bypass graft is punctured by the needle several cm distal
to the proximal anastomosis. The mean pressure with the graft open [Popen] and with a clamp occluding the graft proximal to the needle[Pclamp] are recorded with the
assistance of the anaesthetist. After finishing the measurements, the needle hole is closed with a 6-0 suture.
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Method B: PTFE Femoropopliteal Bypass |
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In a typical PTFE
femoropopliteal bypass, the graft origin is from the common femoral artery. An 8 or 10 mm probe is placed on the common femoral artery just proximal to the bypass
(Position D). Flow in the common femoral artery is then measured with the graft open [Fopen]. or clamped [Fclamp]. Net graft flow is equal to (Fopen - Fclamp). Resistance
is measured as in Method A.
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Method C |
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A 4 or 6 mm probe is placed on the target vessel just distal to the distal anastomosis [position B] and antegrade flow is measured with the
bypass graft open [Fao] and clamped [Fac]. To measure retrograde flow, the probe is placed on the target vessel (Position C) just proximal to the distal anastomosis and
flow is measured with the graft open [Fro] and clamped [Frc]. Care need be taken to ensure that the direction of blood flow is carefully observed and negative and positive
signs correctly employed to accurately measure flow. Net graft flow is calculated as (Fao - Fac) + (Fro - Frc). Again resistance is measured as in Method A.Note: since
pulse is a manifestation of pressure, not flow, an occluded graft may still have a distinct pulse. If two or more of the above criteria are met, it is generally felt that
the graft is not acceptable and should be revised.
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