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How do you identify systole and diastole on the curve?
If you have a good flow pattern, it's easy to
identify systole and diastole by their time duration. The rule of thumb is that systole lasts 1/3 of the cardiac cycle (a single heart beat) and diastole is 2/3. Since
flow to the left coronary circulation is primarily diastolic, the peak that is shorter in amplitude and duration is systolic flow, and the higher, broader peak of longer
duration is diastolic flow.
In grafts to the right side of the heart, the
flow is more equally distributed between systole and diastole. This produces a biphasic flow waveform where the peaks are more or less of equal time duration. We observed
this in the case above: the grafts to the LAD, Diaganol and OM had a predominantly diastolic peak of longer duration, while the graft to the acute marginal had two peaks
that were approximately the same duration.
If you don't have a good flow pattern, it's
difficult to identify systole and diastole. However, if the pattern is not good, you already know that you have a problem with the graft.
How do you explain a reading with a
"good waveform" and a 0 ml/min mean flow?
Due to the pressure pulse in the vessel, if
there is an obstruction downstream, it is possible to have blood flow traveling back and forth without a net forward flow. This will appear on the flowmeter's printout as
a curve oscillating around zero. The curve may indicate good pulatility, but the mean flow will be zero. This is why palpation of pulse is not
a reliable indicator of flow. An audio Doppler would also give misleading information in this situation, because it measures motion of the blood, not actual flow.
How do you know if the graft flow is going proximal or distal?
The rationale behind this question is that
measuring graft flow doesn't tell you which direction the flow is going in the native coronary artery. This is not an issue, IF
the surgeon has selected an appropriate target artery for grafting and the distal anastomosis is placed at the correct location, since under these conditions graft flow will naturally flow toward the distal myocardium.
However, given that the flowprobe cannot be
used directly on the native coronary artery, is there any way to assess the direction that flow is going?
Yes, you can occlude the proximal coronary
artery and compare the graft flow measurements with and without occlusion. If graft flow goes down when you occlude proximally, it indicates a high distal resistance and a
tendency for graft flow to be directed proximally. On the other hand, if graft flow increases or remains the same with proximal occlusion, you have confirmed that graft
flow is going in the desired direction à toward the distal myocardium.
For example, if a stenosis exists at the toe of the anatomosis, flow
throught the graft may be mainly retrograde in the coronary vessel and will decrease drastically whenever the proximal coronary snare is applied. (see the paper "Flow
Measurement in Coronary Surgery" by D'Ancona, et al., just published on-line in Heart Surgery Forum; www.hsforum.com/vol2/issue2 for a case example).
Measuring graft flow with the proximal coronary
artery occluded is also a way to assess the degree of competitive flow in the native coronary artery. If graft flow increases during proximal occlusion, there is some
native coronary flow present that is in competition with the graft. See the Transonic Tip Card on Interpreting Coronary Graft Flow Measurements
for more information on the effects of competitive flow.
What you really want to measure is the flow downstream from a kink, right?
Not necessarily. Velocity patterns would be
altered downstream from a kink but would be fairly normal in the proximal segment of the graft. Blood flow, however, would be the same, regardless of where the probe is
placed on the graft. When you have a graft with no side branches (a closed circuit), the flow is the same all along the length of the graft.
What if you have an air bubble in a vein graft or in the distal vessel?
Will the flow measurements detect this?
An air bubble (or any other obstruction to flow, such as a thrombosis or
an anastomotic stenosis) will result in a reduction in mean flow and an abnormal waveform pattern. If graft flow measurements are unacceptable, it is always a good idea to
wait a few minutes and repeat the measurements.
Are there any studies that demonstrate better long term outcomes when
graft flows are measured routinely?
The purpose of intraoperative graft patency
assessment is to detect technical errors that would lead to early graft failure. Flow measurements, per se, do not predict long term outcome, however, they provide an
essential quality assurance check by making sure that grafts are patent when the patient leaves the operating room.
Although a graft could still occlude the
following day or the following week, you certainly improve your patient's chances of having his grafts stay open if you check patency intraoperatively. It's a matter of
reducing the risk of early graft occlusion.
For surgeons who want published references, here are several good papers:
- D'Ancona G, karamanoukian HL, Lalerno TA, Schmid S, and Bergsland J. Flow
Measurement in Coronary Surgery. Heart Surgery Forum, Vol 2 issue 2. Presented at the ISMICS meeting in Paris, May 1999.
10% of the grafts performed off pump were
surgically revised based on intraoperative transit time flow measurements. The authors used a Medistim flowmeter, however, this is the same technology as the Transonic
and CFM 100 Flometers. The parameters the authors used for assessing graft patency include: mean flow, waveform pattern and Pulsatility Index (PI). This is an
excellent reference for the operative procedure for assessing graft flows. Note that you can easily calculate Pulsatility Index (Max – Min/Mean) off the Transonic or
CFM 100 strip chart report.
- Walpoth BH, Bosshard A, et. al, Transit-time Flow Measurement for
Detection of Early Graft Failure During Myocardial Revascularization. Ann Thorac Surg 1998:66:1097-1100
The authors revised 3 out of 46 IMA
grafts that were performed on pump. A low mean flow, a poor waveform pattern and a high pulsatility index were the variables associated with graft failure. The reason
for the low flow situation wasdistal IMA dissection in 1 patient, an obstructing intimal IMA flap in 1 and an intramural hematoma with compression of the IMA
anastomosis in 1 patient.
· Chun H, Doty JR, Salazar JD, Richmond J, and Fonger JD. Noninvasive
Graft Flow and Patency Assessment Following Minimally Invasive Direct Coronary Artery Bypass (MIDCAB) Grafting. Heart Surgery Forum, Vol 2 issue 3. Presented at the ISMICS meeting in Paris, May 1999.
The authors found that intraoperative flow measurements
demonstrated trends that are helpful in indentifying patients at an increased risk for unfavorable events, guiding the use of postoperative recatheterization.
Does it really matter if all the grafts aren't working because it
doesn't change the clinical outcome. For example, if the marginal graft fails, so what? The patient doesn't need it.
If it's worth taking the time to construct a
bypass graft, it's worth knowing whether or not it's patent!
This question also has implications for long term outcome. There are undoubtably a lot of graft failures in conventional on-pump CABG surgery that have gone undetected because the grafts weren't scrutinized carefully in the post-operative period. If the patient wasn't symptomatic, the grafts were presumed to be open. However, if 1 out of 4 grafts occludes early, that patient may need to be brought back for redo CABG sooner.
What if mean graft flows are low because of
small target vessels or low run off?
Even if flow is low, you will still have a good waveform in a patent graft
.
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