Technology |
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Hemodialysis Adequacy & Access Patency Management |
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Behind the Measurements
The
Transonic Flow-QC Hemodialysis Monitor is a three part unit. It includes a laptop computer, the monitor, and a set of matched flow/dilution sensors. Its design allows
greater flexibility than with a one piece apparatus. Parts are interchangeable, increasing the system's utility in multiple device clinics. This also makes replacement of
a failed component no longer under warranty less expensive.
The laptop computer contains the Flow-QC
Monitor's software programs. These programs display flow traces and dilution curves produced from injections in real time, analyze the relationship between the curves, and
display access flow or recirculation results. The laptop records and stores curves as well as dated log files showing the results of numerous measurements for multiple
patients. The computer also gives you the option of playing back recorded measurements for review at a later time. With the press of a single key, results of prior
measurements may be printed out: daily log files may also be printed.
The monitor and laptop computer communicate with each other through a
nine-pin connecting cable. Through this cable the monitor feeds the computer the data it collects on flow and dilution. The heart of the system, though, lies within the
monitor which processes information collected from the flow/dilution sensors before sending it to the computer for analysis and display.
The set of sensors has a plug on one end that fits into an outlet on the
front of the monitor. One sensor is attached to the venous line; the other to the arterial line using a small amount of petroleum jelly on each line to act as ultrasound
couplant. As a system, these three components are self-contained and sufficiently compact to be moved from patient to patient on a cart and remain out of the way while
measurements are being taken. Because of this mobility, the Transonic Flow-QC Monitor can monitor all the patients in your clinic rapidly and efficiently.
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True Delivered Blood Flow (Qb) |
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Fig. 1.1: Blood line inserted into
the groove in the flow/dilutiuon sensor body. Direction of flow ius indicated by arrows. Ultrasound beam is shown emanating from one of four
transducers in the
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This measurement employs ultrasound transit-time
principles. Each sensor emits an ultrasound beam which travels through the tubing and blood alternatively in upstream and downstream
directions (Fig. 1.1). When the ultrasound beam travels in the direction of flow, the time it takes for the beam to traverse the distance through the
tubing and blood (the transit-time) is decreased by a flow-dependent amount. When the beam travels in the opposite direction, against the flow
in the tube, the beam's transit time is increased by a flow dependent amount. By subtracting upstream and downstream transit-times, volume flow is calculated.
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Ultrasound transit-time technology is the current "gold
standard" in non-contact flowmetry.
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Access Recirculation |
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The same clip-on flow/dilution sensors also monitor ultrasound velocity in
blood. Ultrasound travels at 1560 to 1590 m/sec in blood. This velocity is determined primarily by the blood protein concentration: the greater the concentration, the
faster ultrasound will travel in it. By introducing a bolus of isotonic saline (1533 m/sec) into the blood, the protein concentration is diluted and ultrasound velocity is
decreased. The reduced ultrasound velocity is recorded by the sensors; the Monitor converts this into conventional dilution curves. When saline is introduced into the
venous line, it passes the venous sensor producing a venous indicator dilution curve.If Access Recirculation is present, this saline will immediately appear in the
arterial line and produce a second indicator dilution curve. Recirculation is calculated as a ratio of the area under the arterial curve to the area under the venous
curve.
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Fig. 1.2:
The area of the venous curve is proportional to the volume of saline introduced into
the venous line. If a fraction of this saline reappears in the arterial line, dialysis is hampered by recirculation.
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Fig. 1.3:
Access recirculation caused by vneous stenosis. A portion of the dialyzed blood recirculated from the venous needle back into the arterial needle.
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Recirculation usually occurs when access flow (Qa) is less than dialysis pump flow (Qb). This indicates an access in trouble: one or
more flow limiting stenoses jeopardize access patency. Figure 1.3 shows the typical flow of blood in a patient with recirculation due to a venous stenosis.
Access flow is not adequate for the demands of the pump setting. Lack of flow at the arterial needle is compensated for by claiming some part of the freshly
dialyzed blood from the venous line. A paradoxical situation exists when there is a stenosis between the needles (Fig. 1.4).
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Fig. 1.4:
Mid-graft stenosis limits access flow. Pump flow (Qb) bypasses the stenosis.
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The stenosis limits flow through the access but the pump bypasses the stenosis (area of greatest hydrodynamic resistance). In this
case an access flow less than Qb can co-exist with the no-recirculation condition.
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Access Flow |
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Access Flow measurements are made via the Krivitski MethodTM by reversing the blood lines at the needle tubing connections. The dialyzer now removes blood from the patient's venous side of the access and returns it to the arterial side (Fig. 1.5). This creates mixing conditions which the Krivitski Method uses to make an indicator dilution measurement of access flow.
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Fig. 1.5:
Hemodialysis of access flow measurement with lines reversed according to the
Krivitski Method. Line reversal creates an artificial recirculation loop with a mixing site at the arterial side of the access.
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By introducing saline into the venous line in the same manner and at the
same location as for access recirculation measurements, blood protein concentration is diluted and ultrasound velocity is reduced. This change in blood protein
concentration is detected by the venous line sensor, producing a first dilution curve (upper area in Fig. 1.6). The diluted blood from the venous line then enters the
access and mixes with the incoming access flow. Upon reaching the arterial needle, a portion of mixed blood is removed from the access by the dialyzer, via the venous
needle. The diluted blood is detected by the arterial line sensor and a second dilution curve produced (lower area in Fig. 1.6). Access flow is calculated from the ratio
of the venous area to arterial dilution area, and the actual Qb.
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Fig. 1.6: Results of an access flow measureement showing 1290 ml/min flow.
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