Xref: utzoo sci.med:8949 sci.physics:6079 sci.electronics:5315 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!bpa!manta!conrad!sac From: sac@conrad.UUCP (Steven A. Conrad) Newsgroups: sci.med,sci.physics,sci.electronics Subject: Re: Biomedical Measurement "Challenge": Cardiac Output Keywords: biomedical measurements, cardiac output, instrumentation Message-ID: <156@conrad.UUCP> Date: 23 Feb 89 01:44:30 GMT References: <13175@steinmetz.ge.com> Reply-To: sac@conrad.UUCP (Steven A. Conrad) Distribution: usa Organization: Louisiana State University Medical Center, Shreveport Lines: 108 In article <13175@steinmetz.ge.com> oconnor%sungod@steinmetz.UUCP writes: >An article by larry@kitty.UUCP (Larry Lippman) says: >] Now the problem: how can we measure cardiac output without >] major surgery to expose the aorta and attach a flowmeter? > >Well, you could : >Inject the subject with a technicium bound into a pyrophosphate compound >and place the subject in front of a gamma-ray camera. Record the amount >of gamma-rays ( = k*amount of technicium = K*amount of blood ) in the >heart using a timescale much finer than a single heartbeat. Calculate >the difference beteen the maximum amount of blood in the ventricle >during a heartbeat and the minimum amount. Multiply by beats/time-unit >to obtain a flow rate. This technique assumes all the valves in the >heart are functioning correctly. This method is used quite successfully for determining the ejection fraction as you describe, but is rather lousy for actual flow calculations. Because of attenuations, inability to record all emitted radiation, etc. it doesn't cut it. >Use doppler ultrasound on the aorta to measure the velocity, and use >an NMR or CAT scan to determine the cross-section, and multiply to >obtain the flow rate. This method is used clinically. Actually the cross section is obtained with 2D/M mode echocardiography, usually with the same machine used for the Doppler. However, it is subject to quite a bit of error, most commonly due to errors in calculation of aortic area and in the assumptions about the flow. It assumes a flat velocity profile, which truly occurs only in the very base of the aorta. It is much better used for following changes in cardiac output than for measuring the actual value. >Inject a VERY small transmitter ( about the size of a red blood cell >would be nice, even though that will get stuck in the capillaries ) into >a major vein that broadcast a long pseudo-random number and track it >using multiple ( at least three ) recievers, using the delay to each >receiver to precisely locate the unit, and measure it change in position >as it flows through the aorta, then use an NMR or CAT scan to determine >the cross-section, and multiply to obtain the flow rate. A little wild, maybe? Nonetheless, it is well known that a single red blood cell may travel at a variety of velocities, depending on its proximity to the aortic wall, the diameter of the vessel, eddy currents at the valves, etc. Wouldn't be practical. >Use bolus injection of a radioisotope and see how quickly it moves to >and through the aorta using a gamma-camera, then compute the flow >as you did for the radio transmitter. Again, the major problem is calculating aortic diameter. >Compute the velocity of the flow out through the valve by analysis of >the noise it makes as it passes through, then proceed as above to get >the volume of material going thrnough. No good relationship between noise and velocity. >I thought of heating a section of the aorta with microwaves or >a particle accelorator and measuring the cooling rate, monitoring >the temperture with a fiber-optic thermometer ( we could use >the same fiber to heat it with a laser, come to think of it ) or >monitoring the temperature with NMR sensors, but this seems a bit >invasive to me ( although not as bad as inserting a flowmeter ). This is probably the closest to the most common way in which we do measure cardiac output. There are two major ways of measuring C.O. The oldest is the Fick method, in which the oxygen content difference across the pulmonary capillaries is related to the amount of oxygen taken up by the lungs: C.O. = [oxygen consumption] / [arterial-venous oxygen content difference] A more recent introduction are the indicator dilution methods. Cardiogreen dye was first used (by Wood from the Mayo Clinic, I believe). It is injected into the right atrium, and its concentration curve is monitored downstream, in the arterial system. With the introduction of the bedside pulmonary artery catheter in the early 70's with thermistor probes, however, the indicator now used is heat (actually cold, or negative heat). The injection of cold fluid is made into the right atrium, and the temperature is monitored in the pulmonary artery, with the right ventricle mixing the bolus well. It is based on the following simple principle: _ [mass] = _/ F(t)C(t) Mass is replaced by heat quantity, and concentration C by temperature. If we assume flow F to be constant, then we can take it out of the integral and rearrange: _ [cardiac output] = [heat quantity] / _/ T(t) There are some correction constants and others such as specific heat, density of fluids and blood, etc. that don't affect the overall meaning of the above equation. Notice that a number of assumptions are made. However, the method has less than about 15% biological variation, and has been accepted clinically in the critical care unit and cardiac cath lab. Steve. -- Steven A. Conrad, Department of Medicine (Critical Care) Louisiana State University Medical Center, Shreveport, LA UUCP: sac@conrad.UUCP, Internet: conrad@manta.pha.pa.us "Silence is the only successful substitute for brains"