ORiGINAL
ARTICLES
Continuous
Mark L. Yelderman,
Thermodilution Cardiac Output Intensive Care Unit Patients MD, Mike A. Ramsay, Russel C. McKown,
MD, Michael
A new continuous thermodilution cardiac output measurement technique and companion flow-directed pulmonary artery catheter were evaluated in intensive care unit (ICU) patients. Continuous cardiac output was monitored for 6 hours in each patient, and, at selected intervals, a series of bolus thermodilution cardiac output determinations was made and averaged for comparison. A total of 222 data pairs was obtained in 54 patients. The cardiac outputs ranged from 2.8 to 10.8 L/min. The linear regression is represented by the following equation: continuous thermodilution = 0.99 bolus
A
COMPONENT in the evolution of critical care medicine is the technology that enhances the acquisition of reliable and timely patient information. Given the growing constraints on the clinician’s time, as well as increased budget demands, patient monitoring must minimize operator time, frustration, and educational requirements. Concurrently, it must be safe, cost-effective, and packaged for the bedside. Automatic and continuous patient monitoring provides more information on a more timely basis and has the potential to reveal previously undetected physiological events. Fundamental patient parameters, including arterial and venous hydrostatic pressures and oxygen saturation, are automatically and continuously available. Up to this time, the absence of continuous cardiac output (CO) measurements at the bedside has not been because of a lack of desire by the clinician or the commercial providers, but because of significant technological hurdles.
This article
is accompanied
by an editorial.
Please
see:
From InterFlo Medical, 1101 Resource Dr, Piano, TX, and the Department of Anesthesia, Baylor University Medical Center, Houston, TX Conflict of Interest: InterFlo Medical manufactures and sells the InterFlo Continuous Cardiac Output System andprovided the support to perform this study at Baylor University Medical Center. The Baylor physicians have no association with InterFlo Medical. Address reprint requests to Mark Yeldetntan, MD, 5205 Terrace View, Piano, TX 75093. Copyright 0 1992 by W.B. Saunders Company 1053-077019210603-0003$03.00/O Key words: cardiac output monitotings, intensive care medicine 270
D. Quinn,
PhD, and Paula H. Gillman,
Measurement
PhD, A.W.
Paulsen,
in
MD, PhD,
RN, CCRN
thermodilution + 0.02. The correlation coefficient r was 0.94, the S,, was 0.54. The mean relative error was 0.3%, and the standard deviation of the relative error was 11.5%. The absolute measurement bias was 0.02 L, and the 95% confidence limits were 1.07 and -1.03 L. The results demonstrated that the new continuous thermodilution cardiac output measurement technique provided acceptable accuracy and was considerably easier to use in the clinical situations studied in the ICU. Copyright o 1992 by WA Saunders Company
A new thermodilution method has been developed to measure CO automatically and continuously.’ Pulmonary artery catheters (PACs) are modified to locate a lo-cm thermal filament in the right ventricle (RV) during use. Without using any fluid injectate, the thermal filament continually transfers a safe level of heat directly into the blood according to a pseudorandom binary sequence. The resulting temperature change is detected downstream in the pulmonary artery (PA) and cross-correlated with the input sequence to produce a thermodilution “washout” curve. CO is computed from a conservation of heat equation using the area under the curve. A more detailed explanation of the methodology can be found in a previous publication.’ The average heat infused, generally less than 7.5 W, was selected so that the catheter surface temperature remains below 44°C regardless of blood flow conditions. Studies from previous investigators suggest that long-term exposure at 44°C has no detrimental effect on red blood cells,2J myocardium,“-7 or other blood comp0nents.a The actual filament surface temperature is continually measured and the delivered power either reduced or terminated when the average temperature exceeds 44°C.” From the user’s perspective, the PAC is inserted by the standard procedure, requiring no additional catheter positioning protocol. The catheter is connected to the monitor, and the measurement process is initiated. Within several minutes, the first CO measurement is computed and displayed. Every 30 seconds, the displayed CO is updated, reflecting the average flow of the previous 3 to 6 minutes. Special user calibration or catheter positioning is not required. The technique of measuring volumetric fluid flow discussed herein is derived from conservation of mass, or thermodilution, using stochastic system identification techniques. This method is straightforward to implement, measures true volumetric flow, requires no user calibration, and
Journalof Cardiothoracic and VascularAnesthesia, Vol6, No 3 (June), 1992: pp 270.274
CONTINUOUS
271
CARDIAC OUTPUT
is independent of vascular geometry. The method has been evaluated in a laboratory flow bench model’ and in sheep.tO The purpose of this study was to evaluate the performance in clinical intensive care unit (ICU) patients. MATERIALS
AND
METHODS
After obtaining institutional approval and informed consent, flow-directed PACs with thermal filaments were inserted into ICU patients. Adult patients were selected at random for inclusion in the study subsequent to the clinical decision to insert a PAC. The catheter was connected to a prototype monitor and CO was continuously measured. Generally once an hour, the catheter was connected to an Edwards COMl monitor (Baxter Health care, Irvine, CA) and a random selection of 2 to 5 room-temperature, 10 mL bolus thermodilution CO determinations were made in rapid sequence, asynchronously with ventilation. For comparison, an average of the continuous thermodilution method (CTD), before and after the sequence of bolus injections, was compared with the average of the complete set of bolus thermodilution (BTD) determinations. CO measurements were plotted versus time for each patient. Linear regression for the data pairs was performed and the relative error, defined as (100 x [BTD - CTD]/BTD), was computed. The mean and variance of the error were computed.” RESULTS
Fifty-four ICU patients ranging in weight from 54 to 111 kg were each studied for 6 hours. The medical conditions of the patients included: 1 kidney transplant, 3 liver transplants, 1 heart transplant, 42 cardiac surgical procedures, and 7 aortic aneurysms. Over the study period, patients varied from anesthetized, hypothermic, and controlled ventilation to awake, normothermic, with spontaneous and unassisted ventilation. The study period also included episodes of cardiac irregularities and times of rapid fluid administration. None of these states or conditions adversely affected the continuous CO determinations. A wide range of COs was experienced, and three representative cases are provided herein. In the first case (Fig l), 250 mg of aminophylline was inadvertently infused too rapidly. Note the response in CO. In the second case (Fig 2), the patient was changed from controlled to spontaneous
4
-__t--_.~_-.-.-_-c_---t___._-_.~_-..-_-.__.
2
-.-__i______t-___~.-_--~-..---..~.-..-..-
__.
o~,,,i,,,i..,i,,,i.,,i,,,, 0
I
/
1
2
I
3
4
5
6
Time (hr) Fig 2. Patient is changed from controlled to spontaneous ventilation at the marked point. The key is identical to Fig l.-, continuous thermodilution; o, bolus thermodilution.
ventilation, resulting in an increase in CO. In the third case (Fig 3) the patient was awakened in the ICU. As in Fig 2, an increase in CO was noted. A total of 222 data pairs was obtained and no data were eliminated before analysis. The COs ranged from 2.8 to 10.8 L/min, and the heart rate varied from 74 to 158 beats/min with some periods of irregular rhythms. The linear regression is represented by the equation y = 0.99x + 0.02; the correlation coefficient r is 0.94; the S, is 0.54 (Fig 4). The mean relative error is 0.3% and the standard deviation of the relative error is 11.5%. The absolute measure bias is 0.02 L and the 95% confidence limits are 1.07 and -1.03 L and are shown” (Fig 5). No adverse effects were observed in any of these patients. DISCUSSION
Many earlier investigators attempted to apply classic fluid mechanic techniques to CO measurement but were unsuccessful, not fully appreciating the idiosyncrasies in vascular blood flow. Blood is a slurry, and flow is pulsatile, nonstationary, nonuniform, and occurs through elastic vessels. Changes in volume, flow, and myriad other characteristics affect the heart and vessels, making calculation or estimation of heart geometry difficult or impossible.
ol,,,;,,,i.,,i,,,i..,,,,,, 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (hr) Fig 1. Aminophylline (250 mg) is infused over 15 minutes beginning at the marked point. The abscissa is time (hours) and the ordinate is CO (L/min). The circles represent the individual BTD determinations and the solid lines represent the CTD monitor.-, continuous thermodilution; o, bolus thermodilution.
01~..i...i...;...i...,..., 0
1
2
3
4
5
6
Time (hr) Fig 3. Patient is awakened in the ICU at the marked point, and then allowed to sleep. The key is identical to Fig l.-, continuous thermodilution; o, bolus thermodilution.
272
-2.0
, 0
*
g
-
; 4
8
-
-
YELDERMAN
ET AL
,
3
8
-
-
12
(BTD + CTD)/2 (Lmin)
0
4 Bolus Thermodilution
Fig 4. tion.
8
12
Fig 5. Bias and variance between bolus (BTD) and continuous thermodilutlon (CID). The mean and 95% confidence lines are shown.
(Lmin)
Linear regression of bolus versus continuous thermodilu-
Fluid velocity may be measured directly by using ultrasound transducers placed internallyi or externally.13 Fluid velocity may be measured indirectly by measuring time of flight between two points14 or by hot wire anemometry or “hot” thermistor methods.15 For some applications, vessel cross-section is measured1*J3 or estimated. If vessel crosssection can be measured, flow can be calculated from blood velocity and vessel geometry.*6J7 However, great vessel cross-sectional estimation techniques are difficult because the vessel is not a circular cylinder, and the cross-sectional area cannot reliably be estimated from a diameter. The performance of some flow measurement devices is very dependent on the user’s ability. Successful clinical flow measurement techniques include Fick, dye dilution, and thermodilution. The first law of thermodynamics, namely that mass is neither created nor destroyed, is the fundamental principle of these techniques. From this foundation, volumetric flow formulas are derived that are independent of fluid channel geometry.’ Because clinical implementation of Fick and dye dilution techniques is tedious, thermodilution has become the de facto clinical standard.18-20 To develop an automated continuous CO method, the obvious extension is to place a heating element on a catheter and to transfer heat directly into the blood. Previous investigators used various heat infusion methods. Khali121 applied heat as a step function and measured the downstream amplitude change. Normann et al22 and Barankay et a123applied heat as an impulse and measured the area under the washout curve. Philip et al% applied a sinusoidal heat input and measured downstream temperature attenuation. Although all these techniques have a sound theoretical basis, they are severely compromised because of background thermal noise in the pulmonary artery20,25-2sor limitations on maximum heat flux or temperatures.22 The resultant infused indicator signal is small in propor-
tion to the resident pulmonary artery thermal noise; consequently, if classical signal processing techniques are used, the outcome is an erroneous flow calculation. An order of magnitude signal processing enhancement is required to provide the requisite improvement in signal-to-noise ratio necessary to produce a reasonable flow estimate. Such improvement is achieved by stochastic or spread spectral signal processing techniques. These techniques are used by the continuous CO measurement monitor described herein, and have great immunity to thermal noise and tend to minimize the effects of temperature baseline drift, respiratory artifacts, and cardiac irregularities. The result displayed by the monitor represents the average CO from the previous 3 to 6 minutes, which is updated every 30 seconds. In this study, the accuracy and precision of the technique are established over a wide range of COs. BTD has been verified to correlate well with other methods of measuring C0.2g-32BTD is used as the reference measurement in this study because of its ease of implementation, and because it is the clinical standard.18-20s32-34 BTD can have large errors in precision and bias, attributed largely to loss of indicator (heat),*9T34 effects of ventilation, or cardiac irregularities.ig,” Even in the best controlled environment, precision can be no better than 4% to 9%.35-37 Much has been published on benefits of cold versus room-temperature injectate,3433-4 ventilation synchronous4143 or asynchronous injection3’ number of bolus determinations,42*4,45 catheter position,46 rate of injection,1g,47 and possible contamination.48-50 The protocol for this study was designed to minimize the effects of bias. The results of this study demonstrate close agreement between the two methods, considering the limitations of using room-temperature bolus injections as the standard for comparison. In addition, any unstable physiology induces an apparent variance, because the measurement of CO by the control and test method could not be simultaneously performed. The new continuous thermodilution CO measurement technique offers a promising clinical method to measure CO easily without user calibration or tedious catheter positioning requirements,
CONTINUOUS
CARDIAC OUTPUT
273
REFERENCES
1. Yelderman ML: Continuous measurement of cardiac output with the use of stochastic system identification techniques. J Clin Monit 6:322-332,199O 2. Ham TH, Shen SC, Fleming EM, Castle WB: Studies in destruction of red blood cells. IV. Thermal injury. Blood 3:373-403, 1948 3. Moussa NA, Tell EN, Cravalho EG: Time progression of hemolysis of erythrocyte populations exposed to supra-physiological temperatures. J Biomechan Eng Trans ASME 101:213-217, 1979 4. Moritz AR, Henriques FC: Studies of thermal injury. II. The relative importance of time and surface temperature in the causation of cutaneous burns. Arch Pathol43:695-720,1947 5. Henriques FC, Moritz AR: Studies of thermal injury. I. The conduction of heat to and through skin and the temperatures attained therein. A theoretical and an experimental investigation. Arch Pathol43:531-549,1947 6. Henriques FC: Studies of thermal injury. V. The predictability and the significance of thermally induced rate processes leading to irreversible epidermal injury. Arch Pathol43:489-502, 1947 7. Agah R: Quantitative characterization of arterial tissue thermal damage. Master of Science Thesis, University of Texas at Austin, 1988 8. Gillis MF, Smith LG, Bingham DB: Final technical progress report on studies on the effects of additional endogenous heat relating to the artificial heart. National Heart and Lung Institute publication no. PH-43-66-1130-5,1973 9. Yelderman ML, Quinn MD, McKown RC: Thermal safety of a filamented pulmonary artery catheter. J Clin Monit (in press) 10. Yelderman ML, Quinn MD, McKown RC: Continuous thermodilution cardiac output measurement in sheep. J Thorac Cardiovasc Surg (in press) 11. Bland JM, Altman DG: Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1:307-310, 1986 12. Segal J, Pearl RG, Ford AJ, et al: Instantaneous and continuous cardiac output using a Doppler pulmonary artery catheter. J Am Coll Cardiol 13:1382-1392, 1989 13. Abrams JH, Weber RE, Holmen KD: Transtracheal Doppler: A new procedure for continuous cardiac output measurement. Anesthesiology 70:134-138, 1989 14. Vishnoi R, Roy RJ, Avery JG: Transit time continuous cardiac output. Proceedings of the Seventh Annual Meeting. New York, NY, IEEE-EMBS, 1985 15. Sekii S, Tanabe S: Cardiac output measurement system and method. US Patent no. 4,685,470 16. Nicolosi GL, Punoergi E, Cervesato E, et al: Feasibility and variability of six methods for the echocardiographic and Doppler determination of cardiac output. Br Heart J 59:299-303, 1988 17. Robson SC, Murray A, Pearl I, et al: Reproducibility of cardiac output measurement by cross-sectional and Doppler echocardiography. Br Heart J 59:680-684,198s 18. Fitzgerald DC, Engstrom RH, Siegel LC: Intraoperative measurement of cardiac output with transtracheal Doppler and thermodilution. ASA Abstr 71X386,1989 19. Levett JM, Reploge RL: Thermodilution cardiac output: A critical analysis and review of the literature. J Surg Res 27:392-404, 1979 20. Ganz W, Donoso R, Marcus HS: A new technique for measurement of cardiac output by thermodilution in man. Am J Cardiol27:392-396,197l 21. Khalil H: Determination of cardiac output in man by a new method based on thermodilution. Lancet 1:1352-1354,1963
22. Normann RA, Johnson RW, Messinger JE, Sohrab B: A continuous cardiac output computer based on thermodilution principles. Ann Biomed Engl17:61-73, 1989 23. Barankay T, Jansco SN, Petri G: Cardiac output estimation by a thermodilution method involving intravascular heating and thermistor recording. Acta Physiol Acad Sci Hungaricae Tomus 38:167-173, 1970 24. Philip JH, Long MC, Quinn MD, Newbower RS: Continuous thermal measurement of cardiac output. IEEE Trans Biomed Eng 31:393-400,1984 25. Bogaard JM, van Duyl WA, Versprille A, Wise ME: Influence of random noise on the accuracy of the indicator-dilution method. Clin Phys Physiol Meas 6:59-64, 1985 26. Wessel HU, James GW, Paul MH: Effects of respiration and circulation on central blood temperature of the dog. Am J Physiol 211:1403-1412, 1966 27. Wessel HU, Paul MH, James GW, et al: Limitations of thermal dilution curves for cardiac output determinations. J Appl Physiol30:643-652, 1971 28. Olsson B, Pool J: Validity and reproducibility of determination of cardiac output by thermodilution in man. Cardiology 55:136-148, 1970 29. Kohanna FH: Monitoring of cardiac output by thermodilution after open heart surgery. J Thorac Cardiovasc Surg 73:451457,1977 30. Riedinger MS, Shellock FG: Technical aspects of the thermodilution methods for measuring cardiac output. Heart Lung 13:215-221, 1984 31. Stetz CW, Miller RG, Kelly GE: Reliability of the thermodilution method in the determination of cardiac output in clinical practice. Am Rev Respir Dis 126:1001-1004,1982 32. Weisel RD, Berger RL, Hechtman HB: Measurement of cardiac output by thermodilution. N Engl J Med 292:682-684,1975 33. Ganz W, Swan HJC: Measurement of blood flow by thermodilution. Am J Cardiol29:241-246, 1972 34. Runciman WB, Ilsley AH, Roberts JG: An evaluation of thermodilution cardiac output measurement using the Swan-Ganz catheter. Anaesth Intens Care 9:208-220,198l 35. Elkayam U, Berkley R, Stanley A, et al: Cardiac output by thermodilution technique: Effect of injectate volume and temperature on the accuracy and reproducibility in the critically ill patient. Chest 84:418-422,1983 36. Hoel BL: Some aspects of the clinical use of thermodilution in measuring cardiac output. Stand J Clin Lab Invest 38:383-385, 1978 37. Jansen JRC, Schreuder JJ, Versprille A: Reliability of cardiac output measurements by the thermodilution method, in Vincent JL (ed): Update in Intensive Care and Emergency Medicine. New York, NY, Springer-Verlag, 1990, pp 408-441 38. Bilfinger TV, Anagnostopoulos CE: In vitro determination of accuracy of cardiac output measurements by thermal dilution. Surg Res 33:409-414,1983 39. Shellock FG, Riedinger MS, Bateman TM, Gray RJ: Thermodilution cardiac output determination in hypothermia post cardiac surgery patients: Room vs ice temperature injectate. Crit Care Med 11:668-670,1983 40. Shin B, Ayella RJ, McAslan TC, et al: Pitfalls of Swan-Ganz catheterization. Crit Care Med 5:125-127,1977 41. Armengol J, Godfrey CWN, Balsys AJ, et al: Effects of the respiratory cycle on cardiac output measurements: Reproducibility enhanced by timing the thermodilution injections in dogs. Crit Care Med 9:852-854,198l
274
42. Jansen JRC, Schreuder JJ, Boggaard JM, Rooyan WV: Thermodilution technique for measurement of cardiac output during artificial ventilation. J Appl Physiol50:584-591, 1981 43. Snyder JV, Powner DJ: Effects of mechanical ventilation on the measurement of cardiac output by thermodilution. Crit Care Med 10:677-682,1982 44. Jansen JRC, Versprille A: Improvement of cardiac output estimation by the thermodilution method during mechanical ventilation. Intens Care Med 12:71-79, 1986 45. Bassingthwaighte JB, Knopp TJ, Anderson DU: Flow estimation by indicator dilution (bolus injection). Circ Res 27:277-291, 1970 46. Schauble JF, Maruschak GF, Fronek A: Improved perfor-
YELDERMAN
ET AL
mance of thermodilution catheters. Circulation VII, VIII 214, 1973 (suopl IV) 47. Nelson LD, Houtchens BA: Automatic vs manual injections for thermodilution cardiac output determinations. Crit Care Med 10:190-192, 1982 48. Colt JD: Temperature control apparatus for thermodilution cardiac output syringes. Am J Surg 131:777, 1976 49. Plachetak JR, Larson DF, Salomon NW, et al: Comparison of two closed systems for thermodilution cardiac output. Crit Care Med 9:487. 1981 50. Ray C Jr. Carlon GC, Campfield PB, et al: Multiple determinations of cardiac output using a two-bottle technique. Crit Care Med 7:33, 1979
ARTICLES
Continuous
Mark L. Yelderman,
Thermodilution Cardiac Output Intensive Care Unit Patients MD, Mike A. Ramsay, Russel C. McKown,
MD, Michael
A new continuous thermodilution cardiac output measurement technique and companion flow-directed pulmonary artery catheter were evaluated in intensive care unit (ICU) patients. Continuous cardiac output was monitored for 6 hours in each patient, and, at selected intervals, a series of bolus thermodilution cardiac output determinations was made and averaged for comparison. A total of 222 data pairs was obtained in 54 patients. The cardiac outputs ranged from 2.8 to 10.8 L/min. The linear regression is represented by the following equation: continuous thermodilution = 0.99 bolus
A
COMPONENT in the evolution of critical care medicine is the technology that enhances the acquisition of reliable and timely patient information. Given the growing constraints on the clinician’s time, as well as increased budget demands, patient monitoring must minimize operator time, frustration, and educational requirements. Concurrently, it must be safe, cost-effective, and packaged for the bedside. Automatic and continuous patient monitoring provides more information on a more timely basis and has the potential to reveal previously undetected physiological events. Fundamental patient parameters, including arterial and venous hydrostatic pressures and oxygen saturation, are automatically and continuously available. Up to this time, the absence of continuous cardiac output (CO) measurements at the bedside has not been because of a lack of desire by the clinician or the commercial providers, but because of significant technological hurdles.
This article
is accompanied
by an editorial.
Please
see:
From InterFlo Medical, 1101 Resource Dr, Piano, TX, and the Department of Anesthesia, Baylor University Medical Center, Houston, TX Conflict of Interest: InterFlo Medical manufactures and sells the InterFlo Continuous Cardiac Output System andprovided the support to perform this study at Baylor University Medical Center. The Baylor physicians have no association with InterFlo Medical. Address reprint requests to Mark Yeldetntan, MD, 5205 Terrace View, Piano, TX 75093. Copyright 0 1992 by W.B. Saunders Company 1053-077019210603-0003$03.00/O Key words: cardiac output monitotings, intensive care medicine 270
D. Quinn,
PhD, and Paula H. Gillman,
Measurement
PhD, A.W.
Paulsen,
in
MD, PhD,
RN, CCRN
thermodilution + 0.02. The correlation coefficient r was 0.94, the S,, was 0.54. The mean relative error was 0.3%, and the standard deviation of the relative error was 11.5%. The absolute measurement bias was 0.02 L, and the 95% confidence limits were 1.07 and -1.03 L. The results demonstrated that the new continuous thermodilution cardiac output measurement technique provided acceptable accuracy and was considerably easier to use in the clinical situations studied in the ICU. Copyright o 1992 by WA Saunders Company
A new thermodilution method has been developed to measure CO automatically and continuously.’ Pulmonary artery catheters (PACs) are modified to locate a lo-cm thermal filament in the right ventricle (RV) during use. Without using any fluid injectate, the thermal filament continually transfers a safe level of heat directly into the blood according to a pseudorandom binary sequence. The resulting temperature change is detected downstream in the pulmonary artery (PA) and cross-correlated with the input sequence to produce a thermodilution “washout” curve. CO is computed from a conservation of heat equation using the area under the curve. A more detailed explanation of the methodology can be found in a previous publication.’ The average heat infused, generally less than 7.5 W, was selected so that the catheter surface temperature remains below 44°C regardless of blood flow conditions. Studies from previous investigators suggest that long-term exposure at 44°C has no detrimental effect on red blood cells,2J myocardium,“-7 or other blood comp0nents.a The actual filament surface temperature is continually measured and the delivered power either reduced or terminated when the average temperature exceeds 44°C.” From the user’s perspective, the PAC is inserted by the standard procedure, requiring no additional catheter positioning protocol. The catheter is connected to the monitor, and the measurement process is initiated. Within several minutes, the first CO measurement is computed and displayed. Every 30 seconds, the displayed CO is updated, reflecting the average flow of the previous 3 to 6 minutes. Special user calibration or catheter positioning is not required. The technique of measuring volumetric fluid flow discussed herein is derived from conservation of mass, or thermodilution, using stochastic system identification techniques. This method is straightforward to implement, measures true volumetric flow, requires no user calibration, and
Journalof Cardiothoracic and VascularAnesthesia, Vol6, No 3 (June), 1992: pp 270.274
CONTINUOUS
271
CARDIAC OUTPUT
is independent of vascular geometry. The method has been evaluated in a laboratory flow bench model’ and in sheep.tO The purpose of this study was to evaluate the performance in clinical intensive care unit (ICU) patients. MATERIALS
AND
METHODS
After obtaining institutional approval and informed consent, flow-directed PACs with thermal filaments were inserted into ICU patients. Adult patients were selected at random for inclusion in the study subsequent to the clinical decision to insert a PAC. The catheter was connected to a prototype monitor and CO was continuously measured. Generally once an hour, the catheter was connected to an Edwards COMl monitor (Baxter Health care, Irvine, CA) and a random selection of 2 to 5 room-temperature, 10 mL bolus thermodilution CO determinations were made in rapid sequence, asynchronously with ventilation. For comparison, an average of the continuous thermodilution method (CTD), before and after the sequence of bolus injections, was compared with the average of the complete set of bolus thermodilution (BTD) determinations. CO measurements were plotted versus time for each patient. Linear regression for the data pairs was performed and the relative error, defined as (100 x [BTD - CTD]/BTD), was computed. The mean and variance of the error were computed.” RESULTS
Fifty-four ICU patients ranging in weight from 54 to 111 kg were each studied for 6 hours. The medical conditions of the patients included: 1 kidney transplant, 3 liver transplants, 1 heart transplant, 42 cardiac surgical procedures, and 7 aortic aneurysms. Over the study period, patients varied from anesthetized, hypothermic, and controlled ventilation to awake, normothermic, with spontaneous and unassisted ventilation. The study period also included episodes of cardiac irregularities and times of rapid fluid administration. None of these states or conditions adversely affected the continuous CO determinations. A wide range of COs was experienced, and three representative cases are provided herein. In the first case (Fig l), 250 mg of aminophylline was inadvertently infused too rapidly. Note the response in CO. In the second case (Fig 2), the patient was changed from controlled to spontaneous
4
-__t--_.~_-.-.-_-c_---t___._-_.~_-..-_-.__.
2
-.-__i______t-___~.-_--~-..---..~.-..-..-
__.
o~,,,i,,,i..,i,,,i.,,i,,,, 0
I
/
1
2
I
3
4
5
6
Time (hr) Fig 2. Patient is changed from controlled to spontaneous ventilation at the marked point. The key is identical to Fig l.-, continuous thermodilution; o, bolus thermodilution.
ventilation, resulting in an increase in CO. In the third case (Fig 3) the patient was awakened in the ICU. As in Fig 2, an increase in CO was noted. A total of 222 data pairs was obtained and no data were eliminated before analysis. The COs ranged from 2.8 to 10.8 L/min, and the heart rate varied from 74 to 158 beats/min with some periods of irregular rhythms. The linear regression is represented by the equation y = 0.99x + 0.02; the correlation coefficient r is 0.94; the S, is 0.54 (Fig 4). The mean relative error is 0.3% and the standard deviation of the relative error is 11.5%. The absolute measure bias is 0.02 L and the 95% confidence limits are 1.07 and -1.03 L and are shown” (Fig 5). No adverse effects were observed in any of these patients. DISCUSSION
Many earlier investigators attempted to apply classic fluid mechanic techniques to CO measurement but were unsuccessful, not fully appreciating the idiosyncrasies in vascular blood flow. Blood is a slurry, and flow is pulsatile, nonstationary, nonuniform, and occurs through elastic vessels. Changes in volume, flow, and myriad other characteristics affect the heart and vessels, making calculation or estimation of heart geometry difficult or impossible.
ol,,,;,,,i.,,i,,,i..,,,,,, 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (hr) Fig 1. Aminophylline (250 mg) is infused over 15 minutes beginning at the marked point. The abscissa is time (hours) and the ordinate is CO (L/min). The circles represent the individual BTD determinations and the solid lines represent the CTD monitor.-, continuous thermodilution; o, bolus thermodilution.
01~..i...i...;...i...,..., 0
1
2
3
4
5
6
Time (hr) Fig 3. Patient is awakened in the ICU at the marked point, and then allowed to sleep. The key is identical to Fig l.-, continuous thermodilution; o, bolus thermodilution.
272
-2.0
, 0
*
g
-
; 4
8
-
-
YELDERMAN
ET AL
,
3
8
-
-
12
(BTD + CTD)/2 (Lmin)
0
4 Bolus Thermodilution
Fig 4. tion.
8
12
Fig 5. Bias and variance between bolus (BTD) and continuous thermodilutlon (CID). The mean and 95% confidence lines are shown.
(Lmin)
Linear regression of bolus versus continuous thermodilu-
Fluid velocity may be measured directly by using ultrasound transducers placed internallyi or externally.13 Fluid velocity may be measured indirectly by measuring time of flight between two points14 or by hot wire anemometry or “hot” thermistor methods.15 For some applications, vessel cross-section is measured1*J3 or estimated. If vessel crosssection can be measured, flow can be calculated from blood velocity and vessel geometry.*6J7 However, great vessel cross-sectional estimation techniques are difficult because the vessel is not a circular cylinder, and the cross-sectional area cannot reliably be estimated from a diameter. The performance of some flow measurement devices is very dependent on the user’s ability. Successful clinical flow measurement techniques include Fick, dye dilution, and thermodilution. The first law of thermodynamics, namely that mass is neither created nor destroyed, is the fundamental principle of these techniques. From this foundation, volumetric flow formulas are derived that are independent of fluid channel geometry.’ Because clinical implementation of Fick and dye dilution techniques is tedious, thermodilution has become the de facto clinical standard.18-20 To develop an automated continuous CO method, the obvious extension is to place a heating element on a catheter and to transfer heat directly into the blood. Previous investigators used various heat infusion methods. Khali121 applied heat as a step function and measured the downstream amplitude change. Normann et al22 and Barankay et a123applied heat as an impulse and measured the area under the washout curve. Philip et al% applied a sinusoidal heat input and measured downstream temperature attenuation. Although all these techniques have a sound theoretical basis, they are severely compromised because of background thermal noise in the pulmonary artery20,25-2sor limitations on maximum heat flux or temperatures.22 The resultant infused indicator signal is small in propor-
tion to the resident pulmonary artery thermal noise; consequently, if classical signal processing techniques are used, the outcome is an erroneous flow calculation. An order of magnitude signal processing enhancement is required to provide the requisite improvement in signal-to-noise ratio necessary to produce a reasonable flow estimate. Such improvement is achieved by stochastic or spread spectral signal processing techniques. These techniques are used by the continuous CO measurement monitor described herein, and have great immunity to thermal noise and tend to minimize the effects of temperature baseline drift, respiratory artifacts, and cardiac irregularities. The result displayed by the monitor represents the average CO from the previous 3 to 6 minutes, which is updated every 30 seconds. In this study, the accuracy and precision of the technique are established over a wide range of COs. BTD has been verified to correlate well with other methods of measuring C0.2g-32BTD is used as the reference measurement in this study because of its ease of implementation, and because it is the clinical standard.18-20s32-34 BTD can have large errors in precision and bias, attributed largely to loss of indicator (heat),*9T34 effects of ventilation, or cardiac irregularities.ig,” Even in the best controlled environment, precision can be no better than 4% to 9%.35-37 Much has been published on benefits of cold versus room-temperature injectate,3433-4 ventilation synchronous4143 or asynchronous injection3’ number of bolus determinations,42*4,45 catheter position,46 rate of injection,1g,47 and possible contamination.48-50 The protocol for this study was designed to minimize the effects of bias. The results of this study demonstrate close agreement between the two methods, considering the limitations of using room-temperature bolus injections as the standard for comparison. In addition, any unstable physiology induces an apparent variance, because the measurement of CO by the control and test method could not be simultaneously performed. The new continuous thermodilution CO measurement technique offers a promising clinical method to measure CO easily without user calibration or tedious catheter positioning requirements,
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REFERENCES
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