|
|
|
|
|
|
|
|
|
Central venous pressure monitoring: "Swan-Ganz catheters may be lifesaving in Atlanta, but mostly are a nuisance in Houston." 1
The use of CVP rather than PAC monitoring has been advocated for both surgical and non-surgical patients. The proponents of this form of monitoring maintain that the avoidance of a PAC eliminates the specific risks of the device, reduces the cost of treatment and that anyway the results of therapy are the same whether or not a PAC is used.
It is certainly true that CVP monitoring alone obviates the specific risks of the PAC (notably pulmonary artery rupture, pulmonary infarction and some cardiac arrhythmias), but it is often forgotten that all of the other problems of central vascular cannulation occur at the same (and in some cases a greater) incidence.
In those scheduled for surgery, it has been shown on several occasions that CVP measurements are a reasonable indication of PAOP in low-risk patients, or those with normal ventricular function 2, 3. However, it also seems certain that it is difficult to identify low-risk patients on clinical grounds alone and that within cohorts of patients believed to be at low risk, there will be a significant proportion who have serious haemodynamic abnormalities 4, 5. In the setting of cardiac surgery, a conversion rate from CVP to PAC monitoring of ~ 7% can be expected if a policy of initial non-use of PAC's is pursued 6.
It is self-evident that the direct costs of pulmonary artery catheterisation are greater than if a CV catheter alone is used, but the issue of whether or not the use of the PAC is justifiable on the basis of a proper risk:benefit analysis remains unresolved.
Two large outcome studies of the role of PAC in elective coronary bypass surgery have failed to demonstrate a significant outcome benefit in those patients monitored with a PAC 6, 7. However, one of these studies is open to the criticism that it comprised a group of largely 'low risk' patients 7, and in the other, haemodynamic instability forced crossover to the use of a PAC in 7% of patients 6. In selected groups of cardiac surgical patients such as those with left main trunk disease, there is apparently a significant benefit if a PAC is used 8.
Alternative technologies:
Techniques are now available, using less invasive technology, that permit estimation of some of the parameters that were previously measured with the pulmonary artery catheter. The three important technologies are impedance cardiography, transoesophageal echocardiography (TOE) and the derivative Fick technique (using carbon dioxide) for determination of cardiac output.
Impedance cardiography:
The impedance cardiograph is a device which, in theory, monitors cardiac output continuously, non-invasively and without the need for a high degree of operator skill. The measurement of cardiac output is based on the principle that changes in transthoracic impedance occur as a result of the variations in intrathoracic blood volume which accompany each beat of the heart. The monitor works by applying a small alternating current across the chest and simultaneously sensing impedance through an array of electrodes sited along the long axis of the thorax.
The device has been assessed in patients undergoing cardiac 9 and non-cardiac 10 surgery and found to compare poorly with output determinations made by thermodilution. In its present state of development the impedance cardiograph does not appear to be a viable alternative to the PAC.
Transoesophageal echocardiography:
Developments in echocardiographic techniques have allowed estimation of preload and cardiac output in addition to the more 'traditional' assessments such as regional wall motion and valvular function. This subject has been recently reviewed by Beique and Lavoie 11.
Estimation of preload by transoesophageal echocardiography:
The degree of left ventricular filling can be relatively easily assessed by measurement of the end diastolic dimensions (for example end-diastolic area at the mid-papillary level). A comparison of left ventricular preload assessment using the PAC and TOE during graded blood volume reduction has been made by Cheung et al 12. TOE was found to give a more accurate measure of preload (end-diastolic volume) than the PAC (PAOP) which was significantly influenced by changes in ventricular compliance. A similar finding was made in paediatric surgery when a TOE measure of preload was compared with changes in the central venous pressure 13.
Preload status has also been assessed by study of the mitral inflow patterns. Variations in the ratio of the mitral inflow velocity during early ventricular filling ('E' velocity) to the velocity during atrial systole ('A' velocity) have been found to correlate with the pulmonary artery occlusion pressure 14.
It should be noted that, unlike the PAC, TOE cannot be used to measure LV filling pressure accurately although echocardiographic signs of left atrial pressure elevation are described 15.
Estimation of cardiac output by transoesophageal echocardiography:
Measurement of cardiac output using Doppler signals generated at the pulmonary, mitral and aortic valves is possible using TOE. Most studies have reported varying degrees of accuracy when compared to the 'gold standard' of thermodilution - in part because echocardiographic techniques are crucially dependent on an accurate measurement of valve area used in the calculation of output.
Darmon and colleagues 16 were able to obtain a good correlation between thermodilution outputs and Doppler measurements at the aortic valve by using a novel technique for calculating the area of the aortic orifice.
Descending aortic blood flow has also been used as a measure of total cardiac output. The method is attractive because it has the advantage of being considerably easier to measure than ascending aortic flow, but suffers from the disadvantage that it only measures about 2/3 of total flow.
Cariou et al 17 have evaluated the Sometec 'Dynemo-3000' system which estimates both descending aortic diameter and blood flow and concluded that it is "a reliable non-invasive tool for estimating CO and tracking its changes."
Baillard et al 18 have examined the performance of the Abbott 'ODM II'. Its measurements apparently compare well with outputs determined by continuous thermodilution. Unlike other Doppler methods, it does not involve measurement of aortic cross-sectional area, relying instead on the use of a nomogram based on the height, age and weight of the patient.
It is apparent that a considerable degree of expertise in echocardiographic techniques is necessary before the accuracy attainable by thermodilution is achieved with the TOE.
Comparative role of echocardiography in haemodynamic monitoring:
Echocardiography obviously adds a dimension to haemodynamic monitoring which is not available with a pulmonary artery catheter. In particular, analysis of regional wall motion and assessment of valvular function are simply not possible with a PAC. TOE also has the advantage that complications of the technique, whilst not unheard of 19, are extremely rare.
Against this must be set the facts that:
* The technology is extremely expensive, and requires a high degree of operator skill to make the necessary measurements.
* Many of the measurements rely on the interpretative skills of the observer.
* The present generation of probes are not convenient for, or well-tolerated by, unsedated patients.
* In a practical sense, the monitor does not offer the user a continuous data stream.
* Metabolic indices, such as MVO2 cannot be obtained with the device.
Comparative role of pulmonary artery catheter in haemodynamic monitoring:
With regard to the pulmonary artery catheter it can be said that:
* Thermodilution output determinations by a PAC remain the 'gold standard'.
* Metabolic indices, such as MVO2 can only be obtained with the device.
* Once positioned correctly, the PAC is relatively simple to use, delivers continuous, real-time data and does not require particularly advanced manipulative, cognitive or interpretive skills on the part of the operator.
* The capital and disposable costs of the device are relatively low.
Against this must be set the facts that:
* The complications of a pulmonary artery catheter are more common and potentially more serious than for echocardiography.
* In terms of preload assessment, the PAC is unable to distinguish acute changes in ventricular compliance from real changes in end-diastolic volume.
In conclusion, echocardiography should probably be regarded as a complementary rather than an a competing technology to pulmonary arterial catheterisation. It appears unlikely that it will replace the pulmonary artery catheter in the field of haemodynamic monitoring although it may well reduce the use of the device in the future.
Cardiac output determination using the derivative Fick technique:
For many years it has been recognised that cardiac output can be estimated by using the Fick principle (with carbon dioxide as the marker gas) to measure pulmonary capillary blood flow. If a correction factor for the amount of blood shunted through the lungs is then applied, cardiac output can be derived non-invasively.
The first such monitor to employ this principle was introduced in 1999. The device estimates cardiac output, using respiratory gas analysis and pulse oximetry and is marketed by Novametrix Medical Systems Inc. The computer uses the ratio of the change in end-tidal carbon dioxide and carbon dioxide excretion, in response to a 50 sec period of rebreathing, to calculate pulmonary capillary blood flow. Cardiac output is then estimated by adding a correction factor for the intrapulmonary shunt flow, based on pulse oximetry. A review detailing many aspects of the partial CO2-rebreathing technique has recently been published 20.
The device is simple and convenient to use. The technique appears to be tolerably accurate according to some authors 21, but not so according to others 22.
References:1. Keats AS The Rovenstein Lecture 1983 Anesthesiology 60:467,1984
2. Joyce WP, Provan JL, Ameli FM, et al: The role of central haemodynamic monitoring in abdominal aortic surgery. A prospective randomized study. Eur J Vasc Surg 4:633-636, 1990
3. Mangano DT Monitoring pulmonary arterial pressure in coronary artery disease. Anesthesiology 53:364, 1980
4. Del Guercio LRM, Cohn JD. Monitoring operative risk in the elderly. JAMA 243:1350, 1980.
5. Babu SC, Sharma PVP, Raciti A et al: Monitor-guided responses - operability with safety is increased in patients with peripheral vascular disease. Arch Surg 115:1384, 1980
6. Tuman KJ, McCarthy RJ, Spiess BD, et al: Effect of pulmonary artery catheterization on outcome in patients undergoing coronary artery surgery. Anesthesiology 70:199-206, 1989.
7. Bashein G, Johnson PW, Davis KB, Ivey TD Elective coronary bypass surgery without pulmonary artery catheter monitoring. Anesthesiology 63:451, 1985
8. Moore CH, Lombardo TR, Allums JA, Gordon FT Left main coronary artery stenosis; Hemodynamic monitoring to reduce mortality. Ann Thorac Surg 26:445, 1978
9. Sageman WS, Amundson DE: Thoracic Electrical Bioimpedance Measurement of Cardiac Output in postaortocoronary Bypass Patients. Crit Care Med 1993, 21:1139-1142.
10. Perrino AC Jr, Lippman A, Ariyan C, O'Connor TZ, Luther M: Intraopertive Cardiac Output Monitoring Comparison of Impedance Cardiography and Thermodilution. J Cardiothorac Vasc Anesth 1994, 8:24-29.
11. Beique FA, Lavoie J TEE Monitoring Can J Anaesth 1998 45:10 919 - 924
12. Cheung AT, Savino JS, Weiss SJ, Aukburg SJ, Berlin JA: Echocardiographic and hemodynamic indexes of left ventricular preload in patients with normal and abnormal ventricular function. Anesthesiology 1994;81:376-387
13. Reich DL, Konstadt SN, Nejat M, Abrams HP, Bucek J: Intraoperative Transesophageal Echocardiography for the Detection of Cardiac Preload Change Induced by Transfusion and Phlebotomy in Pediatric Patients. Anesthesiology 1993, 79:10-15.
14. Samuelsson S, Brodin LA, Broman M et al Comparison between transoesophageal Doppler echocardiography and nuclear cardioangiography for the evaluation of left ventricular filling during coronary artery bypass grafting. Anesth Analg 1995; 80:41 - 46
15. Kuecherer HF, Muhiudeen IA, Kusumoto FM, Lee E, Moulinier LE, Cahalan MK, Schiller NB: Estimation of mean left atrial pressure from transesophageal pulsed Doppler echocardiography of pulmonary venous flow. Circulation 1990;82:1127-113
16. Darmon P-L, Hillel Z. Mogtader A, Mindich B. Thys D: Cardiac Output by Transesophageal Echocardiography using Continuous Wave Doppler across the Aortic Valve. Anesthesiology 1994, 80:796-805.
17. Cariou A, Monchi M, Joly LM et al Noninvasive cardiac output monitoring by aortic blood flow determination: evaluation of the Sometec Dynemo-3000 system. Crit Care Med 1998:2066-2072
18. Baillard C, Cohen Y, Fosse JP et al Haemodynamic Measurements (Continuous Cardiac Output and Systemic Vascular Resistance) in Critically Ill Patients: Transoesophageal Doppler Versus Continuous Thermodilution. Anaesth Intensive Care 27:33-37, 1999.
19. Savino JS, Hanson CW III, Bigelow DC, Cheung AT, Weiss SJ: Oropharyngeal Injury after transesophageal Echocardiography. J Cardiothorac Vasc Anesth 1994, 8:76-78.
20. Jaffe MB: Partial CO2 rebreathing cardiac output – operating principles of the NICO system. J Clin Monit Comput 1999: 15: 387–401.
21. Binder JC, Parkin WG. Non-invasive cardiac output determination: comparison of a new partial-rebreathing technique with thermodilution. Anaesth Intensive Care 2001 Feb;29(1):19-23
22. Nilsson LB, Eldrup N, Berthelsen PG. Lack of agreement between thermodilution and carbon dioxide-rebreathing cardiac output. Acta Anaesthesiol Scand 2001 Jul;45(6):680-5
Last edited on: 27/07/2001
| Mission | Confidentiality Policy | Contact the Author | Advertising Policy |