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ORIGINAL ARTICLE Table of Contents   
Year : 2007  |  Volume : 10  |  Issue : 2  |  Page : 132-136
Cardiac output estimation after off-pump coronary artery bypass: A comparison of two different techniques

Department Anaesthesiology and Critical Care, Escorts Heart Institute and Research Centre, New Delhi., India

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The present study compares the cardiac output (CO) estimated by a new, non-invsive continuous Doppler device (Ultrasonic cardiac output monitor-USCOM) with that by bolus thermodilution technique (TD). Thirty post off-pump coronary artery bypass graft surgery patients were studied in this prospective non­randomozed study. Right heart CO estimation by USCOM and TD was performed and measured in quadruplet. A total of 120 paired observations were made. The mean CO was 4.63 and 4.76 L/min as estimated by TD and USCOM respectively. For TD and USCOM, the CO had a mean bias (tendency of one technique to differ from other) of -0.13 L/min and limits of agreement (mean bias±2SD) at -0.86 and 0.59 L/min. The study reveals very good agreement between the values of CO estimated by USCOM and TD.

Keywords: OPCAB, Thermodilution technique, Non-invasive cardiac output, Continuous wave Doppler, USCOM

How to cite this article:
Arora D, Chand R, Mehta Y, Trehan N. Cardiac output estimation after off-pump coronary artery bypass: A comparison of two different techniques. Ann Card Anaesth 2007;10:132-6

How to cite this URL:
Arora D, Chand R, Mehta Y, Trehan N. Cardiac output estimation after off-pump coronary artery bypass: A comparison of two different techniques. Ann Card Anaesth [serial online] 2007 [cited 2022 Nov 29];10:132-6. Available from:

   Introduction Top

Cardiac output (CO) estimation is an integral part of haemodynamic monitoring in the early postoperative period after cardiac surgery. Timely diagnosis of low CO along with its appropriate management has been shown to improve outcome in critically ill patients. [1],[2]

CO estimation using thermodilution (TD) pulmonary artery catheter (PAC) remains the current clinical standard [3],[4] despite its pitfalls [5],[6],[7] and potential hazards. [8] There is a consensus regarding the indications for use of PAC despite the absence of prospective randomized studies showing improvement in patient outcome. [9],[10],[11] Transoesophageal echocardiography (TOE) has long been projected as a lesser invasive and safer alternative haemodynamic monitoring tool to PAC in perioperative cardiac surgical patients. [12] The more recent challenge is being mounted by an array of resurgent non-invasive techniques notably thoracic bioimpedence, oesophageal Doppler and indirect Fick's method. [13],[14],[15],[16] Ultrasonic cardiac output monitor (USCOM) is a new Doppler device for non-invasive estimation of CO.

The aim of this study was to compare the CO estimated by USCOM with that estimated by TD technique during postoperative period in patients undergoing elective off-pump coronary artery bypass surgery (OPCAB).

   Methods Top

Thirty patients scheduled to undergo elective OPCAB gave their written informed consent to participate in this study approved by the Institutional Review Board. The anaesthetic and surgical techniques were standard for all patients. On arrival in the operating room, two lead ECG monitoring and pulse oximetry were connected. Thereafter, a peripheral intravenous infusion was started via a 16G cannula. Before the induction of anaesthesia, femoral artery was cannulated with a 16 G catheter and a 7.5 F 5 lumen thermodilution PAC (Edwards Life Sciences, Irvine, CA, USA) was inserted through the right internal jugular vein under local anaesthesia. For induction of anaesthesia and orotracheal intubation, midazolam, fentanyl, thiopentone and vecuronium were used. Anaesthesia was maintained with oxygen-air mixture with isoflurane and intermittent doses of midazolam, fentanyl and vecuronium. An ambient room temperature of 20 o C, fluid warmer, warming blanket and forced air heating were used to prevent intraoperative hypothermia. All patients underwent uneventful OPCAB. After transfer to the recovery room (RR), haemodynamic parameters were stabilized with volume and inotropes at the discretion of the RR staff. Postoperative analgesia and sedation were achieved with intravenous morphine sulphate 3 to 5 mg as and when required, and propofol infusion (0.3-0.6 mg/kg/hr). Neuromuscular blockade, when needed was achieved with vecuronium 2-4 mg.

CO estimations were performed in these patients on the first postoperative day under stable haemodynamic conditions and after extubation. CO estimations were performed four times in each patient at intervals of thirty minutes. The exclusion criteria are listed [Table 1].

Bolus TDCO estimation was performed by injecting 10 ml of 0.9% saline at room temperature in the right atrial lumen of the PAC. Each injection was completed within 3 seconds. The morphology of thermodilution curve was visualized and inspected for accuracy to exclude any artifact. An average of three measurements, all within 10% range were recorded as TDCO.

Non-invasive CO estimation was performed with USCOM (Ultrasonic Cardiac Output Monitor­ USCOM Private Limited, Sydney, NSW, Australia), a new stand alone Doppler device for non-invasive estimation of CO. USCOM has a non-imaging transthoracic probe and uses continuous wave (CW) Doppler signals to obtain a velocity time integral (VTI) at ascending aorta or pulmonary artery. [Figure 1] shows a typical VTI curve. Stroke volume (SV) is measured by cross section area (XSA) x VTI and CO by SV x heart rate (HR). The XSA of the vessel is predicted by a height-based nomogram [17] that is incorporated in the USCOM software. Non-invasive CO estimation using USCOM was performed at the pulmonary artery. The signal from the pulmonary artery was obtained from 3-5 intercostal space in left parasternal area. The correct alignment of the CW Doppler signal was denoted by a sharp, well defined waveform seen on the monitor and by a characteristic crisp sound. CO was estimated from an average of three systolic outflow cycles.

Same clinician performed non-invasive CO estimations and the nurse attending the individual patients performed TDCO estimations. Both invasive and non-invasive estimations were made almost simultaneously i.e. in a very short time span, allowing for the time spent in locating the optimum Doppler signals. Both the observers were unaware of the findings obtained by each one of them.

The results obtained were analyzed using the Bland and Altman analysis 18 with the help of SPSS 10.0 Chicago USA software package.

   Result Top

Of the thirty patients, twenty-six were male and four were female. Mean age of the patients was 59.20 ±10.14 years, body weight 72.06±15.57 kg, height 166.58 ± 9.23 cm and left ventricular ejection fraction 47.46 ±10.98% [Table 2].

A total of 120 paired observations were made, mean CO was 4.63 and 4.76 as estimated by TD and USCOM respectively. Bland and Altman analysis (where the differences of estimated CO values, from the two methods are plotted against their mean values) revealed the following. For TD and USCOM, the CO had a mean bias (the tendency of one technique to differ from other technique) of -0.13 L/min and limits of agreement (mean bias ± 2 SD) at - 0.86 and 0.59 L/min. [Figure 2].

   Discussion Top

The results of the present study show that the CO estimated by USCOM has a good equivalence with the CO estimated by TD technique after OPCAB. Although USCOM consistently overestimated CO as compared to TD, the difference was minor and within the acceptable limit. [18] It is also possible to estimate systemic vascular resistance (SVR) noninvasively by dividing the mean blood pressure by the minute distance (VTI x heart rate) [19], although this potential was not explored in the present study.

On review of literature, the authors could find only one study in English literature comparing these two techniques by Tan et al [20] and the findings are quite similar to the present study. They also concluded that the USCOM monitor has a place in intensive care monitoring. It is accurate, rapid, safe, well tolerated, non-invasive and cost-effective. Chand et al [21] while estimating CO, cardiac index and SV after OPCAB found similar results. However, there is enough literature in support of equivalence of CO estimated by conventional CW Doppler echocardiography and TD technique. [22] Therefore it is but fair to presume a similar equivalence between USCOM and TD technique, because USCOM uses the same technology as in conventional Doppler echocardiography. However, USCOM uses a non imaging probe, which is claimed to be superior to imaging probes in search of highest velocities. [23] Phillip and coworkers have demonstrated an excellent correlation between USCOM and conventional CW Doppler results and have suggested that CW Doppler validation data can be used for USCOM validation. [24]

While monitoring CO using the USCOM Doppler probe, a number of assumptions are made, the 45 o angle between the ultrasound beam and the direction of blood flow must be accurate and should remain fixed, the XSA of the vessel must remain constant throughout systole despite changes in blood pressure and / or CO. Doppler technique measures the blood flow in the pulmonary artery and calculates the CO, assuming that the cross section is always round and that the amount of blood leaving the vessel is constant. Moreover, potential for inaccuracy while measuring XSA using nomograms does exist and a small error in the measurement of the pulmonary artery diameter can lead to significant miscalculation of CO. Other sources of error will be intracardiac shunts, regurgitant lesions, and arrhythmia such as frequent premature contractions or atrial fibrillation. [4]

The present study attempted to evaluate the utility of USCOM in terms of monitoring CO. Many cardiac centres do not use PAC for conventional coronary artery bypass grafting, although most of the centres do use them for OPCAB. In centres performing OPCAB without using PAC, USCOM may be a good alternative to measure CO, if patient becomes unstable.

The shortcoming of the present study is that although it demonstrates good agreement in the CO values estimated by TD technique and USCOM, the reliability of this agreement over a wider clinical spectrum viz. hypotension, left ventricular failure and sepsis remains untested.

The acceptability of any new technique is in reproducibility, ease of use, lack of operator bias and of course the cost. USCOM is a simple tool easy to use in the postoperative recovery units and preoperative patients. But its use during intra­operative period is difficult in cardiac surgical patients due to difficulty in placing the probe in the left 3-4 intercostal space. There is minimal observer bias and no recurring cost after the initial investment in hardware (monitor and the probe).

In conclusion, USCOM appears to be a potentially useful tool for reliable non-invasive CO estimation and trending in cardiac surgical patients during postoperative period. Other areas of possible use are preoperative patient evaluation and prognostication, haemodynamic monitoring during surgery in patients undergoing non-cardiac surgery and in non-cardiac intensive care units.

   References Top

1.Berlauk JF, Abrams JH, Gilmour IJ, et al. Preoperative optimization of cardiovascular hemodynamics improves outcome in peripheral vascular surgery. Ann Surg 1991; 214: 289-297.  Back to cited text no. 1  [PUBMED]  [FULLTEXT]
2.Bishop MH, Shoemaker WC, Appel PL, et al: Prospective, randomized trial of survivor values of cardiac index, oxygen delivery, and oxygen consumption as resuscitation endpoints in severe trauma. J Trauma 1995; 38: 780-787.  Back to cited text no. 2    
3.Streisand JB, Clark NJ, Pace NL. Pulmonary arterial catheterization before anesthesia in patients undergoing cardiac surgery. Placement of the pulmonary arterial catheter before anesthesia for cardiac surgery: safe, intelligent and appropiate use of invasive hemodynamic monitoring. J Clin Monit 1985; 1: 193-197.  Back to cited text no. 3    
4.Zhao X, Mashikian JS, Panzica P, et al. Comparison of thermodilution bolus cardiac output and Doppler cardiac output in the early post-cardiopulmonary bypass period. J Cardiothorac Vasc Anesth 2003; 17: 193-198.  Back to cited text no. 4  [PUBMED]  [FULLTEXT]
5.Wallace DC, Winslow EH. Effect of iced and room temperature injectate on cardiac output measurements in critically ill patients with low and high cardiac output. Heart Lung 1993; 22: 55-63.  Back to cited text no. 5  [PUBMED]  
6.Bazaral MG, Petre J, Novoa R: Errors in thermodilution cardiac output measurements caused by rapid pulmonary artery temperature decreases after cardiopulmonary bypass. Anesthesiology 1992; 73: 21-37.  Back to cited text no. 6    
7.Latson TW, Whitten CW, O'Flaherty D: Ventilation, thermal noise, and errors in cardiac output measurements after cardiopulmonary bypass. Anesthesiology 1993; 79: 1233-1243.  Back to cited text no. 7    
8.Connors AF, Seperoff T, Dawson NV, et al: The effectiveness of right heart catheterization in initial care of the critically ill patients. JAMA 1996; 276: 889-897.  Back to cited text no. 8    
9.Practice Guidelines for Pulmonary Artery Catheterization (Approved by House of Delegates on October 21, 1992 and last amended October 16, 2002) http:/ and services/ pulm_artery.pdf .  Back to cited text no. 9    
10.Chernow B. Pulmonary artery flotation catheters: A statement by the American College of Chest Physicians and the American Thoracic Society. Chest 1997; 111:261-266.  Back to cited text no. 10  [PUBMED]  [FULLTEXT]
11.Tuman KS, McCarthy RJ, Spiess BD, et al: Effect of pulmonary artery catheterization on outcome in patients undergoing coronary artery surgery. Anesthesiology 1989;70: 199-206.  Back to cited text no. 11    
12.Troianos CA, Porembka DT. Assessment of left ventricular function and hemodynamics with transesophageal echocardiography. Crit Care Clin 1996;12: 253-72.  Back to cited text no. 12  [PUBMED]  
13.Sageman WS, Riffenburgh RH, Spiess BD. Equivalence of bioimpedence and thermodilution in measuring cardiac index after cardiac surgery. J Cardiothorac Vasc Anesth 2002; 16: 8-14.  Back to cited text no. 13  [PUBMED]  [FULLTEXT]
14.Jaeggi P, Hofer CK, Klaghofer R, et al. Measurement of cardiac output after cardiac surgery by a new transesophageal Doppler device. J Cardiothorac Vasc Anesth 2003; 17: 217-220.  Back to cited text no. 14  [PUBMED]  [FULLTEXT]
15.Mielck F, Buhre W, Hanekop G, et al. Comparison of continuous cardiac output measurements in patients after cardiac surgery. J Cardiothorac Vasc Anesth 2003; 17: 211-216.  Back to cited text no. 15  [PUBMED]  [FULLTEXT]
16.Robinson GJB, Peyton PJ, Vartuli GM, et al. Continuous measurement of cardiac output by inert gas throughflow: comparison with thermodilution. J Cardiothorac Vasc Anesth 2003; 17: 204-210.  Back to cited text no. 16    
17.Nidorf SM, Picard MH, Triulzi MO, et al. New perspectives in the assessment of cardiac chamber dimensions during development and adulthood. J Am Coll Cardiol 1992; 19: 983-8.  Back to cited text no. 17  [PUBMED]  
18.Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1: 307-310.  Back to cited text no. 18  [PUBMED]  
19.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 1999; 27: 33-37.  Back to cited text no. 19  [PUBMED]  
20.Tan HL, Pinder M, Parsons R, et al. Clinical evaluation of USCOM ultrasonic cardiac output monitor in cardiac surgical patients in intensive care unit. Br J Anaesth 2005; 94: 287-291.  Back to cited text no. 20  [PUBMED]  [FULLTEXT]
21.Chand R, Mehta Y, Trehan N. Cardiac output estimation with a new Doppler device after off-pump coronary artery bypass surgery. J Cardiothorac Vasc Anesth; 2005; 20; 315-319.  Back to cited text no. 21    
22.Zoghbi WA, Quinnones MA. Determination of cardiac output by Doppler echocardiography: a critical appraisal. Herz II 1986, 11: 258-268.  Back to cited text no. 22    
23.Quinones MA, Otto CM, Stoddard M, et al. Recommendations for quantification of Doppler echocardiography: A report from the Doppler quantification task force of the nomenclature and standards Committee of the American Society of Echocardiography. J Am Soc Echocardiogr 2002; 15: 167­-184.  Back to cited text no. 23    
24.Phillips RA, West MJ, Burstow DJ: Transcutaneous continuous wave Doppler monitoring is feasible producing reliable and reproducible signals. J Am Coll Cardiol 2002; 30 (Suppl B): 684.  Back to cited text no. 24    

Correspondence Address:
Yatin Mehta
Department of Anaesthesiology and Critical Care, Escorts Heart Institute and Research Centre, Okhla Road, New Delhi - 110025.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0971-9784.37939

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  [Figure 1], [Figure 2]

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