Transesophageal echocardiography (TEE) is an examination particularly useful in patients undergoing cardiac surgeries due to dissecting aortic aneurysm, embolism of pulmonary artery trunk and branches, atrial and ventricular septum defects and acquired valvular defects. Pre-and post-operative assessment using TEE is extremely important in mitral and tricuspid valvuloplasty. In patients subjected to transesophageal examinations, the left cardiac auricle and the atrial septum are routinely assessed [1].

In off pump coronary artery bypass (OPCAB) procedures, it is important to assess changes in valvular incompetence during heart balancing and rotation. The abnormalities of segmental contractility of the left ventricular muscle can be accurately assessed. The evaluation of contractility of the inferior and posterior myocardial walls, invisible in the operating field, is particularly useful.

The basic and most commonly used parameter of systolic left ventricular function is the ejection fraction [2]. This parameter during TEE may be assessed by observation – in real time, and after stopping the cardiac image in systole and diastole - once suitable measurements have been performed [3]. Moreover, transesophageal echocardiography enables intraoperative measurement of cardiac output [4]. 

The objective of the present study was to compare two methods of cardiac output measurements in patients scheduled for cardiac surgeries.

METHODS

With the approval of the Bioethics Committee by the Medical University of Lublin, the study involved patients scheduled for elective OPCAB. The inclusion criteria were good systolic function of the left ventricle (ejection fraction >50%), regular heart rhythm and normal function of the aortic and tricuspid valves. Patients in whom the 20º angle between the bloodstream axis in the outflow tract of the left ventricle and ultrasound beam axis was not obtained were excluded.

Cardiac output was measured by thermodilution using the Swan-Ganz catheter (131 HVF, Edwards Lifesciences, USA). Three measurements were carried out using
10 mL 0.9% NaCl of room temperature. If the difference between the results was higher that 10%, two additional measurements were conducted and extreme readings were excluded. The mean of all recorded measurements was considered as a correct value.

Cardiac output was measured echocardiographically using the multiplane transesophageal probe (GE Medical System, Model 6T, GE Vingmed Ultrasound, Norway) and the echocardiograph (Vivid 3, GEMS Ultrasound, Israel). The examination with continuous wave was performed in deep transgastric projection directing the ultrasound beam centrally through the aortic valve. Measurements with pulse waves were carried out 0.5cm below the aortic valve ring (Fig. 1) centrally in the left ventricular outflow tract (LVOT). Assuming that LVOT is circularly shaped, the outflow area was calculated using the formula β x (LVOT D/2)2. An alternative was imaging of the aortic valve (Fig. 2) and determination of its area using planimetric methods - aortic valve area (AVA). If the outflow tract from the left ventricle was poorly visualized in the central projection, LVOT measurements were performed in the central-oesophageal long - axis projection (Fig. 3). 

Once the optimal spectrum of flow through the outflow tract from the left ventricle and aortic valve was obtained, the velocity time integral (VTI) was calculated, which corresponded to the area under the velocity curve (Fig. 4).
The mean of three VTI values was used for further calculations. 

Cardiac output assessed echocardiographically was calculated according to the formula  CO = HR x (VTI x LVOT or AVA area) [5].

Four or five series of measurements were carried out in each patient, using individual methods depending on the duration of surgery under the same haemodynamic conditions.

Echocardiographic and Swan-Ganz catheter measurements were compared. The differences were presented in percentages. Data were statistically analysed using the Pearson correlation coefficient. P<0.05 was considered as statistically significant.

RESULTS

The study included 30 patients (21 males and 8 females) aged 44-82 years. In total, 127 measurements of cardiac output were performed using each method. 

Cardiac output measured with the Swan-Ganz catheter was 4.59±1.25 L min-1 whereas its values assessed echocardiographically using continuous and pulse waves were 4.57±1.29 L min-1 and 4.49±1,14 L min-1, respectively. The arithmetic mean of cardiac output assessed using both echocardiographic methods was 4.53±1.1 L min-1. The differences in cardiac output assessed with individual methods and correlations between them are presented in Table 1.

There were no complications related to measurements performed during surgery or the early postoperative period. 

DISCUSSION

Assessment of cardiac output is an essential part of intraoperative management in patients undergoing cardiac surgical procedures. Irrespective of the method used, cardiac output measurements are associated with some risk of error, which may significantly affect therapeutic decisions [6]. This concerns both the echocardiographic and thermodilution technique, although the latter still appears to be the reference method [7]. The reliability of the method used is also determined by the presence of heart defects. It is well known that in patients with tricuspid valve incompetence, cardiac output measured by thermodilution is underestimated [8] whereas in cases of marked defects of the aortic valve, the echocardiographic measurements are less reliable [9]. 

Recent popularity of transesophageal echocardiographic assessment of cardiac output results from its low invasiveness [3, 4]. Both continuous and pulse waves of blood flow are used [8, 9, 10]. In both cases, the examination is carried out in the same transgastric long axis (119±80) [11] or transgastric deep projection (00) [4]. Both projections visualize the blood flow through the left ventricular outflow tract and aortic valve and enable to direct the ultrasound beam parallel to the blood flow. Our experience suggests that the transgastric deep projection is more effective. The angle between the blood flow and ultrasound beam should not exceed 20º; otherwise, the blood velocity and flow are underestimated [9, 12].

Once the continuous wave is used, the optimal spectrum of flow through the aortic valve should be carefully searched for. If the spectrum is not properly determined, VTI calculations are false, hence cardiac output is miscalculated. In this method, however, errors in planimetric measurements of the open aortic valve area are unlikely.

When cardiac output is measured with pulse waves, LVOT diameters should be very accurately measured. Even a slight mistake results in miscalculations of the flow area, which, in turn, leads to miscalculations of cardiac output.

For the reasons mentioned, the authors take the view that the risk of errors in echocardiographic measurements of cardiac output may be optimally limited when both methods are used and the mean value is considered reliable.

Our findings showed a strong correlation between cardiac output values measured with various techniques. Even if differences in individual measurements exceeded 10%, the dynamics of changes always headed in the same direction. It should be stressed, however, that examinations were performed in patients with good systolic function of the left ventricle.

Transesophageal echocardiography is a recognized method of cardiac output measurements in patients undergoing cardiac surgical procedures. The contraindications include varicoses and diverticula of the oesophagus, earlier surgeries as well as oesophageal constrictions and tumours [4].  During procedures under deep hypothermia, the duration of probing should be limited to a minimum. Once these rules are followed, the method is extremely useful and safe in patients operated on due to cardiac diseases.

CONCLUSIONS

1. Echocardiographic measurements of cardiac output using continuous or pulse waves are comparable with thermodilution measurements.

2. The mean cardiac output value from both echocardiographic measurements is best correlated with thermodilution measurements of cardiac output.

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REFERENCES

1.    Cahalan MK: Transesophageal echocardiography for the occasional cardiac anesthesiologist. Anesth Analag (Suppl. IARS) 2006, 17-22.

2.    Hoffman P, Kasprzak. J:  Echokardiografia. Via Medica. Gdańsk, 2004: 23-41.  

3.    Andres J, Wąsowicz M: Wybrane zagadnienia anestezjologii i intensywnej terapii w chirurgii serca i naczyń. Danbert. Kraków,  2002: 93-105.

4.    Kucewicz E, Czech B,Juszczyk G,Jackowski R, Szapiel G, Bartosiewicz J, Siemiątkowski A: Ocena porównawcza dwóch metod śródoperacyjnego pomiaru rzutu minutowego serca (termodylucja i echokardiografia przezprzełykowa) u chorych poddawanych zabiegom pomostowania tętnic wieńcowych z użyciem krążenia pozaustrojowego. Anestezjologia  Intensywna Terapia 2003; 35: 3-7.

5.    Podolec P, Tracz W, Hoffman P: Echokardiografia praktyczna. Medycyna Praktyczna. Kraków, 2004: 103-105.

6.    Grow MP, Singh A, Fleming NW, Young N, Watnik M: Cardiac output monitoring during off – pump coronary artery bypass grafting. J Cardiothorac Vasc Anesth 2004; 18: 43-46.

7.    Ranucci M: Which cardiac surgical patients can benefis from placement of pulmonary artery catheter? Crit Care 2006; http://ccforum.com/content/10/S3/S6.

8.    Balik M, Plasil P, Pazout J, Othal M, Fric M, Pachl J: Correlation of cardiac output measurement with transesophageal echocardiography and bolus termodilution technique in patients with various of tricuspid regurgitation. Anest Intenziv Med  2004;15: 204-208.

9.    Royse CF, Royse AG, Blake DW, Groggs LE: Measurement of cardiac output by transesophageal echocardiography: a comparison of two doppler methods with termodilution Anaesth Intensive Care 1999; 27: 586-590.

10.    Swanevelder J: The role of transesophageal echocardiography for the cardiac anaesthetist. Anestezjologia  Intensywna Terapia 2001; 33 (Suppl.):10-13.  

11.    Feinberg MS, Hopkins WE, Davila-Roman VG, Barzilai B: Multiplane transesophageal echocardiographic doppler imaging accurately determinates cardiac output measurements in critically ill patients. Chest 1995; 3: 769-773.

12.    Marik P: Pulmonary artery catheterization and esophageal doppler monitoring in the ICU. Chest 1999;116: 1085-91.    

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Address:

*Jarosław Bródka
Oddział Kardiochirurgii
SPSW im. Papieża Jana Pawła II w Zamościu
Al. Jana Pawła II 10, 22-400 Zamość
tel: 0-84 6773186 
e-mail: jbrodka@poczta.onet.pl

Received: 21.05.2009
Accepted: 01.10.2010