The Feasibility of Epicardial Echocardiography for Measuring Aortic Valve Area by the Continuity Equation

Hilberath, Jan N. MD*; Shernan, Stanton K. MD*; Segal, Scott MD*; Smith, Brian MD*; Eltzschig, Holger K. MD, PhD†‡

Section Editor(s): Houge, Charles W. Jr; London, Martin J.; Levy, Jerrold H.

doi: 10.1213/ANE.0b013e318187ed0a
Cardiovascular Anesthesiology: Cardiovascular and Thoracic Education: Hemostasis and Transfusion Medicine: Research Reports

BACKGROUND: Measuring the aortic valve area (AVA) remains an important component of a comprehensive intraoperative echocardiographic examination in patients undergoing aortic valve surgery. Epicardial echocardiography (EE) represents an accessible alternative to transesophageal echocardiography (TEE), however, its agreement and correlation with other imaging modalities for measuring AVA has not been systematically validated.

METHODS: EE was used in 85 patients undergoing cardiac surgery to measure AVA (AVA-EE) using the continuity equation. AVA-EE was compared to measurements obtained by intraoperative transesophageal echocardiography (AVA-TEE) in the same population. In a subset of patients, AVA-EE was also compared to AVA measurements from either preoperative transthoracic echocardiography (AVA-TTE) (n = 65) or cardiac catheterization (AVA-Cath) (n = 35) that were acquired within 4 wk before the date of surgery.

RESULTS: Adequate trans-AV Doppler recordings were obtained in 94% of patients for AVA-TEE and 100% of patients for AVA-EE. EE measurements of AVA showed close agreement with TEE measurements (mean difference [bias] ± 95% CI = −0.09 cm2 ± 0.18 cm2, r2 = 0.83, P < 0.0001). AVA-EE also agreed well with AVA-Cath (mean difference ± 95% CI = −0.03 cm2 ± 0.12 cm2, r2 = 0.87, P < 0.0001) and AVA-TTE (mean difference ± 95% CI = −0.06 cm2 ± 0.22 cm2, r2 = 0.81, P < 0.0001).

CONCLUSIONS: EE measurements of AVA by the continuity equation show high agreement and closely correlate with established techniques of AVA assessment.

IMPLICATIONS: Measurement of the aortic valve area is an important component of a comprehensive intraoperative echocardiographic examination during cardiac surgery involving the aortic valve. We demonstrate that epicardial echocardiography can be used as an intraoperative alternative or adjunct to transesophageal echocardiography for measuring the aortic valve area.

From the *Department of Anesthesiology, Perioperative and Pain Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts; †Department of Anesthesiology and Perioperative Medicine, University of Colorado Health Science Center, Denver, Colorado; and ‡Department of Anesthesiology and Intensive Care Medicine, Tübingen University Hospital, Tübingen, Germany.

Accepted for publication June 5, 2008.

Address correspondence and reprint requests to Holger K. Eltzschig, MD, PhD, Mucosal Inflammation Program, Department of Anesthesiology and Perioperative Medicine, University of Colorado Health Science Center, 4200 E. Ninth Ave., Campus Box B112, Denver, CO 80262. Address e-mail to

Article Outline

Epicardial echocardiography (EE) was initially introduced into clinical practice in the 1970s during open mitral valve commissurotomy.1 Despite the increasing availability and improved technological development of transesophageal echocardiographic (TEE) transducers, EE remains a useful intraoperative diagnostic technique during cardiac surgery.2,3 Accordingly, the American Society of Echocardiography and Society of Cardiovascular Anesthesiologists have jointly published developed guidelines on performing a comprehensive intraoperative EE examination,4 and currently recommend EE as a core component of advanced training.5

The evaluation of valve function is a critical component of a perioperative echocardiographic examination in patients with valvular disease.6–8 For example, intraoperative assessment of the aortic valve (AV) is necessary to confirm the severity of known disease in patients who present for AV surgery, to diagnose previously unknown AV pathology in patients initially scheduled for only coronary artery bypass grafting, and to guide surgeons to replace or repair only moderately diseased AVs.9 Estimating the aortic valve area (AVA) by TEE using the continuity equation has become a uniformly accepted technique for evaluating the severity of AV stenosis.10–12 However, this approach may not be feasible in all patients in whom the placement of a TEE probe is either difficult or contraindicated,2,13–15 or when the image quality is insufficient due to the constraints imposed by limited transducer manipulation. Thus, while EE is often considered an adjunct to TEE, it may, in fact, be a superior echocardiographic technique in some patients.16,17

The degree of agreement with other imaging modalities in determining AVA, however, has not been systematically validated, and several examples in the literature have shown that alternative echocardiographic approaches may not always provide similar AVA and velocity measurements.18,19 Therefore, we retrospectively compared measurements of AVA using the continuity equation obtained by the EE approach (AVA-EE) with those acquired concurrently by TEE (AVA-TEE) in 85 patients undergoing cardiac surgery. In addition, we compared AVA-EE to AVA measurements from preoperative cardiac catheterization (AVA-Cath) (n = 35) and transthoracic echocardiography (AVA-TTE) (n = 65) in a subset of patients who underwent these examinations at our institution within 4 wk before the date of surgery.

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Patient Population

During the 11 mo study period, 1139 TEE examinations were recorded. The study population consisted of all 85 patients undergoing cardiac surgery at the Brigham and Women’s Hospital in whom intraoperative recordings of TEE and EE examinations were obtained and included measurements of AVA by continuity equation. The decision to perform intraoperative echocardiography and the selection of each individual imaging technique including AVA assessment via EE was made at the discretion of the attending cardiac anesthesiologist and cardiac surgeon. For the purpose of this study, all TEE and EE examinations were analyzed retrospectively and off-line, by two cardiac anesthesiologists certified in perioperative TEE. Approval for reviewing the patients’ medical records and intraoperative TEE and EE examinations was obtained from the IRB.

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TEE Examinations

Comprehensive intraoperative TEE examinations were performed using multiplane probes (Siemens, Mountain View, CA; Philips Healthcare, Andover, MA) after induction of general anesthesia. The left ventricular outflow tract (LVOT) diameter was measured in a midesophageal long axis view at a multiplane angle between 110°–135°. The AV and LVOT velocity time integrals were obtained in the deep transgastric long axis views at a multiplane angle of 0°, by aligning the respective continuous–wave and pulse-wave Doppler beams parallel to systolic blood flow. The AVA was calculated using the continuity equation. In patients with sinus rhythm, the mean of three determinations was obtained. In patients with atrial fibrillation, 10 consecutive beats were analyzed and averaged. All TEE examinations were performed by cardiac anesthesiologists certified in perioperative TEE.

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EE Examinations

EE examinations were performed as part of the routine intraoperative echocardiographic examination by attending cardiac anesthesiologists, who guided the cardiac surgeon in the positioning of the epicardial transducer (7 MHz transducer, V7, Acuson, Mountain View, CA; 8 MHz probe, S8, Philips Healthcare, Andover, MA). Examinations were performed after completion of the median sternotomy and pericardiotomy, but before initiation of cardiopulmonary bypass. Warm saline solution was poured into the pericardial space to improve contact between the transducer and the epicardial surface and to optimize image resolution. The transducer probe was inserted into a sterile sheath containing ultrasonic gel between the transducer surface and the inner portion of the sheath to eliminate potential air space, and thus improve surface contact. Imaging was obtained by placing the probe on the aortic root to permit interrogation of the AV as well as the LVOT (Fig. 1). From this probe position, the Doppler beam was oriented parallel to the long axis of the AV to measure transvalvular and LVOT blood flow velocities via a transgastric long-axis equivalent view (Figs. 2 and 3). The AVA was calculated by the continuity equation as above.2,3,17

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Cardiac Catheterization

AVA measurements from preoperative cardiac catheterization at the Brigham and Women’s Hospital obtained within 4 wk before the date of surgery were available in a subset of 35 patients (41%). Cardiac output was determined using the Fick method, and AVA calculated using the Gorlin formula.20

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TTE Examination

TTE examinations from our institution that were obtained within 4 wk before the date of surgery were available in a subset of 65 patients (76%). Measurements of AVA were performed as previously described.21

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Statistical Analysis

Results are presented as mean ± sd. Intra- and interobserver variabilities were assessed for EE and TEE measurements in a sample group of 20 study patients. The correlation between methods was tested by linear regression analysis and the agreement between techniques by the Bland-Altman method.22 Bland-Altman 95% limits of agreement are derived from the absolute differences between pairs of measurements made by two methods on a group of subjects. It is defined as ±1.96 sd of the differences around the bias (i.e., the 95% CI). Bias refers to any mean difference between the two groups of measurements.23 Error was expressed as the mean difference ± 1.96 sd between the methods. A P value <0.05 was considered significant.

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Demographic data and surgical procedure are displayed in Table 1. In 80 of 85 patients (94%), successful assessment of the AVA by TEE continuity equation could be achieved. Consistent with the literature, the deep transgastric views were inadequate to measure AVA by continuity equation in 5 patients (6% of the entire study population) due to difficulties in obtaining appropriate images and in aligning the Doppler beam parallel to blood flow.24 The EE assessment of AVA was successful in 85 patients (100%). Intraobserver and interobserver variability of AVA measurements from EE were 4.8% ± 4.2% and 6.2% ± 5.3%, respectively. For measurements made with TEE, intraobserver and interobserver variability were 4.4% ± 3.9%, 5.3% ± 4.7%, respectively.

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AVA Bias and Correlation Between EE and TEE

AVA means ± sd are presented in Table 2. The mean difference (bias) was −0.09 cm2. The 95% CI was −0.09 ± 0.18 cm2. Correlations between EE measurements and the respective data obtained by TEE were statistically significant (r2 = 0.83; P < 0.0001, Table 2 and Fig. 4).

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AVA Bias and Correlation Between EE and Cardiac Catheterization

Thirty-five of the 80 patients included in the statistical analysis had a preoperative cardiac catheterization performed at our hospital. AVA means ± sd are presented in Table 2. The mean difference (bias) was −0.03 cm2. The 95% CI was −0.03 ± 0.12 cm2. AVA obtained during cardiac catheterization correlated closely with EE data of the respective patients (r2 = 0.87, P < 0.0001, Table 2 and Fig. 5).

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AVA Bias and Correlation Between EE and TTE

Sixty-five of the 80 patients included in the statistical analysis had a preoperative TTE performed at our institution. AVA means ± sd are presented in Table 2. The mean difference (bias) was −0.06 cm2. The 95% CI was −0.06 ± 0.22 cm2. AVA obtained during TTE correlated closely with EE data of the respective patients (r2 = 0.81, P < 0.0001, Table 2 and Fig. 6).

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The use of EE in assessing AVA has not been systematically tested. Therefore, EE measurements of the AVA via the continuity equation were compared to measurements obtained during TEE examination, preoperative cardiac catheterization, and TTE. We found high agreement and close correlation of EE measurements with AVA-TEE, AVA-Cath, and AVA-TTE, suggesting that epicardial measurement of the AVA by continuity equation represents a reliable alternative to TEE in the perioperative quantitative assessment of AVA.

The present study highlights that EE and epiaortic echocardiography are not limited to the assessment of the ascending aorta and proximal aortic arch for atheromatous disease. Although TEE has a history of safety, rare associations with serious complications have resulted in the recommendation of certain contraindications.13,15,25 Alternatively, EE permits the performance of a comprehensive ultrasonic examination when TEE probe placement is specifically contraindicated or cannot be performed.13 The reported incidence of serious complications or contraindications related to EE is virtually nonexistent, except for rare reports of hemodynamically insignificant dysrhythmias.26 Thus, it is not surprising that the American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists have recently published guidelines pertaining to performing a comprehensive EE examination.4

EE may also have some distinct advantages over TEE. Direct application of the high-frequency transducers to the surface of the heart reduces the signal-to-noise ratio and facilitates the acquisition of high quality images with superior resolution.27 Placement of the epicardial probe directly on the anterior surface of the heart permits optimal visualization of anterior structures such as the pulmonic valve and the AV. In fact, the data from the current study demonstrate a 100% success rate for Doppler interrogation of the AVA and close correlation with other techniques (TEE, cardiac catheterization, and TTE), most likely relating to the fact that the AV is in an ideal anatomic position for EE. In addition, a study of mechanically ventilated patients undergoing TEE evaluation of the AVA reported an 11% failure rate associated with inadequate Doppler beam alignment due to eccentric jets or the presence of mitral annular calcification that obscured AV visualization.24 Thus, obtaining a calculated estimate of AVA by TEE may be complicated by difficulties in accurately aligning the Doppler beam parallel to blood flow through the AV which may result in under-estimation of flow velocities and pressure gradients.14 In our study, AVA-EE averaged about 0.1 cm2 smaller than the AVA calculated from TEE measurements. This discrepancy may be the result of the above-mentioned potential systematic under-estimation of AV velocity time integrals by TEE. Epiaortic echocardiography may therefore provide an advantage over TEE by allowing more freedom to maneuver the probe position on the ascending aortic surface, thus facilitating alignment of the Doppler beam even in the presence of severe AV stenosis, heavy calcification or eccentric jets. Finally, TEE examination of the AV may yield conflicting results that could be clarified by EE. However, several examples in the literature illustrate that alternative echocardiographic approaches may not always provide similar AVA and velocity measurements.18,19 The bias of the Bland-Altman analysis in our study showed only a small discrepancy between the AVA-EE measurements and the other imaging devices that appeared clinically tolerable. Moreover, the measurement variability seemed consistent across the graph, and the scatter around the bias line did not display a significant trend, e.g., did not get larger as the average got higher.

The EE approach does present certain limitations and disadvantages compared with TEE. For example, direct application of an epicardial probe to the cardiac surface requires a sternotomy and may be limited in patients undergoing minimally invasive surgery. The image acquisition can be challenging towards the posterior, extreme inferior and lateral cardiac tissues. Furthermore, EE examination requires an inevitable interruption of the operative procedure during the examination. However, this brief time requirement should not significantly interfere with surgical progress of the operation.27 Finally, since an epicardial probe cannot be maintained in an imaging position throughout the operation, it is difficult to use this technique as a continuous monitor of valvular function. In contrast, TEE provides a relatively nonintrusive continuous monitor of cardiac function, neither requiring entry into the sterile field, nor interruption of the surgical procedure.

Although the fact that EE, TEE, TTE, and cardiac catheterization data were not obtained simultaneously could be considered a limitation of this investigation, in principle, determination of AVA by the continuity equation should be unaffected by any changes in hemodynamics that may have occurred in the interim between diagnostic evaluations. A second potential limitation involves the realization that quantitative echocardiographic measurements may include a qualitative component which can be subject to investigator bias. A blinded, prospective and randomized study design would be superior to limit the incidence of bias, and ultimately determine exact differences between alternative techniques used to grade the severity of AV stenosis.

In conclusion, the data from the present study reveals that EE represents a viable alternative or complimentary technique to TEE for quantitative assessment of AVA. This may become particularly important when other imaging modalities are not available, contraindicated, or lead to conflicting results that potentially influence surgical decision making and patient care.

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