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).
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).
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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