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ORIGINAL ARTICLE
Year : 2019  |  Volume : 13  |  Issue : 2  |  Page : 30-34

Transthoracic echocardiography: a surrogate tool for predicting pulmonary hypertension using pulmonary artery to aorta ratio


1 Department of Cardiac Anesthesiology, Cardiothoracic Sciences Center, All India Institute of Medical Science, New Delhi, India
2 Department of Cardiothoracic and Vascular Surgery, Cardiothoracic Sciences Center, All Institute of Medical Science, New Delhi, India
3 Assistant Professor, Cardiothoracic and Vascular Surgery, Cardiothoracic Sciences Center, AIIMS, New Delhi, India

Date of Submission19-May-2019
Date of Decision13-Jul-2019
Date of Acceptance15-Sep-2019
Date of Web Publication15-Nov-2019

Correspondence Address:
Minati Choudhury
Department of Cardiac Anesthesiology, Cardiothoracic Sciences Centre, All India Institute of Medical Science, New Delhi 110029
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ejca.ejca_10_19

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  Abstract 

Background Owing to the nonspecific and subtle nature of physical signs and symptoms of pulmonary hypertension (PH) in early stages, treatment is usually delayed despite advancement over the past decade. Use of transthoracic echocardiography (TTE) as the initial noninvasive modality in the screening and evaluation of PH can provide key information about the etiology and the prognosis of PH, thereby avoiding the complications associated with the invasive methods. The aim of this study was to determine the usefulness of main pulmonary artery (mPA) and ascending aorta (AscAo) ratio using TTE.
Patients and methods In this prospective observational study, 30 adult patients undergoing elective coronary artery bypass grafting surgery were enrolled. Postanesthetic induction mPA and AscAo transverse diameters were measured using TTE. The mean pulmonary arterial pressures (mPAPs) were recorded using a direct pulmonary artery puncture after sternotomy. Correlations were established using the Pearson correlation coefficient. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated.
Results mPA : AscAo ratio demonstrated significant linear correlation with mPAP (i.e. r=0.553, P=0.00152). Sensitivity, specificity, positive predictive value, negative predictive value were 80, 100, 80, and 100%, respectively, for an mPAP of up to 25 mmHg.
Conclusion TTE-guided measurement of mPA : AscAo ratio can be used as a simple and easily reproducible noninvasive method in predicting PH not only in cardiac but also in other noncardiac settings.

Keywords: pulmonary artery hypertension, screening tool, transthoracic echocardiography


How to cite this article:
Sharma A, Choudhury M, Chauhan S, Hote MP, Ramakrishnan P. Transthoracic echocardiography: a surrogate tool for predicting pulmonary hypertension using pulmonary artery to aorta ratio. Egypt J Cardiothorac Anesth 2019;13:30-4

How to cite this URL:
Sharma A, Choudhury M, Chauhan S, Hote MP, Ramakrishnan P. Transthoracic echocardiography: a surrogate tool for predicting pulmonary hypertension using pulmonary artery to aorta ratio. Egypt J Cardiothorac Anesth [serial online] 2019 [cited 2020 Feb 18];13:30-4. Available from: http://www.ejca.eg.net/text.asp?2019/13/2/30/271077




  Introduction Top


Pulmonary artery (PA) hypertension can complicate most cardiovascular and respiratory diseases [1]. It has been associated with significant morbidity and mortality [2],[3],[4]. Owing to the nonspecific and subtle nature of its physical signs and symptoms, especially in the early stages, treatment is delayed despite advancement over the past decade. Nowadays, various noninvasive techniques are available for detecting pulmonary hypertension (PH) before proceeding to gold standard and definitive right heart catheterization for confirmation. Thus, the use of transthoracic echocardiography (TTE) as one of the initial noninvasive modality in the screening and evaluation of PH can provide key information about the underlying etiology and the prognosis besides avoiding the complications associated with the invasive methods [5].

Echocardiographic assessments of various variables are available, of which Doppler examination is used frequently to estimate the PA pressures in clinical settings. However, dependency on multiple factors has questioned its role as a screening tool for the diagnosis of PH in terms of accuracy [6],[7]. Based on various studies done using computed tomography (CT), magnetic resonance imaging (MRI), and transesopheageal echocardiography (TEE), measurements of the main pulmonary artery (mPA) to ascending aorta (AscAo) (mPA : AscAo) ratio had provided a fairly good correlation with the presence or absence of PH. This ratio can serve as an easily reproducible alternative parameter for evaluating the at-risk patients for PH.

The objective of this prospective observational study was to determine the usefulness of easily measurable mPA : AscAo ratio using TTE that would correlate with mean pulmonary artery pressure (mPAP) obtained by direct puncture of PA. The presence of mPA : AscAo ratio of at least one could serve as a screening parameter in identifying PH (mean PAP of≥25 mmHg).


  Patients and methods Top


After obtaining institutional ethics committee approval and informed consent from patients, 30 adult patients undergoing elective coronary artery bypass grafting, with or without cardiopulmonary bypass, were enrolled in this prospective observational study. Sample size for the study was calculated using the formula, N=[(+)/C]2+3, where N is total sample size, and and are standard normal deviate for α and β, respectively (valued as =1.960 and =1.282). Required sample size obtained was 24 with two-tailed α value of 0.05, β value of 0.100, and expected correlation coefficient of 0.61. We included a total of 30 patients for the study. Patients with valvular heart diseases, patients with any congenital heart diseases, patients who underwent redo surgeries/emergency surgeries, patients with BMI of at least 30 kg/m2, patients who were chronic smokers, or patients with lung disease were excluded from the study.

All the patients were premeditated with 0.1 mg/kg morphine sulphate and 0.5 mg/kg promethazine intramuscularly 30 min before the induction of anesthesia. In the operating room, all Standard American Society of Anesthesiologists monitoring guideline was used. General anesthesia was induced with etomidate, 0.3 mg/kg, fentanyl, 3 μg/kg, and midazolam 0.01 mg/kg. Endotracheal intubation was facilitated with rocuronium bromide 1 mg/kg, intravenously, along with intermittent positive-pressure ventilation. Invasive monitoring included radial arterial blood pressure and right atrial pressure via central venous access in right internal jugular vein. Anesthesia was maintained with oxygen-air mixture and continuous infusion of atracurium and dexmedetomidine in titrated doses. All the measures were taken to maintain normoxia, normocarbia, and avoidance of acidosis.

A S5-1 transthoracic probe was used and connected to the echocardiography console (iE 33; Philips, Bothell, Washington, USA) to obtain the images. A comprehensive TTE examination was performed in all patients by an experienced echocardiographer. mPA and AscAo transverse diameters were measured in the modified parasternal short-axis view ([Figure 1]) and parasternal long-axis view ([Figure 2]), respectively. Three measurements were taken 5, 10, and 15 min after induction at stable hemodynamics. Images were saved offline for analysis. Soon after the sternotomy, PA pressures were measured by direct PA puncture done by the operating cardiac surgeon who was blinded to the values of great vessels obtained echocardiographically. Hemodynamic and ventilatory parameters were kept near baseline to avoid errors in measurements while acquiring the echocardiographic images and obtaining the PA pressures. Ratio of mPA : AscAo was calculated from the average of the aforementioned three values taken with TTE, which was later correlated with the PA pressures achieved by direct PA puncture.
Figure 1 Measurement of the diameter of mPA (just above bifurcation) in modified parasternal short-axis view with TTE. LPA, left pulmonary artery; mPA main pulmonary artery; RPA, right pulmonary artery; RVOT, right ventricular outflow tract; TTE, transthoracic echocardiography.

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Figure 2 Measurement of the diameter of ascending aorta (above sino-tubular junction) in parasternal long axis view with TTE. LA, left atrium; LV, left ventricle; MV, mitral valve; TTE, transthoracic echocardiography.

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Statistical analysis

Mean±SD was used to expressed normally distributed values (Kolmogorov–Smirnov test). Correlation between mPA : AscAo ratio and mPAP was established using Pearson linear correlation coefficient (r). Receiver operating characteristic analysis assessed the usefulness of mPA : AscAo ratio in predicting the presence of PH, defined as mPAP of at least 25 mmHg, for the diagnostic purpose. Statistical analysis done using IBM SPSS statistics version 24. A P value less than 0.05 was considered statistically significant.


  Results Top


All 30 adult patients included in the study who underwent elective coronary artery bypass grafting surgery were analyzed. Baseline demographic patient characteristics are summarized in [Table 1] and [Table 2].
Table 1 Baseline patient characteristics

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Table 2 Patient characteristics

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mPA : AscAo ratio demonstrated a significant linear correlation with mPAP measured by direct PA puncture (i.e. r=0.553, confidence interval=0.241–0.761, P=0.00152; P<0.05; [Table 3]).
Table 3 Correlation between mean pulmonary artery pressure and main pulmonary artery : ascending aorta

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Receiver operating characteristic curves were performed to evaluate sensitivity and specificity of mPA : AscAo ratio of at least one for diagnosing PH, defined as mPAP of at least 25 mmHg. Area under the curve for mPA : AscAo ratio was 0.838 (95% confidence interval: 0.658–0.946, P=0.0024), with a sensitivity of 80%, specificity of 100%, positive-predictive value of 80%, negative-predictive value of 100%, positive-likelihood ratio of 11.0, and negative-likelihood ratio of 0.0 for an mPAP of at least 25 mmHg ([Figure 3]).
Figure 3 Receiver operating characteristic curves of mPA : AscAo ratio and mPAP with AUC of 0.838. AscAo, ascending aorta; AUC, area under curve; mPA main pulmonary artery; mPAP, mean pulmonary arterial pressure.

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  Discussion Top


PH is a pathophysiological disorder that may involve multiple clinical conditions. In the presence of nonspecific early symptoms and subtle physical signs, it requires a high clinical index of suspicion to detect the disease before irreversible pathophysiologic changes occur [8]. As it is the leading cause of morbidity and mortality in patients with heart disease, early identification of signs of PH needs utmost attention and prompt measures can reduce pulmonary artery pressure (PAP).

A number of studies based on CT/MRI have been described for the diagnosis of PH based on changes in various dimensions of mPA [9]. One of the largest study to date describing the normal reference values for mPA and mPA : AscAo ratio by CT imaging in an asymptomatic community-based population was the Framingham Heart Study [10]. Based on the diameter of the mPA relative to the AscAo (mPA : AscAo ratio), few of the previously done CT imaging-based studies for describing the diagnosis of PH had also shown mPA : AscAo ratio of more than one in predicting the presence of PH [mPAP>20 mmHg ([Table 4])] [11],[12],[13],[14],[15].
Table 4 Computed tomography-guided study measurements in patients with pulmonary hypertension

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However, CT imaging is a costly modality associated with radiation exposure, and echocardiography has the advantage of being widely available, cost effective, and safe, and can serve as a pivotal screening tool for PH. To the best of our knowledge, there has been no study performed till date to evaluate PH using TTE based on the relative diameters of the mPA and AscAo.

In this study, we measured the diameters of the mPA in relation to AscAo under TTE guidance in the modified parasternal short-axis view for mPA diameter and parasternal long-axis view for AscAo diameter and finally compared the mPA : AscAo ratio with the direct PA puncture to differentiate patients with PH from normal PA pressures. Cutoff of 25 mmHg was taken as per definition of PH. To keep the measurements totally noninvasive, we chose TTE over TEE. Secondly, TTE measurements not only are of help in patients from cardiac field but also those who have noncardiac problems, for example, hepatic transplant.

We observed that patients with PAH (mPAP≥25 mmHg) had an mPA : AscAo ratio of at least one, whereas patients with normal PA pressures (mPAP<25 mmHg) had an mPA : AscAo ratio of less than one, except in two cases, where despite ratio less than one, they had an mPAP of more than 25 mmHg.

Still it was found that the TTE-measured mPA : AscAo ratio of at least one could identify patients with PAH with a better sensitivity of 100%, specificity of 90.91%, positive predictive value of 80%, and negative predictive value of 100%, as compared with TEE and CT-based studies.

Although Doppler echocardiographic assessment of tricuspid regurgitation (TR) jet velocity is traditionally used to estimate PA pressures, its association with the shortcomings like the inability to align the Doppler beam along the tricuspid valve (dependency on angle of interrogation) or even absence of a TR jet in most patients with coronary artery disease and in presence of PH may limit its use [10],[16],[17]. Thus, in case of inadequate or absence of TR envelope, there is another method to determine PA pressures, that is, pulmonary artery acceleration time (PAAT). Unlike most other indices, PAAT needs to be corrected when the heart rate is more than 100 bpm or less than 70 bpm. Thus, mPAP is calculated using the formula mPAP=79–(0.45×PAAT), and was also reported to be less reliable by several studies. Its dependency on heart rate and RV function again limits the use of PAAT in routine practice. Moreover, these methods are cumbersome and time-consuming as well [5].

In terms of accessibility, simplicity, and being entirely noninvasive, TTE-guided measurement of mPA : AscAo ratio is quite feasible, and it is an easily reproducible method to identify patients with PH. We may also emphasize the routine screening of mPA and AscAo diameters while performing TTE and determination of mPA : AscAo ratio for early suspicion or recognition and thus treatment of PH, thereby decreasing the associated morbidity and improving the outcome.

Although the right-heart catheterization is a gold standard method for the measurement of PA pressure to diagnose PH, being invasive it confers both risk and expense, and hence can delay the diagnosis. Moreover, this procedure needs expertise, and technical difficulties to guide the catheter into the PA may be challenging [18],[19].

Alternative to TEE measurement of mPA : AscAo ratio, TTE-guided determination of similar ratio can too serve as a simple and reliable tool for identifying PH in other noncardiac settings as well.


  Conclusion Top


TTE-based ratio of mPA : AscAo for predicting PH can be used as a straightforward, easily reproducible, and feasible noninvasive method with similar reliability and can guide the clinician to screen patients with possible PH.

Limitation

Performing TTE needs expertise in obtaining adequate images as one can encounter poor window, intraobserver as well as interobserver variability. Although every possible measures were taken to obtain echocardiographic parameters at near baseline hemodynamics, some errors may have occurred owing to the effects of anesthesia. Another potential limitation of the study is that the echocardiographic and PA pressure measurements were not performed simultaneously. Studies comparing this parameter with CMR imaging and with a larger sample size may provide further better validation of the subject.

Acknowledgements

The authors thank all the anesthesia technicians and colleagues who significantly assisted in the research.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

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