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European Journal of Echocardiography 2004 5(2):111-117; doi:10.1016/S1525-2167(03)00052-0
© 2004 by European Society of Cardiology
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Copyright © 2003, The European Society of Cardiology

Usefulness of Blalock–Taussig shunt Doppler flow velocity profiles in the assessment of pulmonary artery pressure and flow

M. Chaudhari{dagger}, C. Balmer, J.T. Heng, J. Wright and O. Stümper*

The Heart Unit, Birmingham Children's Hospital — NHS Trust, Steelhouse Lane, Birmingham, UK

Received 2 December 2002; received in revised form 3 June 2003; accepted after revision 4 June 2003.

* Address correspondence to: Dr Oliver Stümper, Consultant Paediatric Cardiologist, Birmingham Children's Hospital — NHS Trust, Steelhouse Lane, Birmingham B4 6NE, UK. Tel: +44-121-333-9442; Fax: +44-121-333-9441. oliver.stumper{at}bhamchildrens.wmids.nhs.uk


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Notes
 References
 
Aim: To assess the utility of continuous wave Doppler evaluation of velocity profiles across a Blalock–Taussig (BT) shunt in the evaluation of pulmonary artery pressure and pulmonary blood flow.

Methods and Results: Eleven children with complex congenital heart disease with a BT shunt as the sole source of pulmonary blood supply were studied prospectively (median age 5 (0.3–21) months). Doppler evaluations of shunt flow velocity profile and cardiac catheterization were carried out simultaneously. Pulmonary artery pressure and flow were estimated using the modified Bernoulli equation and velocity time integral of shunt flow. There was a positive correlation between (1) the Doppler estimates for mean pulmonary artery pressure, using the diastolic flow velocity, and the mean pulmonary venous wedge pressure (r=0.93, SEE = 1.0 mmHg; P<0.001) and (2) the Doppler derived and calculated pulmonary blood flow (r=0.9, SEE = 0.19 l/min; P<0.001). In contrast, the Doppler estimates for mean pulmonary artery pressure using the peak or mean Doppler gradients were poor.

Conclusion: The diastolic BT shunt flow velocity can be used reliably to predict mean pulmonary artery pressure when a BT shunt is the sole source of pulmonary artery flow. There was agreement between Doppler predicted pulmonary blood flow and catheter based calculations. These findings may prove a useful tool for perioperative management.

Keywords: congenital heart disease; Doppler ultrasound; BT shunt; pulmonary artery pressure


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Notes
 References
 
Critically ill neonates with complex congenital heart disease frequently require the creation of a modified Blalock–Taussig (BT) shunt as the first step for surgical palliation. Early postoperative hemodynamic problems, if they occur, are frequently related to either excessive pulmonary blood flow or elevated pulmonary vascular resistance. These two scenarios are often difficult to assess on purely clinical grounds or by the use of standard monitoring techniques.

Previous studies have demonstrated the utility of ductal Doppler flow velocity profiles in estimating systolic, diastolic and mean pulmonary arterial (PA) pressures[1–7]. The present study addresses the potential utility of continuous wave Doppler flow velocity profiles obtained across a modified BT shunt for estimating pulmonary artery pressures and pulmonary blood flow in neonates and infants in whom a BT shunt is the sole source of pulmonary blood supply.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Notes
 References
 
Patients
Eleven consecutive patients with complex congenital heart disease, in whom a modified BT shunt was the sole source of pulmonary blood supply, were studied prospectively. Rightsided modified BT shunts were created using a short (1.5–2.5 cm) GORE TEX tube (W.L. Gore & Associates (UK) Ltd, Livingston, Scotland) with a diameter of 3–4 mm (Table 1). Median age at study was 5 months (range 0.3–21 months). The primary diagnoses were hypoplastic left heart syndrome in eight, double inlet left ventricle with hypoplastic aortic arch in two and congenitally corrected transposition of the great arteries with pulmonary atresia in one. All patients underwent cardiac catheterization in preparation for further surgical intervention (bi-directional cavopulmonary shunt in 10 patients, and complete repair in one patient). Simultaneous continuous wave Doppler ultrasound measurements were performed. The study was in agreement with the local ethics committee and informed consent was obtained.


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  		Table 1 Patient data.

 
Cardiac Catheterization
All patients underwent cardiac catheterization under general anesthesia and positive pressure ventilation using inspired oxygen concentrations of less than 30%. Sampling for oxygen saturations was conducted at predetermined sites, including the superior vena cava, right atrium, left atrium, pulmonary veins and aorta. Oxygen uptake was estimated to be 7 ml/kg bodyweight per minute under general anesthesia[8,9]. Pulmonary blood flow was calculated using the Fick principle. In patients with a BT shunt representing the sole source of pulmonary blood supply, PA oxygen saturation was assumed to be equal to aortic oxygen saturation. Thus

Qp = VO2/O2 (content PV – O2 content Ao) with O2 content = SO2) *Hb*0:134:

Pressure measurements included pressure recordings within the right atrium, left atrium, the systemic ventricle, the aorta and the innominate artery just above the modified BT shunt. Pullback pressure measurements were recorded between the atria and across the aortic arch. In all cases a 4 or 5 French endhole catheter was used to obtain pulmonary venous (PV) wedge pressures of the right lung. All traces were acquired with temporary apnea and recording of mean pressures for at least 5 s. Angiography included innominate vein injection, ventriculogram and angiogram of the right innominate artery so as to visualize the shunt and to exclude either shunt or PA obstructions.

Doppler Ultrasound Examination
Near simultaneously with the catheter pressure recording of the PV wedge pressure, the Doppler velocity profiles across the BT shunt were recorded using a non-imaging continuous wave Doppler probe (1.9 MHz probe on a Hewlett–Packard imaging system model SONOS 2000). An attempt was made to obtain the clearest and strongest signal with minimum amplification, while keeping filter settings at constant values. The best signals could normally be obtained from either the suprasternal notch or the medial aspect of the supra-clavicular region. Continuous ECG recording was used in all patients. Heart rate, maximum peak, minimum diastolic, mean flow velocities and the velocity time integral over one cardiac cycle were assessed from three consecutive beats and the results were averaged.

Calculations
Peak, diastolic and mean pressure gradients across the shunt were estimated from the shunt flow measurements using the modified Bernoulli equation. Corresponding PA mean pressure estimates were obtained by subtracting the pressure gradient from the systolic, diastolic and mean aortic pressure recorded just above the BT shunt according to the formula

PA pressure = Ao pressure – 4*V2 (shunt).

The calculated PA pressure thus obtained was then correlated with the mean PV wedge pressure of the ipsilateral lung obtained during simultaneous cardiac catheterization (Table 2).


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  		Table 2 Catheter and Doppler measurements.

 
Further, pulmonary blood flow was calculated from the velocity flow profile according to the formula[10]

Doppler Qp = Heart rate * velocity time integral * shunt area,

assuming a circular diameter of the BT shunt. The Doppler derived estimate of pulmonary blood flow was correlated with the estimated pulmonary blood flow using the Fick principle (Table 2).

Statistical Analysis
Differences between mean values were compared by Student's t-test, and a P value of <0.05 was taken as statistically significant. Comparative data plots were constructed and the linear regression equation was calculated to compare the two techniques. Bland–Altman analysis of mean differences and limits of agreement for comparison of pulmonary artery pressure and flow as calculated by the two techniques was carried out.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Notes
 References
 
There was a highly positive correlation (r=0.93, SE=1.0) between the Doppler derived PA mean pressure, using the minimum diastolic flow velocity for calculation, and the mean PV wedge pressure obtained at simultaneous cardiac catheterization [Fig. 1(c)]. The correlations between the Doppler derived PA mean pressure calculated from either the peak or the mean flow velocity and the mean PV wedge pressure were poor (r=0.79, SE=5.4 and r=0.69, SE=4.7, respectively) [Fig. 1(a) and (b)]. The Bland–Altman analysis of mean differences for assessment of pulmonary artery mean pressure using invasive pulmonary vein wedge pressure measurement and diastolic shunt flow velocity documented good limits of agreement (Fig. 2).


Figure 1
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  		Figure 1 Correlations between PV wedge pressure at cardiac catheterization and Doppler estimated pulmonary artery pressure using the Doppler evaluation of BT shunt velocity profiles. Solid line=line for linear regression, dashed line=line of identity. (a) Correlation using the systolic shunt flow velocity, (b) correlation using the mean shunt flow velocity, (c) correlation using the diastolic shunt flow velocity.

 


Figure 2
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  		Figure 2 Bland–Altman plot depicting the mean differences and limits of agreement in estimation of PA mean pressure using the diastolic shunt flow velocity and PV wedge pressure. The solid line represents the mean difference between the two techniques and the hashed lines represent mean±1.96 SD.

 
There was a highly positive correlation between the Doppler derived estimate of pulmonary blood flow (Qp) and the calculated pulmonary blood flow at cardiac catheterization. (r=0.9, SE=0.19; Fig. 3). Bland–Altman analysis documented acceptable limits of agreement (Fig. 4).


Figure 3
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  		Figure 3 Correlation between pulmonary blood flow determined at cardiac catheterization using the Fick principle and Doppler estimated pulmonary blood flow using the Doppler evaluation of BT shunt velocity profiles. Solid line=line for linear regression, dashed line=line of identity.

 


Figure 4
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  		Figure 4 Bland–Altman plot depicting the mean differences and limits of agreement in estimation of pulmonary blood flow using the Doppler shunt flow time velocity integral and catheter based Fick principle. The solid line represents the mean difference between the two techniques and the hashed lines represent mean±1.96 SD.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Notes
 References
 
For more than a decade continuous wave Doppler ultrasound has been used in daily clinical practice to estimate intracardiac pressure gradients[11,12]. Numerous studies have documented the utility and reliability of this technique in the noninvasive assessment of right ventricular systolic pressure and also for the estimation of PA pressures in the presence of a patent ductus arteriosus[1,3–7,13–16]. In vitro[17,18] studies of BT shunt Doppler derived gradients have also been carried out recently. The purpose of this study was to assess the clinical usefulness of Doppler measurements across the BT shunt in a growing group of patients with complex congenital heart disease with the shunt being the only source of pulmonary blood flow.

Despite the advances in pediatric cardiac surgical techniques the creation of a modified BT shunt remains one of the most important and effective surgical procedures in the initial palliation of critically ill neonates with complex cyanotic congenital heart disease. In other instances, the creation of such a shunt constitutes an integral part of the initial surgical procedure, such as first stage of the Norwood operation. Early postoperative problems, if they occur, are frequently related to either excessive pulmonary blood flow or persistent elevation of pulmonary vascular resistance. Cardiac catheterization is the gold standard for accurate assessment of pulmonary artery pressure and pulmonary vascular resistance. However, in patients in whom a modified BT shunt is the sole source of pulmonary blood supply, direct catheterization of the pulmonary arteries and direct measurement of pulmonary artery pressure are technically difficult and can be dangerous because of the risk of acute shunt failure. Also, passage of a diagnostic catheter across a shunt reduces the effective cross-sectional diameter of the shunt, and thereby will influence distal pressure measurements. We therefore elected to utilize the catheter derived mean PV wedge pressure as an indirect measurement of pulmonary artery mean pressure[19–22]. In clinical practice, PA systolic and diastolic pressure cannot be assessed reliably by recordings of PV wedge pressure. Hence we have only attempted to correlate Doppler velocities with the indirect measurement of PA mean pressure.

The poor correlation of estimated pulmonary artery pressures using peak or mean Doppler gradients is not fully explained. Local hemodynamic factors such as partial shunt obstruction[18] may, in part, be the cause. In vitro measurements showed underestimation of Doppler gradients across small shunts due to significant viscous losses within the shunt, which are not taken into account in the simplified Bernoulli equation[17]. By underestimating the gradient across the shunt, the pulmonary artery pressure would be overestimated. This trend is also reflected in our findings regarding estimation of pulmonary artery pressure using the systolic or mean Doppler velocities [Fig. 1(a) and (b)]. In contrast, estimation of pulmonary artery pressure using diastolic Doppler gradients correlated well with the catheter derived mean PV wedge pressure [Figs. 1(c) and 2Go]. This estimate of the pulmonary artery mean pressure as well as the Doppler estimate of pulmonary blood flow provides a sensitive noninvasive tool in the assessment of PA mean pressure and pulmonary blood flow. This technique potentially offers the opportunity to monitor acute changes in the patients' condition and also the effect of therapeutic interventions in manipulating pulmonary vascular resistance. Preliminary studies in our intensive care unit in a prospective group of nine patients who underwent a modified Norwood stage I procedure documented poor outcome in both patients in whom Doppler calculated PA mean pressures could not be reduced to below 30 mmHg despite maximum therapeutic interventions.

The accuracy of the described method of estimating PA mean pressure using the diastolic flow velocity across a BT shunt would appear to depend on obtaining reliable arterial pressure measurements just proximal to the shunt. Care has to be taken to exclude obstruction across the aortic arch or individual head and neck vessels. Also, in the presence of obstructions to either the proximal or distal anastomosis of the shunt the technique would appear to be limited. In none of our patients did we encounter shunt stenosis and invasive pressure recordings were obtained just proximal to the shunt.

We conclude that predictions of pulmonary artery pressures using peak and mean Doppler velocities are inaccurate and should therefore be avoided in clinical practice. In contrast diastolic prediction of the pulmonary artery pressure showed good correlation with the catheter derived mean pulmonary vein wedge pressure. This estimate as well as the Doppler estimate of pulmonary blood flow was found to be sensitive noninvasive tools to assess PA pressure and pulmonary blood flow in infants with complex cyanotic congenital heart disease. In our experience, the technique is of clinical value to evaluate changes in the patients' condition and to monitor the effect of therapeutic interventions to alter pulmonary vascular resistance.


   Notes
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Notes
References
 
{dagger} Present address: The Freeman's Hospital, Freeman Road, Newcastle upon Tyne NE7 7DN, UK. Back


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Notes
 References
 

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