Assessment of Aortic Blood Flow Velocities With Continuous Wave Doppler Ultrasound in the Neonate and Young Child

Transcription

1 JCC Vol. 5, No. I January 1985:113S-9S ll3s ssessment of ortic Blood Flow Velocities With Continuous Wave Doppler Ultrasound in the Neonate and Young Child LIV, MD Trondheim, Norway ortic flow velocities can be recorded with pulsed or continuous wave Doppler ultrasound. In the absence of obstruction or regurgitation, changes in flow can be assessed and stroke volume obtained. Continuous wave Doppler ultrasound has the advantage that there is no limit to the velocities that can be recorded. In left ventricular outflow obstruction and coarctation, the pressure drop across these can be calculated from increases in maximal velocity using a modification ofthe Bernoulli equation. Other systolic high velocity jetssuch as mitral regurgitation or ventricular septal defect may also be recorded with continuous wave Doppler ultrasound from the suprasternal notch, but they can be distinguished from aortic flow velocities by their timing and duration when the flow signals are recorded together with the electrocardiogram and phonocardiogram. In aortic regurgitation, reversal of flow across the aortic valve in diastole can be shown and with high velocity in the regurgitant jet. (} m Coil Cardiol 1985;5: ) Blood flow velocities in the aorta in children and neonates can easily be recorded with continuous wave Doppler ultrasound. With the continuous wave Doppler technique, velocities are recorded all along the ultrasonic beam. lthough there is the advantage that high velocities can be recorded, a signal cannot be localized in depth with continuous wave Doppler techniques. Pulsed Doppler ultrasound has the advantage that a signal can be localized in depth, but there is a limit to the velocities that can be recorded. From the velocity of blood flow, it is possible to assess changes in volume flow. n increase in flow causes an increase in velocity, and velocity decreases with a decrease in flow. By measuring both the velocity of flow and the cross-sectional area, estimation of volume flow is possible (1-3). The shape of the velocity curve is, in addition to volume flow, influenced by left ventricular function and the capacity and resistance in the systemic circulation (4,5). Until recently, the clinical information this may give has been utilized little (6,7). Such assessment of aortic blood flow velocities can be performed equally well with pulsed and continuous wave Doppler ultrasound. Obstruction to flow causes much larger increases in velocity than are seen with increases in flow. The increase in From the Section of Cardiology. Regional Hospital, and Department of Medicine, University of Trondheim, Trondheim, Norway. ddress for reprints: Liv Hatle, MD, Section of Cardiology, Regional Hospital, Trondheim, Norway by the merican College of Cardiology velocity is related to the pressure drop across the obstruction and can be calculated from the maximal velocity using a modified Bernoulli equation (8). By recording maximal velocity in the ascending aorta, left ventricular outflow obstructions can be diagnosed and assessed (9). In obstructions to blood flow velocities usually exceed the limit for most pulsed Doppler systems, and to assess obstructions the continuous wave Doppler mode, therefore, is preferable. ortic regurgitation is diagnosed by the continuous wave Doppler ultrasound recording across the aortic valve, while reverse diastolic flow in the aorta in regurgitation and shunts is better recorded with pulsed Doppler ultrasound (10). Instrumentation Stand-alone Doppler systems. separate continuous wave Doppler system can be used. Equipment with carrier frequencies of 2 to 5 MHz is commercially available. The higher frequencies are in instruments intended for peripheral vascular disease and these can be used for neonates and infants. Instruments with the lower frequencies can be used in older children as well as in infants and neonates. The output from the Doppler system can be recorded either with spectral analysis where all the frequencies in the signal are seen or with analog curves giving the outline of the spectral curve or the mean of the frequencies in the signal. Even in children in whom good Doppler signals are easily obtained, the spectral display has clear advantages. It provides a much better visual documentation of the Doppler signal recorded, /85/$3.30

2 114S JCC Vol. 5. No. I January 1985:113S-9S estimator artifacts are avoided and it is possible to distinguish between the various components of a mixed signal. With a combined pulsed and continuous wave Doppler system, a signal recorded in the continuous mode can also be localized in depth by changing to the pulsed mode. This can help to ascertain the origin of signal recorded with continuous wave Doppler ultrasound (10). Combined Doppler and two-dimensional echocardiography. The combination of continuous wave and pulsed Doppler ultrasound with two-dimensional imaging makes localization of flow signals easier and quicker and avoids ambiguity, especially in patients with complex lesions (11). It is most helpful when recording from apical or subcostal positions because from the suprasternal notch and right sternal border, access is often easier with a smaller separate Doppler transducer than a larger one for combined imaging and Doppler ultrasound. Simultaneous imaging and continuous wave Doppler ultrasound is especially useful in neonates and infants in whom minimal movements may change the site of recording completely. Electrocardiography and phonocardiography. When Doppler ultrasound is used without imaging, correct identification of a flow signal is essential. This can be obtained by recording the valve movements, electrocardiogram and phonocardiogram simultaneously with the Doppler signal (Fig. 4). This can be useful also if continuous wave Doppler ultrasound is used with imaging, as when two high velocity jets of similar direction are present. ortic Flow Velocity Doppler recording technique. Flow velocity in the ascending aorta can be recorded from the suprasternal notch, a high right parasternal position, the apex and a subcostal position. In the descending aorta, flow velocities are best recorded from the suprasternal notch. The Doppler ultrasound signal contains the frequency shift of the backscattered ultrasound. The velocity is obtained from the frequency shift by the Doppler equation that includes the cosine ofthe angle between the ultrasound beam and the velocity. With angles between 20 and 0, the cosine will be from 0.94 to I and can, therefore, be disregarded. If the angle is larger and it is disregarded, velocity will be underestimated. If ignored, an angle of 25 to 30 will cause a 9 to 13% underestimation of velocity and with larger angles this underestimation increases rapidly. The audio signal that represents the frequency shift is used to find where the angle to a flow signal is smallest; as the angle becomes smaller, the signal becomes more high frequent. More than one position and beam direction should, therefore, be tried to find the signal with the highest frequencies, which will give the velocity closest to that present. Evidence in support of the technique is shown in studies on reproducibility (12,13) and simultaneous velocity and pressure measurements in obstructive lesions (8,9,14). ngle correction. When Doppler ultrasound is used together with two-dimensional imaging, another possibility is aortic flow velocity... Il..,.ntl " aortic stenosis pulsed ~ CW Doppler Figure 1. Pulsed Doppler recording from a normal subject (top) shows a narrow band of frequencies. By switching to the continuous wave (CW) mode, the signal also contains some lower frequencies. In aortic stenosis (bottom), the continuous wave mode shows the high velocities exceeding the limit for the pulsed mode which shows aliasing.

3 JCC Vol. 5, No.1 January 1985:113S-9S 115S to correct for the angle to the velocity as measured from the image. The drawbacks are that velocity may not be parallel to the imaged walls, the image is only two-dimensional so that there may be a considerable angle in the third plane that is not seen and the possibility of overestimation ofvelocity by angle correction is also introduced. different direction of velocity from that assumed from the image occurs especially with flow through stenotic and regurgitant valves, but perhaps not as often in infants and young children as in adults. Operating at an angle to flow also increases the error introduced by a certain angle. Therefore, an attempt to obtain alignment between ultrasound beam and velocity is preferred. Pulsed versus continuous wave Doppler ultrasound. With aortic flow velocities within normal limits, there is little difference between the use of pulsed and continuous wave Doppler ultrasound. With pulsed Doppler ultrasound a signal with a more narrow band of frequencies is obtained and the maximal velocity may be little higher than that obtained with continuous wave Doppler ultrasound in the same direction because of a better signal to noise ratio with the pulsed mode. The signal obtained with the continuous wave Doppler technique contains a larger variation of frequencies from all along the beam (Fig. 1). The best Doppler signal is usually quicker to find with the continuous wave mode because various depths will not have to be checked for each beam direction. With increased flow or force of ejection, the velocity profile in the aorta may no longer be flat and, thus, the pulsed mode will be more sensitive to the location of the sample volume within the cross-sectional area of the aorta (15). For practical purposes, however, the information obtained with pulsed and continuous wave Doppler recording from the aorta is similar when there is no left ventricular outflow obstruction. Obtaining cardiac output. In the absence of aortic obstruction or regurgitation, aortic flow velocity can be used to obtain cardiac output. Integrating the area under the velocity curve and multiplying with the cross-sectional area of flow gives the stroke volume. Good correlation with other methods has been shown (1-3,16,17). The critical measurement in children as well as in adults is the diameter measurement of the aorta. Other problems include underestimation of velocity and the fact that the assumption that the velocity profile is flat may not always hold, especially when there is increase in flow. With the good reproducibility shown for aortic flow velocity measurements, relative changes in velocity can be useful in assessing changes in flow. Obstruction to Flow With obstruction to blood flow, the maximal velocity of flow across the obstruction increases. The increase is related to the drop in pressure drop. With a modified Bernoulli equation, the pressure drop can be calculated from the increase in the maximal velocity (8,9). Obstruction to flow produces a high velocity jet that extends for some centimeters beyond the obstruction (18). long the jet, a gradual decrease in velocity occurs, and beside it there may be disturbed flow with low velocities and of various directions. The Doppler signal from the beginning of a jet is pure with high frequencies only; along the jet it is more mixed with lower frequencies. Beside the jet, only low frequencies are present, but these are often of high intensity. To obtain the maximal velocity, the recording must be made from the jet and at a small angle. The audio signal is again used to find the highest frequencies (Fig. 2). Underestimation of velocity will result in a larger underestimation of the pressure drop because the velocity is squared when calculating the pressure drop. Velocity before an obstruction is often so low that it can be ignored in the calculation, but if it is increased it must be considered. With continuous wave Doppler ultrasound, the velocity patterns from both sides of the valves will be present in the signal, and with spectral display they may be clearly separated. For flow across the aortic valve, the velocity in the left ventricular outflow tract can usually be recorded separately with the continuous wave Doppler technique with a slight change in beam direction from an apical or subcostal position. The velocity below the valve can also be recorded with the pulsed Doppler technique. Left Ventricular Outflow Obstructions Calculation of pressure drop. It was shown experimentally (19-21) that pressure drop calculated from maximal velocity across an obstruction comes close to that recorded by pressure except for very small orifice sizes «0.1 cm 2 ) where underestimation of lower pressure drops occurs. To date, practical experience has been that even in neonates with severe aortic stenosis, a small orifice size has not led to underestimation of the pressure drop. From the maximal aortic jet velocities recorded with continuous wave Doppler ultrasound, a peak pressure drop can be calculated. This corresponds to the instantaneous peak pressure difference during systole (Fig. 3). With use of spectral display and fluid-filled catheters, the delay in the velocity compared with the pressure is so small that a mean pressure drop can also be calculated. With maximal frequency estimation, the velocity will be a little more delayed and may lead to some underestimation of the mean pressure drop. The course of the velocity curve shows whether the pressure drop is sustained during systole or if it decreases rapidly as is the case in mild obstructions. The velocity curve then shows an early peak and a rather low velocity in late systole. This helps in assessing severity, especially

4 II6S JCC Vol. 5, No. I January 1985: 113S-9S m/s Figure 2. Left, Moderately severe aortic stenosis with a recording from the jet showing high velocities () and low velocities (B) beside the jet. Right, Mild aortic stenosis. If recording at a large angie across the beginning of a jet, a signal with a narrow band of frequencies (B) can also be obtained in the continuous wave mode, bui a recording more in the direction of the jet will show higher velocities (). when there is significant increase or decrease in flow across the valve. Correlation of pressure drop and maxihial velocity measurements. In simultaneous measurements of pressure and maximal velocity in the aortic jet, the results with the two methods have been very similar (9). Good correlations between peak pressure drop from the pressure and from maximal velocity have been shown (9,22). With continuous wave Doppler ultrasound, the level of obstruction is not determined. The velocity curves in the ascending aorta are similar whether there is a valvular, supravalvular or fixed subvalvular obstruction. The additional use of the pulsed Doppler mode can show the level of obstruction, whether this is used with imaging or together with the valve sounds. ortic flow velocities can easily be recorded in all children, and the diagnosis of obstruction can be made as easily in neonates as in older children. The method is sensitive because even obstructions mild enough to show equalization of ventricular and aortic pressures toward end-systole and, therefore, no peak to peak pressure drop will s,how an increase in velocity early in systole. With very mild obstructions, the maximal velocity may be in the range seen with increase in flow. The pulsed Doppler mode is then useful to show if there is a clear increase in velocity at one level peak pressure drop,. calculated pressure drop c - mean Figure 3. From maximal velocity (), both a peak (B) and a mean (C) pressure drop can be calculated. In B, it can be seen that this peak pressure drop is higher than the peak to peak pressure difference usually measured from pressure curves. o = aorta; LV = left ventricular; p2-p I = pressure drop; v = maximal velocity. velocity pressure p2-p1=4.v 2

5 JCC Vol. 5, No. I January 1985: 113S-9S 117S or if it is high all the way as when flow is increased. This may also be seen wpen the continuous wave poppler technique is used with a spectral display. Differentiation of aortic jet and oth~r high velocity jets. ' high velocity jet toward the transducer in systole recorded from the suprasternal notch will most often be due to valvular or fixed subvalvular aortic stenosis. It is unusual for other high v~locity jets to have a clearly superior d~rection toward the suprasternal notch, but it occurs occasionally with both pulmonary stenosis and mitral regurgitation. It Can also occur with a ventricular septal defect jet, and tricuspid regurgitation may show a direction toward a high right parasternal position. To avoid possible confusion with other high velocity jets, even if this is rare, the timing of the flow signal should be recorded either by electrocardiography and comparison with the other flow signals or by also recording the valve movements or the phonocardiogram. Figure 4 shows the high velocity of ajet due to mitral regurgitation recorded from the suprasternal notch and the longer duration of the regurgitation than of flow through the' aortic valve. With timing, an aortic jet can, therefore, be distinguished from other high velocity jets. Clinical applications. The easy diagnosis of aortic obstruction together with adequate assessment of severity makes the Dopp'ler ultrasound a useful technique. It helps in the planning of follow-up, timing of surgery, in postoperative evaluation and further follow-up. In older children, it is also possible to record maximal velocity in the aorta and obtain a pressure drop during exercise. The method should also be useful in obtaining more information on the progression of the obstruction in aortic stenosis. The experience in patients followed up to 6 years has been that none of the patients with mild to moderate lesions have shown signs of progression of stenosis. In patients with moderately severe obstruction, the pressure drop has ~Iso changed little during follow-up studies despite agreat change in body surface area, but one patient became symptomatic. In the patients with the highest pressure drop, these were recorded already on the first examination and with little change during followup study. Some of these patients have become symptomatic. ortic Regurgitation Differentiation from otller conditions. With continuous wave Doppler echocardiography and the beam direction across the aortic valve, the high velocity of aortic regurgitation can be recorded in diastole. With both obstruction and regurgitation, the beam direction to record these may Figure 4., high velocity jet toward the transducer in the suprasternal notch. B, Normal aortic ~ow velocity with aortic valve opening (.,) and closure ( e ). The timing of the valve movements shows that the high velocity jet starts at the beginning of the first heart sound and continues past aortic closure. It represents mitral regurgitation which could also be recorded from the apex (C). Me = mitral valve closure; M o = mitral valve opening; dotted lines = start and end of high velocity jet. suprasternal notch r~~ I.1 CW Doppler B, ",. ' '...\\ " 'I', U." tn' \itt '.'1 ' nn f' I -I,tlit II,.ff" apex c m/s 4 2 '->cw-.j~---:;~'lj~ "_\=v--.;:::ll~~w~ o c t Me..... _----::ri~~--:;::::k..:._o/i_ ~--.:.:._''w_':>:...;;;:::._.:,_

6 118S JCC Vol. 5, No. I January 1985: II3S-9S be similar or may differ. The two jets will be continuous, but they will have opposite directions. With continuous wave Doppler echocardiogrllphy and the beam passing the aortic orifice, flow velocities from the coronary arteries might also be recorded in diastole. This is unusual, prohably because flow in these arteries will be at an almost transverse angle. These two diastolic flow velocity patterns can also be distinguished by the high velocity in the aortic regurgitation compared with the low velocity and sound ofnormal flow from the coronary arteries. Pulmonary regurgitation will also show lower velocities unless there is severe pulmonary hypertension with diastolic pressure in the pulmonary artery approaching that in the aorta. The diagnosis of aortic regurgitation is, therefore, specific and it is sensitive especially when aortic flow velocities are recorded from the apex. ortic regurgitation is easily distinguished from left ventricular inflow obstruction by the higher velocity and the longer duration of the regurgitation. Degree of regurgitation. This has been assessed with pulsed Doppler ultrasound from its extension in the left ventricle and from the reverse diastolic flow velocities in the descending or ascending aorta (23-28). For the latter, both continuous wave and pulsed Doppler ultrasound have been used, but the pulsed Doppler mode is probably preferable. With continuous wave Doppler ultrasound, the intensity of the signal from the regurgitation compared with that offorward flow can give some indication ofthe severity, as can the decrease in velocity of the regurgitation during diastole (11). With the combined use of pulsed and continuous wave Doppler ultrasound, semiquantitation of aortic regurgitation can be obtained. Reverse diastolic flow is also seen in patent ductus arteriosus, shunts and in aorticopulmonary window. The first two conditions can be distinguished from aortic regurgitation by showing reverse diastolic flow in the descending but not ascending aorta. With the pulsed Doppler technique, they can all be distinguished from regurgitation by showing the level where reversal of flow starts. Coarctation of the aorta. With continuous wave Doppler recording from the suprasternal notch toward the descending aorta, the high velocity jet across a coarctation can be recorded. This is easily done in patients with mild to moderately severe coarctation and in residual lesions after surgery. In severe coarctation, the jet may be more difficult to find. This may be due, in part, to a more eccentric jet and, in part, to less flow because the major flow may be through collateral vessels. In these patients, combined Doppler ultrasound and two-dimensional imaging is especially helpful. Small angle to the jet can usually be obtained and the pressure drop across the coarctation can be calculated. This has been helpful in the assessment ofcoarctation, especially after surgery. With combined Doppler ultrasound and two-dimensional imaging the presence ofan obstruction can be excluded. Other Changes With heart failure, changes in aortic flow velocity curves are seen in children as well as in adults. Clear respiratory changes are seen in constrictive pericarditis (10). In hypertrophic cardiomyopathy, the increased ejection in early systole is shown by an early peak velocity and low velocities in late systole (29). Similar changes can be seen in severe mitral regurgitation (30), but left ventricular ejection time is then shortened, whereas in obstructive cardiomyopathy it is prolonged. Value and Limitations The use of continuous wave Doppler ultrasound to record aortic flow velocities is quite easy in neonates and children. The method is inexpensive and measurements can easily be repeated. ssessment of aortic stenosis and aortic flow velocities to estimate volume flow can be performed with a stand-alone continuous wave Doppler instrument. Combined with imaging, the information may be obtained quicker in some patients and more complete information on associated lesions can be provided. The main drawback is the possibility of underestimation of the velocity by recording at an angle that is too large. Being aware of this and using various transducer positions and directions, current experience indicates that significant underestimation can be avoided, especially in children and neonates. The other drawback is possible confusion with other high velocity jets if care is not taken to identify the flow signal by recording the aortic valve's movements or the phonocardiogram as described. The level of obstruction cannot be shown without the use of pulsed Doppler ultrasound or two-dimensional imaging, or both. With some pulsed Doppler systems, higher velocities can be recorded in the pulsed mode (10,31). This makes it possible to assess obstructions with the pulsed Doppler technique, especially in infants and children in whom the flow signals are recorded at a low depth. Continuous wave Doppler ultrasound has the advantage of being easier to use because it is quicker to find both a jet and the optimal direction to the jet. There are no known hazards. The ultrasonic effect of Doppler instruments is higher than in instruments used for imaging and higher for continuous wave than for pulsed Doppler echocardiography. The level can be up to 5 to 10% of that for therapeutic use. To date, biologic effects have not been demonstrated for ultrasound used for diagnostic purposes, but further research is necessary to exclude this (32). Conclusions. With continuous wave Doppler echocardiography, aortic stenosis, regurgitation and coarctation can be diagnosed and assessed. In the absence of obstruction or

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