proximal isovelocity surface area (pisa) as a noninvasive method for the estimation of the shunt...

Proximal Isovelocity Surface Area (PISA) as aNoninvasive Method for the Estimation of theShunt Quanti�cation in PerimembranousVentricular Septal DefectsMehmet Eren, M.D., Bahadir Dagdeviren, M.D., Osman Bolca, M.D., Mustafa Polat, M.D.,Yekta Gurlertop, M.D., Tugrul Norgaz, M.D., and Tuna Tezel, M.D.

Siyami Ersek Cardiovascular and Thoracic Surgery Center, Istanbul, Turkey

This study was designed to assess the reliability of the proximal isovelocity surface area (PISA)method for the estimation of shunt quanti�cation in perimembranous ventricular septal defects(PVSD). The study group was composed of 30 patients (age 11 6 7 years, 13 female) with PVSD. Theshunt �ow (Qp-Qs) and the ratio of the pulmonary �ow to the systemic �ow (Qp/Qs) were calculatedby spectral Doppler and catheterization. The Qp-Qs, the defect area (DA), and the shunt volume (SV)were obtained by the PISA method. The PISA method estimated the DA (cm2/m2), the SV (cm3/m2),and the Qp-Qs (L/min/m2) to be equal to (2 3 p 3 R2 3 NL)/(Vmax 3 Body surface area), DA 3TVIshunt, and to SV 3 Heart rate, respectively (R is the distance of the maximal PISA from the �rstaliasing line to the left ventricular side of the defect, NL is the nyquist limit, and Vmax and TVIshuntare the peak velocity and time-velocity integral of transdefect Doppler tracing obtained by continu-ous-wave Doppler). The PISA method (3.4 6 1.5 L/min/m2) underestimated the Qp-Qs according tospectral Doppler (r 5 0.96, P , 0.001; mean difference 2 0.74 6 0.61 L/min/m2; SEE 5 0.11L/min/m2, P , 0.001) and catheterization (r 5 0.92, P , 0.001; mean difference 2 0.45 6 0.7L/min/m2; SEE 5 0.13 L/min/m2, P , 0.001). The correlations between the PISA �ndings (Qp-Qs,DA, SV) and the catheterization Qp/Qs (r 5 0.86, 0.84, and 0.86; P , 0.001, respectively), or betweenthese and the spectral Doppler Qp/Qs (r 5 0.80, 0.80, and 0.79; P , 0.001, respectively) weresigni�cant. The accuracies of the PISA �ndings in identifying large defects were high (0.90, 0.93, and0.90 for cut-off values of Qp-Qs 5 3.67 L/min/m2, DA 5 0.44 cm2/m2, and SV 5 43 cm3/m2,respectively). As a result, the PISA method can be a simple and reliable alternative to the spectralDoppler method in the identi�cation of large shunts in PVSD. (ECHOCARDIOGRAPHY, Volume 18,February 2001)

perimembranous ventricular septal defect, proximal isovelocity surface area, shunt quanti�cation

The functional disturbance caused by a ven-tricular septal defect (VSD) depends primarilyon its size and the status of the pulmonaryvascular bed. Echocardiography is of signi�-cant importance in the evaluation of patientswith VSD. Doppler echocardiography providesmethods for calculating the pulmonary to sys-

temic �ow ratios, as well as estimating thegradient across the VSD and the pressurewithin the chambers of the heart.1-3 Dopplercolor-�ow mapping echocardiography has beenused to show even small or multiple VSDs.4,5

By the end of the 1980s, a better understand-ing of �uid mechanics and color �ow imagingled to more reliable quantitative assessmentsof intracardiac �ows.6,7 In 1991, Recusanishowed that by using the �ow convergence re-gion in patients with mitral regurgitation, thevolume of �ow and the regurgitant ori�ce areacould be measured.8 The same method wasused to assess the severity of stenosis in ste-notic cardiac valves.9-12 In patients with VSD,the proximal isovelocity surface area (PISA)can be visualized and the shunt �ow (Qp-Qs)

Presented in part as an oral presentation at the XIVthNational Congress of the Turkish Society of Cardiology(1998 Antalya, Turkey) and as a poster presentation at theEuroecho-2 Meeting of the European Society of Cardiology(1998 Trieste, Italy).

Address for correspondence and reprint requests: MehmetEren, Icadiye Mah. Makastar Sok., Cihat Apt. No. 6/6,Uskudar (81200), Istanbul, Turkey. Fax: 90-216-341-6399;E-mail: [email protected].

Reprinted with permission fromECHOCARDIOGRAPHY, Volume 18, No. 2, February 2001

Copyright ©2001 by Futura Publishing Company, Inc., Armonk, NY 10504-0418

137ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech.Vol. 18, No. 2, 2001

can be calculated by using the PISA.13 Al-though it has been shown that the Qp-Qs ob-tained by the PISA correlates well with cathe-terization �ndings, a very limited number ofstudies13,14 have been carried out on this sub-ject. To evaluate the reliability of this method,more studies need to be done. This study wasplanned to test the accuracy of the estimationof the shunt quanti�cation obtained by PISA inpatients with perimembraneous VSD (PVSD).

Methods

Patients

This study was performed on 40 consecutivepatients, diagnosed as PVSD, in our echocardi-ography laboratory between November 1997and January 1999. Ten patients were excluded,one with congenital complete atrioventricularheart block, two with secundum atrial septaldefect, two with partial venous return abnor-mality, two with moderate aortic regurgitation,two with in whom the image of parasternalechocardiographic window was inadequate,and one patient in whom the �rst aliasing linewas not visualized. The remaining 30 patients,13 of whom were female, formed the studygroup.

In addition, 18 patients (8 female, age 28 617 years) without regurgitation and a cardiacshunt had the spectral Doppler method per-formed to assess the accuracy of stroke volumemeasurements.

Spectral Doppler Study

Two-dimensional and spectral Doppler echo-cardiographic studies were carried out on allpatients without sedation. A Hewlett-PackardSonos 1500 (Hewlett-Packard, Andover, MA,USA) echocardiography instrument with a 2.5–3.5 MHz transducer at a pulsed repetition fre-quency of 4 kHz was used. The images wererecorded on videotape to allow a frame-by-frame review of images.

To measure the systemic �ow, the aorticvalve annulus diameter (Dao) was measuredfrom the parasternal long-axis view. The time-velocity integral of the aorta (TVIao) was ob-tained by using the tracing, recorded by plac-ing the sample volume of pulsed-wave Dopplerat the level of the aortic annulus in the apical�ve-chamber view. The systemic �ow (Qs) wascalculated by using the formula:15

Qs(L/min)

5 Dao2 3 (p /4) 3 TVIao 3 Heart rate

and was standardized according to body sur-face area (BSA).

To measure the pulmonary �ow (Qp), thepulmonary valve annulus diameter (Dp) wasmeasured from the parasternal short-axisview. The TVI of the pulmonary (TVIp) wasobtained by placing the sample volume ofpulsed-wave Doppler at the level of the pulmo-nary valve annulus from the same view. Thesame formula was used to calculate the Qp15

and was standardized according to BSA. TheQp-Qs (L/min/m2) and the Qp/Qs were also cal-culated. In four patients in whom the pulmo-nary annulus was not visualized and in twopatients who had infundibular pulmonary ste-nosis, mitral annulus was used for the Qp mea-surement.13

The Color Flow Imaging Study

The color �ow imaging was performed byusing a Hewlett-Packard 1500 Sonos deviceand 2.5–3.5 MHz transducer for all patients.The color sector size was 30°, the pulse repeti-tion frequency was 4 KHz, the nyquist limit(NL) velocity was 52 6 11 (37–77) cm/s, thewall �lter was kept at minimum, and the gainlevel was kept just below that which producedstatic noise in the color signal. The color �owimages and the PISA were best obtained fromparasternal long-axis or oblique four-chamberviews. The �rst aliasing line, which is theboundary of the PISA composed of a series ofisovelocity rings with color �ow changes of redto blue, was detected in order to obtain thewidest PISA on the period coinciding with thepeak or the downslope of T wave of the electro-cardiogram, implicating a peak systole. The“R” radius was measured as the distance of themaximal PISA from the �rst aliasing line tothe left ventricular side of the defect (Fig. 1).The hemispheric PISA was calculated accord-ing to the formula “PISA 5 2 3 p 3 R2.”8,16,17

According to the “continuity equation,” the �owpassing through this area should be equal toQp-Qs passing through the VSD into the rightventricle. The Qp-Qs, which is a pulsatile �ow,was calculated by using the formula:18

Qp-Qs(L/min)

5 (PISA 3 NL 3 TVI 3 HR)/Vmax

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138 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. Vol. 18, No. 2, 2001

The DA and the SV were calculated by usingthe formulae:

DA(cm2)

5 (PISA 3 NL)/Vmax SV(cm3) 5 DA 3 TVI

The Qp-Qs, DA, and SV were standardized ac-cording to BSA. The abbreviations used in theabove formulae are: PISA is the maximal PISAin one cycle (cm2), NL is the nyquist limit ve-locity (cm/sec), TVI is the time-velocity integralrecorded by continuous-wave Doppler ultra-sound at the defect (cm), HR is the heart rate(beats/min), and Vmax is the peak velocity re-corded by the continuous-wave Doppler of�ow through the interventricular septal defect(cm/sec).

The diameter of the defect was directly mea-sured in color-Doppler imaging, and by assum-ing a circular defect shape, the DA was calcu-lated as (defect diameter)2 3 p /4.

Cardiac Catheterization

Cardiac catheterization was performed on allpatients during the subsequent 30 days by acardiologist who was not aware of the echocar-diographic �ndings. The adult patients weresedated with 5 mg diazepam PO. The proce-

dure was performed under general anesthesiain infants. For this purpose, ketamine 5 mg/kg 1 atropine 0.3 mg/kg IM was given. Byusing the methods of oximetry, ventriculogra-phy, and angiography it was shown that therewas no extra shunt. Two blood samples wereobtained from all chambers for oxygen satura-tion and the mean of these samples was used.The Qp and Qs were calculated by Fick’s rulebased on estimated oxygen consumption.19 Theventricular septal shunt �ow was Qp-Qs andthe pulmonary to systemic �ow ratio was Qp/Qs. The �ows were standardized according toBSA.

Statistical Analysis

Numeric values were given as mean 6 stan-dard deviation. The paired-Student’s t-test wasperformed in the comparison of numeric val-ues. The Pearson linear regression analysiswas performed to correlate between the vari-ables. The Bland-Altman analysis was per-formed for the agreement between �ows.20 TheMann-Whitney U test was used to differentiatebetween patients with severe or nonsevereshunts, according to their PISA �ndings. TheSPSS program was used to obtain the statis-tics. The speci�city, sensitivity, positive, andnegative predictive values of the color Doppler�ndings (Qp-Qs, DA, and SV) for large VSDwere calculated and the receiver-operatingcharacteristic (ROC) curves were drawn.21 P ,0.05 was considered statistically signi�cant.

Results

Of the excluded patients, three had unsatis-factory PISA images. Thus, the feasibility ofthe PISA method was 91% (30/33). The meanage of the patients accepted for the study was11 6 7 years and 13 (43%) of the patients werefemale. The heart rates of patients during cath-eterization were slightly higher than those ob-tained during echocardiography (98 6 13 vs93 6 13 beats/min, P 5 0.03). The mean rightventricle pressure was 32 6 15 mmHg withcatheterization. The mean interventricularpressure gradient was 82 6 17 mmHg by con-tinuous-wave Doppler, and this was similar tothat obtained by catheterization (82 6 16mmHg, P 5 0.95). The mean value of NL, themaximum R diameter, the directly measureddefect diameter, the effective diameter (Deff) ofDA obtained by PISA, and the R/Deff ratio were52 6 11 cm/sec, 0.67 6 0.12 cm, 0.46 6 0.08 cm,

Figure 1. The Doppler color-�ow mapping study from theparasternal long-axis view in a perimembranous VSDshowing the PISA on the left septal surface. The line in thePISA where the color changes from red to blue is the �rstaliasing line. The distance from this line to the midpoint ofthe left ventricular surface of the defect is the “R” diameterof the PISA that is thought to be hemispheric. The turbulent�ow is visible in the right ventricle. The R diameter ismeasured simultaneously with the peak of T wave electro-cardiogram. Ao 5 aorta; IVS 5 interventricular septum;LA 5 left atrium; LV 5 left ventricle; R 5 diameter of PISA;RV 5 right ventricle.

THE PISA METHOD IN PVSD


0.69 6 0.17 cm, and 1.07 6 0.26, respectively(Table I).

Using spectral Doppler, the shunt �ow was4.1 6 1.9 L/min/m2, with PISA 3.4 6 1.5 L/min/m2, and with catheterization 3.8 6 1.8 L/min/m2. The Qp-Qs obtained by PISA had goodcorrelations with those of the spectral Doppler(r 5 0.96, P , 0.001) and the catheterization(r 5 0.92, P , 0.001) (Figs. 2A and 3A). PISAhad a tendency to systematically calculate alower Qp-Qs value relative to the spectralDoppler (mean difference 2 0.74 6 0.61 L/min/m2, SEE 5 0.11 L/min/m2, P , 0.001) (Fig. 2B).There was a linear relationship between thedifference and the mean of the Qp-Qs obtainedby either methods (r 5 2 0.70, P , 0.001) (Fig.2B). There was no agreement between theshunt �ow rates obtained by PISA and cathe-terization, and the shunt �ow rate obtained byPISA was lower than that of catheterization(mean difference 2 0.45 6 0.7 L/min/m2, SEE 50.13 L/min/m2, P , 0.001) (Fig. 3B). It waspossible to measure the defect diameter in 27(90%) of the patients directly by color �ow im-aging; the DA calculated using this diameterwas found to be 0.16 6 0.08 cm2. The DA cal-culated by the PISA method was 0.4 6 0.17cm2/m2. Although there was a signi�cant cor-relation between the two defect areas (r 5 0.62,P , 0.001), the difference between those wastoo high for agreement (mean difference2 0.20 6 0.15, SEE 5 0.04, and P , 0.001). TheSV obtained by PISA (37 6 17 cm3/m2) was

lower than that of the spectral Doppler (48 620 cm3/m2), but there was a signi�cant corre-lation (r 5 0.96, P , 0.001; mean difference2 8.96 6 7 cm3/m2, SEE 5 1.23 cm3/m2, andP , 0.001). The Qp-Qs, DA, and SV calculatedby PISA had an excellent correlation with thecatheterization Qp/Qs (r 5 0.80, 0.84, and 0.87,respectively; P , 0.001) (Fig. 4). Similar corre-lations were obtained between the PISA �nd-ings and the Qp/Qs calculated by the spectralDoppler (r 5 0.81, 0.80, and 0.81, respectively).

Eleven patients had Qp/Qs $ 2:1, deter-mined at catheterization. The Qp-Qs, DA, andSV obtained by PISA were useful in differenti-ating small/medium (Qp/Qs , 2:1) defects fromlarge (Qp/Qs $ 2:1) defects (P , 0.001) (Fig. 5).The ROC curve analysis was performed to com-pare ability of the PISA �ndings and the Qp/Qsobtained by spectral Doppler to predict largedefects (Fig. 6). The areas under these curveswere similar to each other. The accuracy of thePISA �ndings in identifying large defects washigh (0.90, 0.93, and 0.90 for cut-off values ofQp-Qs 5 3.67 L/min/m2, DA 5 0.44 cm2/m2,and SV 5 43 cm3/m2, respectively).

A signi�cant correlation was found betweenthe shunt �ow rates obtained by spectral Dopp-ler and catheterization, but the difference be-tween these was too high for agreement (r 50.93, P , 0.001; mean difference 2 0.29 6 0.7L/min/m2, SEE 5 0.13 L/min/m2, P 5 0.03).There was no linear relationship between thedifference and the mean of the Qp-Qs obtained

TABLE I

Patients’ Demographic, Echocardiographic, and Catheterization Findings

Demographic Parameters Color Flow Imaging

No 30 NL (cm/sec) 52 6 11Female/male (no) 13/17 R (cm) 0.67 6 0.12Age (years) 11 6 7 Qp-Qs (L/min/m2) 3.4 6 1.5Body surface area (m2) 1.07 6 0.39 Defect area (cm2/m2) 0.4 6 0.17

Spectral DopplerShunt volume (cm3/m2)R/Deff

37 6 171.07 6 0.26

Heart rate (beats/min) 93 6 13Vmax (shunt) (cm/sec) 449 6 54 Catheterization Findings

IVPG (mmHg) 82 6 17 Heart rate (beats/min) 98 6 13TVI (shunt) (cm) 97 6 16 RVP (mmHg) 32 6 15Qp-Qs (L/min/m2) 4.1 6 1.9 IVPG (mmHg) 82 6 16Qp/Qs 1.89 6 0.51 Qp-Qs (L/min/m2) 3.8 6 1.8Shunt volume (cm3/m2) 48 6 20 Qp/Qs 1.86 6 0.53

Deff 5 the effective diameter of the circular assuming defect area obtained by PISA; IVPG 5 interventricular pressuregradient; NL 5 nyquist limit; Qp 5 pulmonary �ow; Qs 5 systemic �ow; R 5 diameter of PISA; RVP 5 right ventricularpressure; TVI 5 time-velocity integral of interventricular �ow Doppler tracing; Vmax 5 maximum interventricular �owvelocity.

EREN, ET AL.


by either method (r 5 0.10, P 5 0.58). Therewas a signi�cant correlation and agreementbetween the Qp/Qs obtained by either method(r 5 0.89, P , 0.001; mean difference 2 0.03 60.26, SEE 5 0.05, and P 5 0.52). There wasalso good agreement between the aortic (3.44 60.75 L/min/m2) and pulmonary �ow rate(3.39 6 0.78 L/min/m2; mean difference 0.05 60.28 L/min/m2, SEE 5 0.07 L/min/m2, and P 50.43). The pseudo Qp-Qs and Qp/Qs were0.05 6 0.28 L/min/m2 and 0.98 6 0.09, respec-tively.

The intra- and interobserver variabilitieswere evaluated in ten randomly selected pa-tients with simple linear regression and Bland-Altman analyses by using videotape records.Excellent inter- and intraobserver agreementswere obtained for the measurement R diameter(r 5 0.96, P , 0.001; mean difference 0.004 60.02 cm, SEE 5 0.007 cm, P 5 0.57; and r 5

0.93, P , 0.001; mean difference 0.002 6 0.04cm, SEE 5 0.011 cm, and P 5 0.8, respectively).

Discussion

The present study shows:

(1) PISA was able to be visualized in mostpatients with PVSD,

(2) the PISA �ndings correlated well withthe spectral Doppler and catheterization �nd-ings,

(3) there was a high accuracy for large de-fects, and

(4) although the Qp-Qs obtained by PISAunderestimates the Qp-Qs obtained by spectralDoppler and catheterization, it may be used asan index for severe shunts.

PISA

With the PISA method, the �ow ratethrough an ori�ce is calculated as the prod-

Figure 2. A. The regression analysis between the shunt�ow rate (Qp-QsPISA) obtained by PISA and the Qp-QsSD

obtained by the spectral Doppler (SD) method. B. Theagreement between both the Qp-Qs according to the methodof Bland and Altman.

Figure 3. A. The regression analysis between the Qp-QsPISA (ordinate) and the Qp-QsCATH obtained by catheter-ization (CATH) (abscissa). B. The agreement between boththe Qp-Qs according to the method of Bland and Altman.



uct of the isovelocity surface area and itscorresponding velocity based on the continu-ity concept.8,10,18,22-25 To accomplish this, it isessential to obtain an accurate isovelocitysurface area and the appropriate aliasing ve-locity. The PISA was originally assumed to behemispherical.8,26 However, with an ori�ce of�nite size, as occurs in a clinical situation,there is a progressive �attening of the isove-locity surfaces as they approach the regurgi-tant ori�ce.22 Some studies have also shownthat the shape of the PISA is transformed bythe sequence, from hemispheroidal to hemi-spheric and then to hemielliptic, as it con-verges on an ori�ce.24,27-29 Thus, the hemi-spheric formula consistently produces an un-derestimation when close to the ori�ce30 anda signi�cant overestimation when far fromthe ori�ce.31 In the present study, the Qp-Qsobtained by the PISA method was systemat-ically underestimated. The R diameters were

Figure 4. Shows the regression analyses between the pulmonary to systemic �ow ratio (Qp/QsCATH) determined at cardiaccatheterization and A. the Qp-QsPISA (L/min/m2), B. defect area (DA) (cm2/m2), and C. shunt volume (cm3/m2) calculatedfrom the PISA.

Figure 5. Diagrams showing that the large defects (Qp/Qs . 2:1) and medium to small defects (Qp/Qs , 2:1)classi�ed by the pulmonary to systemic �ow ratio (Qp/QsCATH) obtained by catheterization can be identi�ed byPISA �ndings (Qp-Qs, DA, and SV). There were statisti-cally signi�cant differences between the two groups in re-spect to these parameters (P , 0.001).

EREN, ET AL.


measured close to the ori�ce as a result of thehigh aliasing velocities in this study. Thus,the hemispheric formula used in the calcula-tion of the hemielliptic PISA may have led tothe underestimation of the Qp-Qs. Ut-sunomiya et al. have suggested that the he-mielliptic model can correct this condition.18

However, this is dif�cult to apply in vivosince the measurement of the R diameter intwo orthogonal plans is required. Li et al.obtained the PISA from three-dimensional(3-D) echocardiographic images.32 The �ow rateobtained by this method had a good agreementwith the electromagnetic �ow meter, but un-derestimation occurred with the hemisphericformula, greater than that using the hemiellip-tic formula.

An in vitro study has shown that the settingsof the equipment (gain, wall �lter, frame rate,transmit power, and packet size) do not affectthe volumetric �ow rate calculated by PISA.18

In the same study, the accuracy of the PISAmethod using a hemielliptic model to calculatethe �ow rate was not affected by the ori�ceshape. In contrast, the PISA method using ahemispheric model underestimated the actual�ow rate, especially when used for rectangularshapes. Anayiotos et al.29 has shown that theviscous �ow �eld proximal to any in�nite cir-cular ori�ce is represented by a single normal-ized curve independent of ori�ce size and �ow.Li et al. reported an in vitro model where thePISA can be visualized by 3-D echocardiogra-

phy; the PISA shape is affected by the ori�ceshape.32 Myers et al. showed that the PISAshape was particularly related to the ori�ceaspect ratio.28 The ori�ce aspect ratio loses itseffect on the PISA shape when at a distance ofmore than 1.4 times the diameter of the ori�ce.

It has been shown that the motion of theheart in�uences the quanti�cation of PISA (es-pecially for an ori�ce causing insuf�ciency).33

Moreover, the shape of PISA changes duringsystole. It is hemispheric during peak systole,whereas it is found to be hemielliptic duringthe early and late systole.34 These factors,which in�uence PISA unfavorably, can be elim-inated by two methods.35 First, the mean tem-poral measurement of the radius can be ob-tained by using color Doppler/M-mode (colorM-Q Doppler). However, because of transla-tional movements of the heart, it might be dif-�cult to obtain an accurate measurement of asingle radius throughout systole. The secondmethod is to determine the mean SV, which isa product of the DA and the TVI representingthe variability of the blood �ow during the sys-tole. In the present study, this method wasutilized to take into account the effects of thecardiac cyclic changes on PISA.

Another determinant of the PISA quanti�ca-tion is the NL velocity.8 PISA increases as theNL value decreases.25 The PISA shape alsovaries with the chosen NL.22 The standard NLvelocity proposed for mitral stenosis is 11–39cm/sec36 and 15–29 cm/sec for valvular insuf�-

Figure 6. The diagrams show receiver-operating characteristic curves for A. Qp/QsSD obtained by spectral Doppler and thedefect area (DA), and B. Qp-QsPISA and shunt volume (SV) obtained by the PISA.



ciencies.37 In the present study, high NL(37–77 cm/sec) was used. The isovelocity shellwas visualized close to the ori�ce as a result ofa high NL. Thus, the observed PISA shape wasprobably hemielliptic and the hemispheric for-mula used may have caused an underestima-tion of the Qp-Qs. This underestimation can beminimized by reducing the aliasing velocity ifthe �ow can approach equally from all direc-tions.22,38 However, since PISA of PVSD takesplace in the left ventricular out�ow tract, it isin�uenced by the blood �ow merging towardsthe aorta during systole.39 The effect of theout�ow causes an overestimation of the alias-ing velocity, so the R diameter �tting to thealiasing velocity in the scale of the equipmentappears greater than that of the true aliasingvelocity. Namely, this effect leads to an overes-timation of the Qp-Qs obtained by PISA. In thepresent study, using high NL reduced the ef-fect of the out�ow. However, since the PISAshape close to the ori�ce became �attened, aproblem occurred in which the hemisphericPISA formula underestimated the Qp-Qs. It isessential to �nd the appropriate aliasing veloc-ity visualizing the PISA in which the hemi-spheric model is valid in the �ow convergenceregion. Shiota et al. suggested the centerlinevelocity/distance pro�le using digital color M-QDoppler for this purpose,40 but this methodprovides a rough discrimination between highand low �ow rates rather than an exact quan-ti�cation. Shwammental et al. reported thatthe optimal aliasing velocity is provided byplotting �ow rate calculated by hemisphericmodel versus aliasing velocity, and by selectingdata points in the relatively �at portion of thiscurve.41 An infrequently addressed importantissue is the ability of the color Doppler instru-ment to provide accurate velocities in the prox-imal �ow region. Vandervoort et al.42 reportedan overestimation of color-Doppler measuredvelocities in regions of low velocity compared tothe NL setting. Reportedly, this overestimationwas due to the effects of electronic wall �lter-ing. However, Perry et al.43 showed that wall�lter settings are not the sole factor contribut-ing to velocity accuracy, but that spatial beamexpansion characteristics must also be consid-ered. Myers et al.44 reported that in small ori-�ces, velocity errors are higher in velocitiesclose to the ori�ce and the ori�ce aspect ratiowill have more effect on velocity errors. Anayio-tos et al.45 have shown that a 3-D echo providesinformation on the ori�ce geometry and size.The same study reported that a region between

0.6 and 0.8 diameters is a region where veloc-ities are close to the NL and the velocity calcu-lation is the most accurate. However, closer tothe ori�ce, the velocities are higher than theNL and the velocity gradients are very steep.Far from the ori�ce the velocities are muchlower than the NL, with smaller velocity gra-dients. In the present study, the velocity errorswould be minimal because the wall �lter waskept at minimum, the velocity measurementswere performed approximately at a distance ofthe diameter of the ori�ce from the ori�ce it-self, and the defect sizes were too large to leadto velocity errors.

The angle between the ultrasound beam andthe �ow affects the images since the color �owis composed of multiple-pulsed Doppler’s sam-ple volumes. This was not a limitation in thepresent study, as the defects were visible par-allel to the ultrasound beams in parasternal oroblique four-chamber views.

The Defect Area

The effective ori�ce area has been calculatedin mitral regurgitation by proportioning thevolumetric �ow rate obtained by PISA to themaximum velocity of the regurgitant jet.46 Thepresent study is the �rst study to calculate DAin VSD by the same method. It is possible thatan underestimation of Qp-Qs has led to anunderestimation of DA in the present study.However, the DA calculated by the single plandiameter was found to be less than that of thePISA. This �nding suggests that the defects inthis study were asymmetric. To obtain the trueDA, the defect diameter in the two orthogonalplans, or particularly in 3-D echo imaging,could be utilized.32,45 In spite of these limita-tions, the signi�cant correlations between DAand the spectral Doppler or catheterization�ndings suggest that this parameter can beused at least in the identi�cation of largeshunts.

The Accuracy of Qp-Qs

The Qp-Qs calculated by any of the threemethods is the average of a cardiac cycle. How-ever, the Qp-Qs obtained by PISA has beenfound to be smaller than the Qp-Qs obtained byboth spectral Doppler and catheterization. Themain reason for the underestimation of theQp-Qs obtained by PISA is the hemisphericassumption of the PISA shape. The high NL,which we used in the present study, causedPISA to be visualized close to the ori�ce. In this

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way, the PISA shape became �attened and he-mielliptic. This problem may be resolved byobtaining the true PISA shape. Also, the truePISA shape can be obtained by 3-D echo. Thedetection of the oxygen consumption, using no-mograms when calculating the �ows by theFick method in cardiac catheterization, mayalso result in misleading values. It would belogical to take the Qp/Qs calculated by the ox-ygen saturation samples from the heart cham-bers as a reference for shunt quanti�cation. Itmay be a limitation of the present study thatthe catheterization and echocardiographicstudies were not performed simultaneously,but since the time interval between the twostudies was short and there were no signi�canthemodynamic changes during this period, thislimitation may be negligible. Many experimen-tal and clinical studies1,2,47,48 have shown thatQp-Qs and Qp/Qs values can be calculated byusing spectral Doppler echocardiography forthe quanti�cation of the shunt in VSD. Simi-larly, signi�cant correlations were found be-tween the spectral Doppler and the catheter-ization �ndings in the present study. However,the circular assumption of the annular crosssection may be a limitation of this method. Asthere was a good agreement between the aorticand pulmonary �ow rates calculated by thismethod and the pseudo Qp/Qs and Qp-Qs wereminimal in the normal group of the presentstudy, the accuracy of this method was en-hanced. In addition, there is the advantagethat a spectral Doppler study was simulta-neously performed with the PISA. PISA can bea practical alternative method to the spectralDoppler due to the following: the errors in mea-suring the pulmonary diameter caused by poorlateral resolution, the effect of moderate-to-se-vere regurgitant jets of the aortic and pulmo-nary valves, and the time-consuming nature ofthe method.

Previous Studies

The PISA method in VSD was �rst used byMoises et al.13 Their study patients were com-posed of two groups. In the �rst group, the colorDoppler �ow imaging was performed in 18 pa-tients with a PVSD undergoing cardiac cathe-terization. The maximal shunt �ow rate acrossthe defect was developed from the R diameterof the �rst alias, based on the color Doppler�ow images. The Qp-Qs derived from images of�ow convergence in both the long-axis (n 5 15)and the oblique four-chamber (n 5 10) views

correlated closely with the Qp/Qs (r 5 0.71 to0.92) and the Qp-Qs (r 5 0.69 to 0.97) deter-mined at cardiac catheterization. In the secondgroup (n 5 8), the same PISA indexes corre-lated well with the results of the Qp-Qs deter-mined by spectral Doppler (r 5 0.94). Qp-Qsobtained by PISA was signi�cantly different inthe group of seven patients with Qp/Qs , 2 andthe eight patients with Qp/Qs $ 2.

The second study in this subject was per-formed by Kurotobi et al.14 This study wascomprised of only 14 patients. They calculatedthe Qp-Qs and shunt volume ( 5 Qp-Qs 3 shuntduration time) and shunt fraction by PISA.There was a correlation between shunt vari-ables determined by PISA and those by cathe-terization, including the shunt volume (r 50.78, P 5 0.001) and the shunt fraction (r 50.74, P 5 0.003). The Qp/Qs was also signi�-cantly correlated with the Qp-Qs by PISA (r 50.79, P 5 0.0007). Ten patients had Qp/Qs $2:1, and their Qp-Qs obtained by PISA weregreater than those of the patients with Qp/Qs , 2:1 (P 5 0.004).

The previous two studies overestimatedQp-Qs because they used the PISA formula forconstant �ow, but the present study utilizedthe pulsatile �ow formula, leading to an under-estimation of the Qp-Qs. The common �ndingof the present and previous two studies is thatthe PISA method provides an accurate identi-�cation between large and small shunts.

Conclusions

In PVSD, PISA can be obtained at the leftventricular out�ow tract in most cases. Rela-tively higher values of the NL should be usedfor PVSD in order to overcome the effect of theleft ventricle out�ow, as compared with the NLused in heart valvular disease. The high NLcauses PISA to approach to the ori�ce and totransform its shape from hemispheric to he-mielliptic. Thus, the PISA method leads to anunderestimation of the shunt �ow rate. Theshunt �ow rate obtained by PISA should beconsidered as a determinant of shunt quanti�-cation rather than of the true �ow itself.

As a result, PISA can be a simple and reli-able alternative to the spectral Doppler methodin the identi�cation of large shunts in PVSDs.

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proximal isovelocity surface area (pisa) as a noninvasive method for the estimation of the shunt...

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