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Evaluation of a Noninvasive System for Determining Left Ventricular Filling Pressure
G. V. R. K. Sharma, MD;
Patricia A. Woods, MSN, RN;
Costas T. Lambrew, MD;
Claire M. Berg, MS, RN;
Daniel A. Pietro, MD;
Thomas P. Rocco, MD;
Frederick W. Welt, MD;
Peter Sacchetti, MS;
Kevin M. McIntyre, MD
Arch Intern Med. 2002;162:2084-2088.
ABSTRACT
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Background Measurement of left ventricular filling pressure (LVFP) provides an
accurate assessment of left ventricular failure. Clinical and radiographic
methods of estimating LVFP are unreliable. The noninvasive method of analyzing
the decline in the arterial pressure during the strain phase of the Valsalva
maneuver may be used to directly measure LVFP.
Objective To examine the relationship and the level of accuracy of a noninvasive
system (VeriCor) in directly determining left ventricular end diastolic pressure
(LVEDP) using simultaneously recorded VeriCor and LVEDP measurements obtained
during left heart catheterization.
Methods During elective right and left heart catheterization, LVFP was assessed
by measuring pulmonary capillary wedge pressure (PCWP) and LVEDP in 57 patients
followed immediately by a Valsalva maneuver using the VeriCor assembly to
estimate these pressures noninvasively.
Results VeriCor measurements had a significant correlation with the catheter-measured
LVEDP (r = 0.86), comparable to the correlation of the catheter-measured
PCWP with LVEDP (r = 0.81). The predictive accuracy of VeriCor
for LVEDP, however, appeared to be superior to that of catheter-measured PCWP
for LVEDP: 84% of VeriCor measurements compared with only 41% of PCWP measurements
were within 4 mm Hg of catheter-measured LVEDP, and 93% of VeriCor measurements
compared with only 67% of PCWP measurements were within 6 mm Hg of catheter-measured
LVEDP.
Conclusion VeriCor is a reliable noninvasive tool for measuring LVFP.
INTRODUCTION
MEASUREMENT OF left ventricular filling pressure (LVFP) allows detection
of congestive heart failure (CHF) and helps assessment of the efficacy of
therapy for CHF. Left ventricular filling pressure can be measured directly
by placing a catheter in the left ventricle to obtain the end diastolic pressure
(LVEDP) or indirectly by placing a catheter in the pulmonary artery to measure
the pulmonary capillary wedge pressure (PCWP). Both of these invasive techniques
involve cardiac catheterization with its attendant risks and expense. Clinical
and radiographic methods of detecting CHF, however, are insensitive to alterations
in LVFP and, hence, unreliable.1-2 A
reliable noninvasive method of estimating LVFP would, therefore, be a useful
tool in the detection and management of patients with CHF, since early detection
and treatment of CHF have been shown to reduce the rate of hospitalization
and mortality.3-4
The noninvasive method of assessing PCWP by using the strain phase of
the Valsalva maneuver has been found to be practical and promising.5-8 Preliminary
studies9-11 using
technology (VeriCor; CVP Diagnostics Inc, Boston, Mass) that calibrated the
response to the Valsalva maneuver showed that PCWP could be predicted with
a clinically useful degree of accuracy. Analysis of the mechanism by which
PCWP was estimated by the calibrated Valsalva response suggested that the
arterial pressure during the strain phase of the Valsalva maneuver may directly
reflect the LVEDP. This is a report of a prospective study designed to examine
the relationship and the level of accuracy of VeriCor in directly determining
LVEDP using simultaneously recorded VeriCor and LVEDP measurements obtained
during left heart catheterization. Right heart catheterization for PCWP was
also performed to provide comparisons with this measurement.
MATERIALS AND METHODS
Studies performed in 57 patients scheduled for elective cardiac catheterization
in 2 hospitals (Veterans Affairs Medical Center, West Roxbury, Mass, and Maine
Medical Center, Portland) form the basis of this report. Fifty-five patients
were men and 2 were women, ranging in age from 40 to 82 years (mean, 62.6
years) Fifty-six patients had coronary artery disease and 1 patient had hypertension.
Patients with significant aortic stenosis, mitral stenosis, uncontrolled hypertension,
or arrhythmias were excluded from the study. The research protocol was approved
by the institutional review board at both hospitals. After informed consent
was obtained, all patients received training in the performance of the Valsalva
maneuver and underwent right and left heart catheterization in the supine
position. The PCWP was measured with a 7F, double-lumen, balloon-tipped catheter
(Swan-Arrow or Swan-Abbott) after the wedge position was noted on fluoroscopy
and confirmed by the waveform. When there was doubt, a blood sample was drawn
from the wedge position to confirm arterial saturation. Pressures from the
left ventricle were obtained with a 6F (Cordis-angled pigtail) catheter. Simultaneous
records of the PCWP and the LVEDP were made on a 0-to-40 scale during at least
3 respiratory cycles. All pressures were measured with the transducer set
at the midchest level. Hemodynamic measurements were followed immediately
by the Valsalva maneuver to obtain the noninvasive measurement with the VeriCor
system.
NONINVASIVE METHOD
Arterial pressure response to the Valsalva maneuver generally shows
4 distinct phases. In phase 1, the arterial pressure rises as a result of
the transmission of increased intrathoracic pressure; in phase 2, systolic,
diastolic, and pulse pressures are reduced as a result of decreased venous
return with continuing strain; in phase 3, a sudden drop in arterial pressure
occurs with release of strain; and in phase 4, the arterial pressure overshoots
to levels above baseline, with a widened pulse pressure. The arterial pressure
changes during the Valsalva maneuver are recorded by the VeriCor system (Figure 1).
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Figure 1. Validation of the noninvasive
VeriCor system (CVP Diagnostics, Inc, Boston, Mass) during a Valsalva maneuver,
with simultaneous record of expiratory pressure, radial tonometry, intra-arterial
pressure, and pulmonary capillary wedge pressure (PCWP). The broken vertical
lines indicate the phases of the arterial pressure responses to the Valsalva
maneuver (see "Materials and Methods" section). The broken horizontal lines
indicate the baseline amplitude (1.0) and 1.2 times the baseline amplitude.
The ratio of the terminal to the initial pulse amplitude was 0.42, which correlated
with a PCWP of 7 mm Hg.
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Details of validation of the VeriCor method have been previously described.12 In brief, the VeriCor system uses a noninvasive blood
pressure tonometer applied over the radial artery and a digital pulmonary
manometer for the continuous acquisition of arterial and expiratory pressure
signals during strain. The digital signals are collected and stored on a medical
grade computer. The device acquires and analyzes radial arterial tonometric
data during and after a period of continuously monitored expiratory strain
(Valsalva maneuver).
From the arterial pressure signals during the strain phase, a range
of predictive algorithms was developed by calibration of the various changes
in the arterial pressure response to the Valsalva maneuver. These changes
include the rate of decline of pulse pressure, the rate of change of the slope
of the systolic pressure, rate of change of the dP/dt of the upstroke of the
arterial pressure, and the ratio of the terminal to the initial amplitude
of the strain-phase beats expressed as the pulse-amplitude ratio (Figure 1). A comprehensive analysis of the
algorithms was performed by proprietary software (a customized version of
LabVIEW systems; National Instruments Corp, Austin, Tex). Those algorithms
that were most accurately predictive of PCWP and LVEDP were then applied and
validated in new data sets as additional data sets were developed. The VeriCor
device is mounted on a mobile cart powered by either a self-contained battery
pack or an external power source, allowing testing to be done at the bedside
or in any clinical setting.
The Valsalva maneuver was performed immediately after the recording
of the invasive PCWP and LVEDP. After a normal inspiration, the patient was
asked to exhale with enough force to raise a marker on the computer monitor
to at least 20 mm Hg (range, 20-35 mm Hg). The strain was maintained for 8
to 12 seconds. A printout displaying the expiratory pressure, the arterial
pressure, and the pulse-amplitude ratio during the early strain phase and
the computed PCWP and LVEDP was obtained at the end of the maneuver.
Hemodynamic measurements of PCWP and LVEDP were repeated in 8 patients
along with the VeriCor measurements to verify the reproducibility of the results.
As mentioned before, hemodynamic measurements always preceded the VeriCor
measurements to obviate the effect of the Valsalva maneuver on the hemodynamic
measurement.
ANALYSIS OF THE DATA
Invasive measurements of the PCWP and LVEDP were made by an experienced
cardiologist (G.V.R.K.S.) who was blinded to the results of the VeriCor measurement.
For PCWP, the mean end expiratory value during at least 3 respiratory cycles
was measured. For LVEDP, the post "a" value was measured. Computer estimates
of the pulse-amplitude ratios of the strain phase during the Valsalva maneuver
were checked by a physician experienced in reading the tracings and who was
blinded to the invasive measurements (K.M.M.).
Standard least-squares linear regression analysis was used to examine
the correlation between the VeriCor measurements of PCWP and LVEDP and the
invasive PCWP and LVEDP measurements. For computation of correlations in the
paired data, the nonparametric correlation coefficient  statistic was
used because of the small sample size.
PREDICTIVE ACCURACY OF VERICOR
With the invasive LVEDP measurement as the "gold standard," predictive
accuracy of VeriCor was expressed as the number of VeriCor measurements that
corresponded with invasive measurements of LVEDP within a 4.0- to 7.0-mm Hg
range. A similar approach was used for determining the predictive accuracy
of VeriCor for the PCWP (ie, the number of VeriCor measurements that corresponded
with the invasive PCWP measurements within a 4.0- to 7.0-mm Hg range).
RESULTS
Of the 57 patients who underwent cardiac catheterization and VeriCor
testing, the results had to be discarded in 8 patients because of ectopy during
the Valsalva maneuver that rendered the analysis of the pulse-amplitude ratio
difficult. Of the remaining 49 patients, 46 had simultaneous measurements
of LVEDP and PCWP with VeriCor and 3 patients had only LVEDP with VeriCor
(without PCWP). Eight patients with LVEDP and PCWP had duplicate studies,
bringing the total number of LVEDP measurements to 57 and the PCWP measurements
to 54.
The average time taken for setting up the VeriCor equipment and performance
of the test was about 15 minutes. The time that elapsed between the recording
of the hemodynamic measurements and the VeriCor measurement was 2 to 4 minutes.
There were no complications during the study.
COMPARISON OF VERICOR MEASUREMENTS AND CATHETER-MEASURED LVEDP
VeriCor measurements correlate well (r = 0.86)
with the LVEDP (Figure 2). The following
are the VeriCor measurements that correspond with the catheter-measured LVEDP
values in the 4.0- to 7.0-mm Hg range (n = 57).
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Figure 2. Correlation of VeriCor (CVP Diagnostics,
Inc, Boston, Mass) measurements with catheter-measured left ventricular end
diastolic pressure (LVEDP).
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COMPARISON OF CATHETER-MEASURED PCWP AND CATHETER-MEASURED LVEDP
Correlation of the PCWP measurements with the LVEDP is shown in Figure 3. There was a good correlation of
PCWP with LVEDP (r = 0.81). Figure 4 shows a simultaneous display of the correlation of catheter
LVEDP with VeriCor and the pulmonary artery catheter. VeriCor -determined
LVEDP appears to have a better correlation ( 2 = 0.74) than
that of the pulmonary artery catheter (2 = 0.66)
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Figure 3. Correlation of catheter-measured
pulmonary capillary wedge pressure (PCWP) with catheter-measured left ventricular
end diastolic pressure (LVEDP).
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Figure 4. Simultaneous display of the correlation
between catheter left ventricular end diastolic pressure (LVEDP) and VeriCor
(CVP Diagnostics, Inc, Boston, Mass) and the pulmonary artery catheter–determined
measurements. Vertical lines indicate SD. Dotted line shows line of equivalence.
PCWP indicates pulmonary capillary wedge pressure.
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The following are the catheter-measured PCWP measurements that correspond
to the catheter-measured LVEDP values (n = 54). The predictive accuracy of
the PCWP for estimating the LVEDP appears to be significantly less (4.0 mm
Hg: 41%; 7.0 mm Hg: 83%) than that of VeriCor for estimating the LVEDP (4.0
mm Hg: 84%; 7.0 mm Hg: 98%).
COMPARISON OF VERICOR MEASUREMENTS AND CATHETER-MEASURED PCWP MEASUREMENTS
There was a good correlation between the VeriCor measurements and the
PCWP measurements (Figure 5, r = 0.81). The following are the VeriCor measurements that
corresponded to the catheter-measured PCWP measurements in the 4.0- to 7.0-mm
Hg range (n = 54).
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Figure 5. Correlation of the VeriCor (CVP
Diagnostics, Inc, Boston, Mass) measurements with catheter-measured pulmonary
capillary wedge pressure (PCWP).
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ANALYSIS OF PAIRED MEASUREMENTS OF VERICOR AND CORRESPONDING CATHETER-MEASURED
LVEDP AND PCWP
Correlations between the noninvasive and invasive measurements in 8
patients (n = 16) yielded correlation coefficients of 0.89 for VeriCor vs
catheter LVEDP and 0.73 for VeriCor vs catheter PCWP, indicating good reproducibility.
COMMENT
Although the "gold standard" for the LVFP is the LVEDP, because of the
invasiveness and risks of left ventricular catheterization, the pulmonary
artery catheter and the PCWP measurement have been in widespread use to estimate
the LVFP. However, as pointed out by Rapaport and Dexter,13 PCWP
is an approximation of the true pulmonary capillary pressure and only an indirect
estimate of the LVEDP.
Previous attempts at quantification and correlation of the change in
the arterial pressure during the strain phase of the Valsalva maneuver used
PCWP as an index of left ventricular function.9-11 We
believe that the slope of the arterial pressure during the strain phase is
largely determined by the rate at which the left ventricle depletes the "fixed"
blood volume sequestered in the lungs, the beat-to-beat change in the stroke
volume and pulse pressure being a function primarily of a progressive decrease
in left ventricular filling. A decline in slope with a tendency toward a square
wave pattern of the arterial pressure during the strain phase, therefore,
is associated with a progressive increase in LVEDP.
For this reason, we hypothesized that the relationship previously observed
between the PCWP and the Valsalva response9 was
primarily determined by LVEDP and secondarily determined by factors that influenced
the column of blood between the left ventricle and the PCWP catheter. Thus,
the arterial pressure slope of the early strain phase of the Valsalva maneuver
would likely correlate well with the catheter estimate of both the LVEDP and
the PCWP. However, between the VeriCor determinations of the LVEDP and the
PCWP, the LVEDP should be considered as a critical contributor to left ventricular
filling because it is the final determinant of the left ventricular end diastolic
volume and the resultant stroke volume during the strain phase of the Valsalva
maneuver.
The method and validation of the VeriCor System in the recording of
the peripheral arterial pressure during the Valsalva maneuver have been previously
described in detail by our group.12 Although
peripheral arterial pressure waveforms have been known to differ significantly
from central aortic pressures because of reflected waves and vascular impedance,
using tonometry, O'Rourke et al14 have shown
that noninvasively recorded peripheral arterial pressure waveforms are similar
to those in the ascending aorta. The radial tonometry used by us used the
same technology and transfer function used by Chen et al15 to
reconstruct the central aortic waveforms from tonometric radial waveforms
with clinically acceptable accuracy for systolic and pulse pressure.
The results of our study (Figure 2 and Figure 5) show that VeriCor accurately measures
the 2 important components of the left ventricular filling pressure, the LVEDP
(r = 0.86) and the PCWP (r =
0.81), within a clinically acceptable range of variability. Between these
2 indices, VeriCor appears to have superior predictive accuracy for the LVEDP
(Figure 4), supporting our hypothesis
that VeriCor may sense the LVEDP directly and that the arterial pressure response
during the strain phase of the Valsalva maneuver may be primarily determined
by the LVEDP. VeriCor thus appears to be a valuable noninvasive tool in the
determination of the LVEDP.
Considering the multiple physical, vasomotor, and neurohumoral mechanisms
that govern the Valsalva maneuver, the VeriCor method, which is based on the
arterial pressure response during the early strain phase, appears to be a
fairly robust and reproducible index of the LVEDP, as also substantiated by
the analysis of paired data in our study. Its apparent ability to directly
sense the LVEDP perhaps renders it relatively immune to the influence of intrathoracic
and intra-alveolar pressures that can interrupt the continuity of the column
of blood between the pulmonary artery catheter and the left ventricle and
falsify PCWP as an approximation of LVEDP.
In contrast to the reported insertion time and complications of the
pulmonary artery catheter, the VeriCor method is quick and safe even in critically
ill patients.16-17 Advantages
to obtaining VeriCor LVEDP measurements in preference to catheter-measured
PCWP also stem from the fact that major errors can be committed in the technique
of obtaining and reading PCWP measurements18-19 by
the pulmonary artery catheter. VeriCor technology thus facilitates immediate
reading and display of the 2 components of LVFP (ie, PCWP and LVEDP) on the
monitor screen soon after the completion of the test, eliminating errors in
calculation and interpretation.
Although the VeriCor test is noninvasive and safe and can be repeated
as needed, there are important limitations. In patients with atrial fibrillation
or frequent atrial or ventricular ectopy, there may be significant beat-to-beat
variation in the arterial pressure pulse, which renders analysis of the slope
difficult and erroneous. Patients with arrhythmias were, therefore, excluded
from our study. Patients also need to maintain uniform strain at the desired
level (20-35 mm Hg) during the Valsalva maneuver, if reliable results are
to be obtained. Erratic initial strain ("spikes") can transiently increase
arterial pressure and alter the computation of the pulse amplitude ratio.
Training of the patients helps in this regard.
Despite these limitations, VeriCor measurement of LVFP may prove to
be a useful adjunct in the diagnosis and management of CHF, particularly in
those clinical situations where repeated invasive measurements are impractical
because of the time, safety, and cost considerations. Its advantages over
the pulmonary artery catheter make it an attractive device for the screening
and follow-up of patients in the office setting.
AUTHOR INFORMATION
Accepted for publication April 1, 2002.
This study was supported in part by funding from CVP Diagnostics, Inc
(Dr Sharma).
Corresponding author and reprints: G. V. R. K. Sharma, MD, Division
of Cardiology, VA Medical Center, 1400 VFW Pkwy, West Roxbury, MA 02132 (e-mail: gvrk_sharma{at}hotmail.com).
From the Division of Cardiology, Medical Service, Veterans Affairs
Healthcare System and Harvard Medical School, Boston, Mass (Drs Sharma, Pietro,
Rocco, Welt, and McIntyre); and the Division of Cardiology (Dr Lambrew) and
the Nursing Service (Ms Berg), Maine Medical Center, Portland. Ms Woods is
a consultant nurse-coordinator and Mr Sacchetti is a consultant engineer,
both in Boston. Ms Woods and Mr Sacchetti have acted as paid consultants to
CVP Diagnostics, Inc, Boston, and Dr McIntyre has a financial interest in
CVP Diagnostics, Inc.
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