Author + information
- Received September 7, 2010
- Revision received September 13, 2010
- Accepted September 17, 2010
- Published online February 22, 2011.
- Bryan Williams, MD⁎,⁎ (, )
- Peter S. Lacy, PhD⁎,
- Peter Yan, MBBS†,
- Chua-Ngak Hwee‡,
- Chen Liang, PhD‡ and
- Choon-Meng Ting, MBBS‡
- ↵⁎Reprint requests and correspondence:
Dr. Bryan Williams, Department of Cardiovascular Sciences, University of Leicester, Clinical Sciences Wing, Glenfield Hospital, Leicester LE3 9QP, United Kingdom
Objectives The purpose of this study was to develop and validate the novel application of a simple n-point moving average (NPMA)—a mathematical low pass filter—to noninvasively derive central aortic systolic pressure (CASP) from the radial artery pressure waveform (RAPWF) in humans.
Background CASP may be an important independent determinant of clinical outcomes. Development of simple, well-validated methods to noninvasively derive CASP is necessary to facilitate the routine clinical measurement of CASP.
Methods Three studies in different population cohorts were used to develop and validate the NPMA method to derive CASP in humans: 1) a development study (n = 217), describing the optimal application of the NPMA to derive CASP; 2) a validation study comparing NPMA-CASP with CASP derived using a generalized transfer function (GTF-CASP [SphygmoCor system, AtCor, Sydney, Australia]) using 5,349 RAPWFs from the CAFE (Conduit Artery Function Evaluation) study; and 3) an invasive validation study (n = 20) comparing NPMA-CASP with direct aortic root pressure measurements during cardiac catheterization.
Results In the development study, when using the NPMA, a denominator of n/4 (where n = tonometer sampling frequency) most accurately defined CASP relative to GTF-CASP. Validation of NPMA-CASP using RAPWFs from the CAFE study revealed excellent correlation and agreement (r2 = 0.993, mean difference 0.3 ± 1.0 mm Hg). The agreement remained robust after stratification by sex, age, treatment, and diabetes status. There was also excellent correlation and agreement (r2 = 0.98, p < 0.001) between directly measured aortic root systolic pressures (Millar's catheter) at cardiac catheterization versus NPMA-CASP, derived simultaneously from noninvasive RAPWFs.
Conclusions We show that an NPMA with a denominator of one-quarter of the tonometer sampling frequency accurately defines CASP when applied to noninvasively acquired RAPWFs in man. These novel findings have important implications for the simplification of noninvasive CASP measurement and its wider application in clinical trials and clinical practice.
- central aortic systolic pressure
- generalized transfer function
- n-point moving average
- radial artery pressure waveforms
Blood pressure (BP) has been conveniently measured over the brachial artery using a sphygmomanometer for more than a century. However, brachial BP may not always accurately represent the corresponding pressures in the aorta (1). It is now clear that many factors including age, heart rate, body height, sex, and drug therapies can all influence the relationship between brachial and central aortic pressure (2,3). Furthermore, evidence is beginning to emerge that central aortic systolic and/or pulse pressures may more accurately predict cardiovascular structural damage and cardiovascular outcomes when compared to brachial pressures (1,4–9).
With regard to the variation between central aortic and brachial pressures, it is assumed that the mean arterial and diastolic pressure remains largely unchanged from aortic root to brachial artery, and that it is variation in amplification of the pulsatile pressure component, namely, systolic pressure, that accounts for the central-to-brachial pressure differences (10). Thus, focus has been on the accurate derivation of central aortic systolic pressure (CASP).
Mindful of the potential for central aortic pressures to provide incremental value in predicting cardiovascular morbidity and mortality, the challenge is to develop simple and convenient ways to accurately estimate central aortic pressures in routine clinical practice. The “gold standard” is direct measurement of aortic root pressures using a pressure transducer introduced into the aortic root at the time of cardiac catheterization, but this is clearly invasive and unsuitable for routine clinical practice. A popular alternative approach has been analysis of the radial artery waveform obtained by noninvasive tonometry. In this method, the radial waveform is usually calibrated to brachial blood pressure, measured using standard sphygmomanometry, thereby generating a calibrated radial artery pressure waveform (RAPWF). Mathematical “generalized transfer functions” (GTFs) in the frequency or time domain have then been used to derive central aortic pressures and related aortic hemodynamic indices from the RAPWF (11–13). This method has, however, been criticized because of concerns that it may not be appropriate to apply a GTF generated in 1 cohort of patients to all patients with different disease states, at different ages, and receiving different treatments, and so forth (14,15). Nevertheless, applying a GTF to the RAPWF remains the most commonly used method for the noninvasive assessment of central aortic pressure indices (16).
More recently, an alternative approach to estimating CASP from the RAPWF has been proposed. This requires the accurate identification of an inflection point on the RAPWF that is said to correspond to the superimposition of the peak of the reflected wave onto the outgoing pressure wave (17–19). Numerous recent studies have suggested that this inflection point, so-called SBP2, corresponds to the peak CASP and is a reasonably accurate way of noninvasively assessing CASP, without the need to apply a GTF (20,21).
In this paper, we report for the first time a simple approach for the accurate estimation of CASP in humans, using an n-point moving average (NPMA). A moving average is a mathematical model, used extensively in many applications beyond medicine, that acts as a low pass filter to smooth signal data, to better determine underlying trends. We hypothesized that this methodology could be applied to the RAPWF to generate a representative smoothed waveform, the peak of which would correspond to CASP. This reasoning is based on the fact that amplification of radial systolic pressure from the aortic root is due to changes in the relative timings of the incident and reflected pressure waves with increasing distance from the heart, together with the nonuniform elasticity and viscous damping within the arterial system (22,23). These effects result in a spatial amplification of the arterial pressure wave without additional energy input. This has been described as a “distortion” rather than true amplification of the pressure signal, which translates into the altered morphology of the radial waveform (24). Accordingly, we hypothesized that application of a NPMA might eliminate this effect, revealing the amplitude of the original signal and, thus, CASP. Such an approach, if valid, would have the advantage of utilizing patient-specific data contained within the entire RAPWF, removing the need for a more complex GTF, or the need to identify any specific points on the waveform, namely, an inflection point. It would provide a novel and simple system that could potentially be applied routinely to estimate CASP, wherever a radial waveform is captured by tonometry.
This work involved 3 related studies, each using different population cohorts: 1) an initial development study to define the optimal denominator for the NPMA (i.e., extent of filtering) required to estimate CASP relative to the reference noninvasive GTF method (see the following text); 2) a noninvasive validation study versus the GTF-CASP; and 3) an invasive validation study at cardiac catheterization.
Reference GTF method for noninvasive measurement of CASP
To develop and validate the NPMA method for deriving CASP from the RAPWF, a reference method was required. We used the GTF embedded within the SphygmoCor system (AtCor, Sydney, Australia), which is, to date, the most widely used method for noninvasively deriving central pressures and hemodynamics. The SphygmoCor GTF is reported to be based on a Fourier transformation, namely, a frequency domain method (11,25).
Estimation of CASP from individual RAPWFs using a moving-average method
A moving average generates an array of incrementally averaged data points, based on a specified and constant denominator. We hypothesized that this denominator would be related to the sampling frequency of the tonometer used to acquire the RAPWF. The optimal NPMA denominator to derive CASP from the RAPWF could have been one of a range of fractions of the sampling frequency of the tonometer (n), namely, n/2, n/3, n/4, and so on. Thus, if the optimal fraction for the NPMA was ultimately found to be n/4, then using a sampling frequency of 128 Hz, incremental blocks of 128/4, in other words, 32 data points, would be generated. The resulting array of averages yields a maximum value that we hypothesized would equate to CASP (Fig. 1).
Study to develop and optimize the NPMA method to noninvasively derive CASP in humans
Using a study cohort comprising 217 volunteers, RAPWFs were sampled using the SphygmoCor tonometer (sampling ƒ = 128 Hz), calibrated to seated brachial BP, measured using the MC3100 oscillometric device (AAMI standard-SP10, 1992, Food and Drug Administration approved, Healthstats, Singapore). To define the optimal denominator for the NPMA method to predict CASP, various denominators, namely, proportions of the tonometer sampling frequency ranging from n/3 through to n/6 were tested. The resulting peak value from the moving average arrays was then compared to the GTF-CASP generated by the SphygmoCor device to determine which denominator generated a curve, the peak of which best equated to GTF-CASP (see preceding text and Fig. 1 for details).
The noninvasive NPMA validation study
The optimal NPMA method for deriving CASP as determined in the development study was then tested versus the alternative, 2 currently used approaches for the noninvasive derivation of CASP from the RAPWF, namely, GTF-CASP and SBP2-CASP. The population used for this study was different from that used for the development study. The RAPWFs used for these analyses had been collected for the CAFE (Conduit Artery Function Evaluation) study, which has been previously described in detail (1). We used the RAPWFs collected from patients who attended the host CAFE study center (Leicester, United Kingdom [n = 383]) and had repeated samplings of RAPWFs, at scheduled study follow-up visits, over the course of 5 years, yielding 5,349 individual RAPWFs for these analyses. The RAPWFs had been sampled using radial applanation tonometry (SphygmoCor system) calibrated to brachial blood pressure (Omron 705CP), as previously described (1). Ensemble-averaged waveforms were manually extracted as text files to facilitate comparisons of the signal processing models to estimate CASP. The reference method for these noninvasive validation studies was the GTF embedded within the SphygmoCor system (GTF-CASP) (24), which was compared to CASP generated by our NPMA method and the SBP2 method. The SBP2 method for deriving CASP (SBP2-CASP) relies on the identification of an inflection point on the systolic down slope of the untransformed ensemble-averaged RAPWFs. The SBP2 was defined by the SphygmoCor software using a derivative plot and was taken to represent SBP2-CASP, as previously described (20,21).
The invasive validation study
This study was conducted at the Gleneagles Medical Center in Singapore. Twenty adult patients undergoing routine diagnostic cardiac catheterization were invited to participate. Before cardiac catheterization, a high-fidelity micromanometer-tipped catheter (Millar SPC-454D, Millar Instruments, Houston, Texas) was set to zero and introduced through the femoral artery, and with fluoroscopic guidance, the tip of the catheter was positioned in the proximal aortic root, ∼1 cm above the aortic valve, until stable aortic pressure waveforms were obtained. At the same time as the Millar catheter was introduced, a tonometer attached to a wrist strap (A-pulse, Healthstats, Singapore) was applied to the wrist to allow continuous sampling of RAPWFs. The A-pulse tonometer had a sampling frequency of 60 Hz; thus, the denominator for the moving average was 15 (i.e., 60/4). With both devices in place, the brachial BP was measured over the same arm as the tonometer using an oscillometric device (MC3100, Healthstats). These measurements were used to calibrate the noninvasive RAPWFs. An average of 3 brachial BP readings, not <30 s apart, were used for this calibration. The output from the Millar catheter, by a multichannel monitor, was synchronized in time with the output from the A-pulse tonometer, allowing simultaneous, real-time comparison of the moving average-CASP (NPMA-CASP) with direct aortic root measurement of CASP. Both the invasive and noninvasive devices averaged waveform data in synchronous, time stamped, 10-s blocks; and the first stable 10 blocks obtained after calibration was completed were used for data analysis, over a sampling period of up to 3 min. This allowed comparison of the simultaneous beat-to-beat averaged generation of CASP from the RAPWF through the NPMA versus the actual invasive aortic root pressures.
All studies were approved by local research ethics review boards, and all participants provided written informed consent for participation in these studies.
Comparisons of the reference GTF method with the NPMA or SBP2 for the development and noninvasive validation studies, namely, when data from the same waveforms were processed using different algorithms, used paired Student t tests. The impact of the CAFE study BP-lowering treatments, comparing the reference GTF-CASP with the moving average and SBP2 methods, was analyzed using an unpaired Student t test because 2 different treatment populations were being compared. For the invasive validation study, where waveforms were collected using different sampling devices for direct comparisons between noninvasive NPMA-CASP and invasive aortic root pressures, a paired Student t test was used. Additional comparisons used linear regression and Bland-Altman plots.
Study to develop and optimize the NPMA method to derive CASP in humans
The demographics of the population used for the development study (n = 217) to define the optimal denominator for the NPMA to derive CASP from the RAPWF is shown in Table 1. An NPMA with a denominator of n/4 consistently produced a moving average curve with a peak value (i.e., NPMA-CASP) that most closely approximated to the reference GTF-CASP (Fig. 2). The other denominators either underestimated (n/3) or overestimated (n/5, n/6) relative to GTF-CASP (Fig. 2). Thus, NPMA-CASP was best defined relative to GTF-CASP using a denominator of one-quarter of the sampling frequency of the tonometer.
Noninvasive validation study and comparison of NPMA-CASP with CASP derived using the GTF and SBP2 methods
The characteristics of the patients included in the noninvasive validation study are shown in Table 2. The mean age was 63 years; 80% of patients were male; and ∼90% were white Caucasian. All had treated hypertension, and ∼15% had diabetes mellitus at baseline. The CASP could be derived from all 5,349 RAPWFs analyzed in this validation study, using either the GTF or the NPMA method. By contrast, the systolic inflection point, upon which SBP2-CASP is dependent, could not be identified in 164 (3.1%) RAPWFs. Figure 1B shows an example of the 3 methods for deriving CASP from a typical RAPWF and the remarkable correspondence for CASP according to each of these methods.
Pair-wise comparisons of the NPMA or SBP2 methods to derive CASP, versus GTF-CASP, are shown in Table 3. Using the GTF-CASP as the reference, the NPMA-CASP differed by an average of only 0.3 mm Hg. In contrast, SBP2-CASP differed from GTF-CASP by an average of 1.6 mm Hg.
Regression analyses comparing the reference GTF-CASP with NPMA-CASP or SBP2-CASP are shown in Figure 3. Good correlations were seen with both methods; however, the correlation was improved for NPMA-CASP relative to SBP2-CASP (NPMA-CASP versus GTF-CASP: r2 = 0.99, p < 0.0001; SBP2-CASP versus GTF-CASP: r2 = 0.95, p < 0.001).
Comparison of GTF-CASP versus either NPMA-CASP or SBP2-CASP showed good agreement in the Bland-Altman plots. However, the agreement was stronger and the data less scattered for the GTF versus NPMA-CASP (average difference 0.3 mm Hg, 2 × SD 2.1 mm Hg) (Fig. 4), when compared to the GTF versus SBP2-CASP (average difference 1.6 mm Hg, 2 × SD 6.1 mm Hg). The data also suggested a bias with the SBP2 method, namely, increasing difference from the GTF-CASP with increasing pressure (Fig. 4). There was no such bias in the comparison between the GTF and NPMA-CASP.
Impact of sex, age, and diabetes
There was no impact of sex, age (≤55 years vs. >55 years), or presence versus absence of diabetes on performance of the NPMA in defining CASP relative to the reference GTF-CASP in the noninvasive validation study (see the Online Appendix for supplementary results).
Impact of BP-lowering treatments on CASP derived by different processing methods
The patients in the CAFE study from which the RAPWFs were derived had been randomized to treatment with 2 different blood pressure-lowering strategies (amlodipine ± perindopril, as required vs. atenolol ± bendroflumethiazide, as required) (1). The brachial blood pressures throughout the study on both treatment regimens were similar. Stratification by treatment arm allowed examination of whether the differential effects of treatment (e.g., on heart rate and/or vasodilation) introduced any variation in the relationship between the reference GTF-derived CASP and the NPMA or SBP2 methods. Table 4 shows the average value for brachial BP and the corresponding CASP values derived using the 3 different signal processing methods. With each method, amlodipine-based therapy was associated with lower CASP values versus atenolol-based therapy. There was excellent agreement between GTF-CASP and the NPMA-CASP irrespective of treatment. There was also good agreement between GTF-CASP and SBP2-CASP but with wider scatter.
Invasive validation study comparing direct aortic root systolic pressure with NPMA-CASP
The demographics of the 20 patients recruited for the invasive validation study are shown in Table 5. The data showing a comparison of invasive aortic BP versus the corresponding noninvasive brachial BP at the time of calibration and derived NPMA-CASP are shown in Table 6. It is notable that noninvasive brachial systolic and diastolic pressures are overestimated by conventional oscillometric BP measurement versus direct aortic root pressures (aortic systolic BP 139.6 ± 4.3 mm Hg versus brachial systolic BP 147.1 ± 4.8 mm Hg, p < 0.01) (Table 6); this was most noticeable for diastolic BP (aortic diastolic BP 74.2 ± 2.1 mm Hg vs. brachial diastolic BP 87.7 ± 2.5 mm Hg, p < 0.001). In contrast, there was remarkable correspondence between invasive CASP measured at the aortic root versus CASP derived noninvasively from RAPWFs using the NPMA (invasive CASP 139.6 ± 4.3 mm Hg vs. NPMA-CASP 139.2 ± 4.1 mm Hg) (Table 6).
The data in Figure 5 shows the average CASP for each 10-s block (i.e., 10 data points per patient) for both invasive and noninvasive measurements. Noninvasive NPMA-CASP from the RAPWFs did not differ from invasively measured aortic root pressure (invasive CASP 139.6 ± 1.4 mm Hg vs. NPMA-CASP 139.2 ± 1.3 mm Hg, p = NS), and a high degree of correlation (r2 = 0.98, p < 0.001) (Fig. 5) was seen between these 2 parameters. Bland-Altman analysis also showed good agreement between values with little scatter (mean difference: NPMA-CASP minus invasive CASP = −0.41 ± 2.5 mm Hg) (Fig. 5).
We have demonstrated that the application of a simple moving average method to a noninvasively acquired radial artery waveform, calibrated to brachial blood pressure, is sufficient to accurately derive central aortic systolic pressure in humans. The correspondence between NPMA-CASP and direct measurement of aortic root pressure was remarkable. In the invasive study, the agreement between direct invasive measurement and noninvasive NPMA-CASP was well within agreed standards for validation of blood pressure measuring devices (26,27). Applying these standards, the average of all sampling blocks for each patient revealed that the vast majority (96%) of data points were comfortably within 5 mm Hg variance from the mean (Fig. 5). Moreover, when NPMA-CASP was compared to GTF-CASP, all but 1 of >5,000 data points was within these standards. Thus, in the present study, the NPMA-CASP has been validated against the most commonly used method to derive CASP from the RAPWF, and against direct aortic root pressures.
Methods for deriving central systolic pressure from radial artery pressure waveforms
There has been much debate about the use of a GTF to derive central aortic pressures and hemodynamics from the RAPWF (15) and, in particular, whether a generalized function is appropriate for people at different ages and with different disease states (14,28,29). Most concur, however, that the low frequency harmonics of the pressure wave do allow CASP to be accurately determined by the use of a GTF (30–33). Furthermore, the GTF-derived CASP used in the present study has been validated against direct invasive measurement of central aortic pressures (25,30). The main concern about the GTF has related to its application to higher frequency harmonics upon which other derived central hemodynamic parameters depend, for example, central augmentation index (12,33)—these are much less well validated.
The alternative to a GTF for the derivation of CASP from the RAPWF has been the use of SBP2, first described by Takazawa (17). This method is critically dependent on the accurate determination of the inflection point on the down-stroke of the radial systolic pressure wave. Identifying this point is dependent on the morphology of the radial artery waveform and is not always possible. In our study of >5,000 waveforms, identification of SBP2 was not possible for 3.1% of waveforms. Furthermore, the correspondence between SBP2 and GTF-derived CASP was less robust than that seen with our NPMA-CASP versus GTF-CASP. We also detected a bias with the SBP2 method, which was accentuated at higher pressures. The same bias has also been reported by others (20,21). Nevertheless, despite these limitations, the SBP2 method does allow CASP to be estimated from the RAPWF with reasonable accuracy (19).
A key finding of the present study is the demonstration that there is no need for the added complexity of a GTF or the limitations of SBP2 to derive CASP from the RAPWF. We show that the information required to accurately derive CASP is contained within the RAPWF and can be revealed by a simple low pass filter, in other words, the moving average. We tested a range of denominators as integers for the moving average and determined that one-quarter of the tonometer's sampling frequency is the most accurate denominator for deriving CASP from the RAPWF in humans. Importantly, this applies irrespective of the sampling frequency of the tonometer, as illustrated by the fact that we used 2 different tonometers for the noninvasive and invasive validation studies, sampling at 128 Hz and 60 Hz, respectively.
Use of a low pass filter in assessing CASP
We show that a simple low pass filter, performing a smoothing function, can be used to accurately define the amplitude of the original pressure wave signal, and thus CASP from the RAPWF. It could be argued that a Fourier transformation in the time or frequency domains, as used in a GTF, is also functioning as a low pass filter. However, the noted controversy about the use of a GTF to derive other central hemodynamic parameters has obscured recognition of the simplicity of applying a low pass filter to accurately derive CASP as described here. To our knowledge, this is the first report describing the use of a simple moving average to noninvasively derive CASP from the RAPWF. This application of the moving average for deriving CASP is not dependent on any generalized adjustment or assumptions; it simply filters data that are actually present in the waveform, specific to the patient. Accordingly, we observed no obvious impact of sex or age on the correlations between GTF-CASP and the NPMA-CASP, and no evidence of any blood pressure-related bias.
Why one-quarter × sampling frequency of the tonometer should be the specific denominator required to reveal CASP from the RAPWF in humans requires further evaluation.
Simplicity in the moving average method
A frustrating feature of existing technologies to derive central pressure and hemodynamics noninvasively is that they are expensive and often exist as a “black box” from which is it is difficult to define the specific algorithms used to derive the final data. The NPMA method reported here is simple and will allow CASP to be derived from any RAPWF imported into a spreadsheet, facilitating further evaluation of the method in a variety of patient groups from past and future studies. The main potential error in the method, as with all of the noninvasive methods, comes from the inherent error in the conventional measurement of brachial blood pressure that is required for calibration of the radial artery waveform. This error is generally less for brachial systolic pressure than for diastolic pressure (Table 6) (34). Thus, any error in CASP estimation from the RAPWF should be no greater than the error in conventional brachial BP measurement.
The moving average method is designed to accurately derive CASP and does not generate an aortic waveform. Thus, it cannot be used to derive other indices such as aortic augmentation index, the validation and utility of which remain controversial (12,33,35).
An alternative method for deriving CASP is the use of carotid artery waveforms calibrated to brachial mean and diastolic pressures. This method does not require a GTF or moving average as the carotid waveform is considered to be a central waveform. However, carotid tonometry is more difficult than radial tonometry and is much less suited for routine measurements of CASP when compared to the aforementioned radial waveform approach.
There are a number of limitations to our study that we acknowledge. In the development study and noninvasive validation studies, the reference method for estimating CASP was the GTF embedded within the SphygmoCor device. This noninvasive method has itself, as discussed in the preceding text, been criticized with regard to the extent of its validation. It is, however, the most widely used method, and our study is strengthened by our also reporting an invasive validation of the NPMA method, showing excellent correlation with direct measurement of aortic root pressure.
With regard to the comparison versus GTF-CASP, our noninvasive validation study population was mainly male but did include >800 RAPWFs from female patients, and no sex bias was noted when formally tested (Online Fig. 1A). Another limitation is that our noninvasive validation study population was predominantly white Caucasian, consistent with a general paucity of data on pulse wave analysis in other ethnic groups, which needs addressing. However, our population for the NPMA development and invasive validation studies were Chinese Asian, confirming applicability of the method to these populations as well. As with almost all studies of this kind, our populations for both validation studies were mostly older than 50 years, with manifest cardiovascular disease or at high cardiovascular risk. We saw no evidence of a bias related to extremes of age in the performance of the NPMA method (Online Fig. 2A). It is noteworthy that the demographics and ethnic origins of the 3 cohorts used in these studies of NPMA-CASP were different from each other but still yielded remarkably consistent data—supporting the robustness of the method. However, further studies would be desirable in younger people and children.
Finally, regarding the potential impacts of drug therapy on the performance of the NPMA, our noninvasive validation study cohort, from the CAFE study, were treated with 2 different BP-lowering regimens (atenolol ± bendroflumethiazide or amlodipine ± perindopril) (1). We have previously reported that these treatments had markedly different effects on heart rate and CASP as determined using the SphygmoCor GTF (1,36). It is notable that the differential impact of these treatments on CASP were perfectly replicated by application of the NPMA method to the same RAPWFs (Table 3), indicating no inherent bias or degeneration of the performance of the NPMA-CASP due to treatment and/or heart rate effects.
We report a simple and accurate method for estimating CASP from noninvasively acquired RAPWFs in humans. This method has been compared to existing reference methods and validated against direct invasive measurements of aortic pressures, using different study populations. There is currently tremendous interest in the noninvasive measurement of CASP in clinical studies and clinical practice (31). This has been driven by emerging evidence that CASP may provide a better assessment of the impact of medications on aortic pressures and incremental value, over and above conventional brachial blood pressure measurement, in the assessment of cardiovascular risk (1,4–9). Wider application of CASP measurement in clinical practice has been hindered by a lack of data regarding its clinical relevance, the unnecessary complexity and cost of acquiring the data, and longstanding controversy regarding the use of a GTF. The simplicity of the moving average method, the transparency of the technique, its widespread acceptance as a universal tool for signal processing, and the detailed development and validation studies reported here, could lead to greater acceptance and use of noninvasive CASP measurements in clinical studies and ultimately, clinical practice.
The authors thank Tom Williams, BSc (Hons), University of Warwick United Kingdom, for help in manually extracting wave forms for the noninvasive validation study. Part of this study used radial artery waveforms collected in Leicester, United Kingdom, for the CAFE study, a substudy of the ASCOT study. Therefore, the authors thank the CAFE and ASCOT study investigators for their contributions.
For detailed Methods, Results, and supplementary figures, please see the online version of this article.
Dr. Williams worked in a scientific collaboration with Healthstats, Singapore, to develop the concepts contained in the report, and is a nonsalaried advisor to Healthstats, Singapore; and an NIHR Senior Investigator. Drs. Williams and Lacy and Mr. Ting are faculty members of the Leicester National Institute for Health Research (NIHR) Biomedical Research Unit in Cardiovascular Diseases, which supported this study. Dr. Lacy and Mr. Yan have no conflicts of interest in relation to this work. Chua-Ngak Hwee is a board member of Healthstats. Dr. Liang is an employee of Healthstats. Mr. Ting is founder and Chairman of Healthstats.
- Abbreviations and Acronyms
- blood pressure
- central aortic systolic pressure
- generalized transfer function
- n-point moving average
- pulse pressure
- radial artery pressure waveform
- systolic blood pressure
- Received September 7, 2010.
- Revision received September 13, 2010.
- Accepted September 17, 2010.
- American College of Cardiology Foundation
- The CAFE Investigators for the ASCOT Investigators
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