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* From Itamar Medical Ltd. (Drs. Bar, Dvir, Sheffy, and Schnall), Caesarea; and Sleep Laboratory (Drs. Pillar and Lavie), Bruce Rappaport Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel.
Correspondence to: Peretz Lavie, PhD, Sleep Laboratory, Gutwirth Bldg, Technion City, Haifa 32000, Israel; e-mail: plavie{at}tx.technion.ac.il
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Abstract |
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 TOP Abstract IntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Objectives: To evaluate the efficacy, reliability, and reproducibility of the WP100 device for the diagnosis of OSAS as compared to in-laboratory, standard polysomnographic-based manual scoring.
Design and methods: One hundred two subjects (78 men; 69 patients with OSAS and 33 normal volunteers; mean ± SD age, 41.4 ± 15.2 years; body mass index, 26.8 ± 5.5) underwent in-laboratory full polysomnography simultaneously with WP100 recording. Fourteen subjects also underwent two additional unattended home sleep studies with the WP100 alone. The polysomnography recordings were blindly scored for apnea/hypopnea according to the American Academy of Sleep Medicine criteria (1999), and the polysomnography respiratory disturbance index (RDI) [PSG-RDI] was calculated. The WP100 data were analyzed automatically for the PAT RDI (PRDI) by a proprietary algorithm that was previously developed on an independent group of subjects.
Results: Across a wide range of RDI levels, the PRDI was highly correlated with the PSG-RDI (r = 0.88, p < 0.0001), with an area under the receiver operating characteristic curve of 0.82 and 0.87 for thresholds of 10 events per hour and 20 events per hour, respectively. The PRDI scores were also highly reproducible, showing high correlation between home and in-laboratory sleep studies (r = 0.89, p < 0.001).
Conclusion: The WP100 may offer an accurate, robust, and reliable ambulatory method for the detection of OSAS, with minimal patient discomfort.
Key Words: ambulatory • automatic analysis • obstructive sleep apnea syndrome • peripheral arterial tone • respiratory disturbance index • sleep
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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The earliest ambulatory devices were based on overnight pulse oximetry alone, an easy and simple technology for diagnosing OSAS.4 However, it has been shown that the pulse oximeter suffers from limited accuracy.5 6 Portable full polysomnography and other multiple-channel ambulatory devices are frequently complex and cumbersome.7
The American Sleep Disorders Association (ASDA) has classified sleep study systems into four categories: level 1, in-laboratory attended standard polysomnography; level 2, unattended home sleep study with comprehensive portable devices incorporating the same channels as the in-laboratory standard polysomnography; level 3, unattended devices, which measure at least four cardiorespiratory parameters; level 4, unattended devices recording one or two parameters.8
The Watch PAT100 (WP100) [Itamar Medical; Caesarea, Israel] is a four-channel unattended ambulatory device (level 3) based on the peripheral arterial tone (PAT) signal with three additional channels: heart rate (derived from the PAT signal), pulse oximetry, and actigraphy (both are embedded in the device). The PAT signal measures the arterial pulsatile volume changes of the finger that are regulated by the 
-adrenergic innervation of the smooth muscles of the vasculature of the finger, and thus reflects sympathetic nervous system activity.9
The WP100 indirectly detects apnea/hypopnea events by identifying surges of sympathetic activation associated with the termination of these events. This information is further combined with heart rate and pulse oximetry data that are analyzed by the automatic algorithm of the system (which was developed on a prior group of patients). This detects respiratory events and calculates the PAT RDI (PRDI).
The primary objective of this study was to evaluate the efficacy of the device for diagnosing OSAS, by comparing its results to simultaneous polysomnography recordings. Secondary objectives were to evaluate the feasibility and reproducibility of the WP100 in an unattended home setting.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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-adrenergic receptor blockers (24-h washout period required), and alcohol or drug abuse during the last 3 years. The study protocol was approved by the Ethics Committee of the Rambam Medical Center, and the subjects gave their written informed consent prior to participation.
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The overnight sleep studies were considered acceptable for data analysis if none of the following rejection criteria occurred: (1) polysomnography-related rejection (polysomnography actual sleep time < 1.5 h, technical failure of synchronizing the polysomnography to the WP100, and poor quality of polysomnographic recording); and (2) WP100-related rejection (WP100 valid sleep time < 1.5 h). Patient demographic and medical information, as well as the ESS questionnaire, were acquired by interview prior to the sleep study.
Equipment
WP100 Device:
The WP100 used in the unattended sleep studies is comprised of a battery-powered, forearm-mounted console unit placed just above the wrist, and two finger-mounted probes: pulse oximetry and PAT probe (Fig 1
).
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Pulse Oximeter Probe:
A standard-type pulse oximeter probe (Nonin OEM 2 oximetry module, 8000J; Nonin; Ellös, Sweden) is applied to a finger in a conventional manner according to the instructions of the manufacturer.
PAT Probe:
The second finger probe measures the PAT of the patient's fingertip. Previous applications of this technology have been described9
10
11
12
; however, in this study the method was specifically adapted for ambulatory unattended use.
Essentially, the WP100 finger sensor applies a uniform pressure field over the distal two thirds of the finger, including the fingertip. Previous plethysmographic devices that enveloped the fingertip, such as venous occlusion plethysmography collection cuffs, tended to be pushed off the finger when pressurized. However the split-thimble design of the PAT probe allows it to actively clamp itself to the finger while applying a pressure field that facilitates the unloading of arterial wall tension without causing distal venous pooling and distension, thus avoiding the induction of venoarterial-mediated vasoconstriction.13
A transmission mode photoelectric plethysmograph situated at opposing lateral sides at about the middle of the distal phalanx is used to measure the optical density changes associated with pulsatile blood volume changes of the finger. The proximal two thirds of the pressure field buffers the sensing region from extraneous and artifactual signals such as perturbations in the venous system.
Isobaric, Volume-Displacement PAT Probe Design:
A unique feature of the PAT finger probe is its ability to generate its own pressure field at a fixed level of pressure irrespective of the size of the finger.14
The pressure field is created by the insertion of a finger into the probe (Fig 2
, top, A). When the finger is inserted into the probe, a proportionate amount of air is shifted from the inner compartment of the probe to its outer compartment, causing the pretensioned outer membrane to be pushed off the wall of the inner shell and thus apply pressure to the air within the probe. The elastic properties of the balloon-like outer membrane are such that over a wide range of volumes it creates a constant pressure.
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The physical basis for the ability of the probe to generate a fixed pressure is described by the law of Laplace, which relates the pressure within a distensible hollow object to the wall tension and the radius such that the pressure is proportional to wall tension divided by radius. In the case of elastic balloons in general, and in the specific case of the elastic outer membrane of the probe, wall tension varies in direct proportion to radius, and thus pressure remains constant over a large range of volumes.
Automatic Algorithm:
The automatic algorithm of the WP100 is based on the PAT signal amplitude, heart rate, and oxygen saturation. The sleep/wake detection is based on data recorded by the built-in actigraph.
Polysomnography
Data Collection:
Standard in-laboratory, overnight polysomnography was performed using a computerized polysomnography system (Embla; Flaga Medical; Reykjavik, Iceland), with the following channels: EEG (C3-A2 and O2-A1), electrooculogram (right and left), chin electromyogram, arterial oxygen saturation, nasal-oral airflow (thermistor), ECG, chest and abdominal wall motion (piezo electrodes), bilateral tibialis electromyogram, body position, and auxiliary channel with a synchronizing signal from the WP100 device.
Scoring:
The polysomnography recordings were scored manually for sleep stages15
and respiratory events (apnea/hypopnea) according to the American Academy of Sleep Medicine criteria, 1999.2
An apnea/hypopnea event was defined as an airflow amplitude reduction of > 50% from the baseline lasting at least 10 s, or having a less significant reduction in the airflow amplitude, but with the presence of arousal or oxygen desaturation of at least 3%. The PSG-RDI was calculated as the number of apnea/hypopnea events per hour of sleep. The scorer had no access to the WP100 data or results while scoring the polysomnography data.
Statistical Analysis
The correlation between the PRDI and PSG-RDI was assessed using the Pearson correlation coefficient and Bland-Altman plots. Receiver operating characteristic (ROC) analysis was carried out to evaluate the WP100 diagnostic capability. A threshold of PSG-RDI > 10 was used as the cut-off point for OSAS diagnosis,16
and PSG-RDI > 20 was defined as the commonly used cut-off point for intended CPAP treatment.17
Based on these threshold definitions, ROC curves were derived, and areas under the curves (AUCs) were calculated.
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Figure 3 shows a scatter graph that demonstrates the high and statistically significant correlation coefficient between the PSG-RDI scores and the PRDI scores (r = 0.88, p < 0.0001, n = 99). Figure 4 shows a Bland-Altman plot of the differences between the PSG-RDI and the PRDI values vs the corresponding averages of the two RDI indexes. There was a slight tendency for the PAT to underscore events in the mild range of OSAS, and to overscore events in the severe range.
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Over 80% of patients with moderate-to-severe OSAS are undiagnosed and untreated, which can subsequently lead to the severe sequelae of the syndrome. Kapur et al24 concluded that patients with undiagnosed OSAS had considerably higher medical costs than age and sex-matched non-OSAS subjects, and that the OSAS severity was associated with the magnitude of medical costs. The long waiting lists for in-laboratory polysomnography studies, which are expensive and time consuming, have led to an intensive search for ambulatory alternatives. According to the ASDA standards of practice (1994), sleep study devices are classified into four levels, from the attended, in-laboratory, full polysomnography (level 1) to the unattended, at-home, single/double-channel recording (ASDA level 4).8 In general, there appears to have been a tradeoff between the amount of information and the simplicity of the device. Simple devices tend to have a high level of feasibility and saving of time and expenses, but provide less information. For example, Portier et al7 evaluated unattended, at-home polysomnography studies (ASDA level 2) vs in-laboratory polysomnography studies, using the same recorded channels. They found that the reliability of the studies was associated with the quality of data obtained under the unattended conditions, where 33% of the recordings were not adequate for data analysis, and a further 11% of the studies had discordant results. The rejection rate of the WP100 device (10%) is in the lower-to-middle range of the ASDA grade level III devices, which were estimated to have rejection rates in the range of 4 to 24%.20 21 22 23 24 Other ambulatory devices from this category could also demonstrate relative simplicity and low rejection rate, but showed unsatisfactory accuracy. Esnaola et al25 studied 150 individuals with suspected OSAS, who underwent full polysomnography simultaneously with level III devices and reported AUC values from 0.67 to 0.76. Cirignotta et al26 reported that the automatic scoring of such a device was unreliable in assessing patients with complicated OSAS. In contrast to these reported devices, the WP100 appears to have both the advantage of simplicity (recording only from two finger probes) and high accuracy.
The device is based on the PAT signal, a measurement of the pulsatile volume changes in the vascular bed at the fingertip, providing continuous monitoring of the pulse rate and detecting digital vasoconstriction events, both of which are affected by the sympathetic nervous system. Grote et al9
has shown that the control of the finger arteries is almost exclusively mediated by sympathetic receptors, so that PAT-detected episodes of vasoconstriction actually reflect sympathetic activation. Pitson and Strad-ling27
reported that repeated occurrence of sleep apnea and hypopnea events causes arterial oxygen desaturation, as well as arousal from sleep with periodic episodes of increased sympathetic nervous system activation. These authors used heart rate change and the pulse transit time as indices of sympathetic activation and found that these were less well correlated with the apnea/hypopnea index (r = 0.51 and 0.65, respectively).
The PAT technology is a unique and relatively new concept of noninvasive measurement of sympathetic activation levels that appears to be very accurate for detecting sleep-disordered breathing events. The self-contained pressurizing mechanism of the probe allows it to be lightweight and silent, essential for a practical ambulatory device. Its ability to reliably apply a predetermined and constant level of pressure over a broad range of finger sizes is essential for the accuracy, robustness, and reproducibility of the method, and was also vital for the development of the algorithm.
The first study that demonstrated the PAT signal diagnostic capability for patients with OSAS was performed using a bedside version of the PAT system, which was automatically analyzed using PAT signal attenuation and pulse rate criteria alone. In that study, Schnall et al10 found a high correlation between standard polysomnography scoring of total apnea-hypopnea events and PAT-vasoconstriction events with concurrent tachycardia. Later, Pillar et al11 showed that detection of apnea and hypopnea events based on combined data from PAT and pulse oximetry was highly correlated with standard polysomnography scored results, a finding that was confirmed by Pittman et al12 utilizing both manual and automatic analysis. O’Donnell et al28 further explored the PAT response in patients with OSAS. They experimentally induced upper airway obstruction and have shown that airflow obstruction in patients with OSAS leads to a PAT signal attenuation in a "dose-response" manner, ie, greater airflow obstruction causes greater PAT attenuation.
The criteria of the automatic algorithm for respiratory disturbance event in the present study were set as either a substantial digital vasoconstriction (> 50%) or substantial arterial oxygen desaturation (> 4%), or a milder degree of vasoconstriction (> 30%) with concurrent pulse rate acceleration (> 10%) or subthreshold arterial oxygen desaturation (> 3%). Based on the actigraphic data, periods of sleep and wakefulness were identified, and the automatic algorithm RDI was calculated per hour of detected sleep. Although actigraphically determined sleep time is not as accurate as polysomnography, this feature is an improvement over other modified portable devices, which provide RDI values per total recording time rather than per the actual sleep time, which may lead to a biased RDI.
It should be noted that the population we studied did not include patients with central sleep apnea. We therefore assume that, at this stage, the current algorithm would not differentiate between obstructive and central apneas. However, given the very low prevalence of central apnea among the referred population to diagnostic sleep laboratories and the fact that the treatment of choice is usually the same, we do not consider this to be a major disadvantage. It should be also noted that sympathetic activation during sleep is not exclusively associated with the resumption of respiration after disordered breathing events, but may also arise in association with a variety of other conditions that may cause sleep arousals such as gross body movements, periodic leg movements, or changes in upper airway resistance. As outlined before, in order to maximize the specificity of the WP100 automatic algorithm to detect respiratory disturbance events, it was designed to be based not only on the presence of vasoconstriction episodes but it also takes into account their periodic nature, typical length, and combination with heart rate and oxygen desaturation changes. All these parameters were tuned in an optimizing process over a prior training set of results and were shown to give robust correlation with the polysomnography results in the validation set of the present study. Recently, we built a separate algorithm which utilizes the PAT signal to detect ASDA-defined arousals during sleep regardless of their source. This latter algorithm was based on a combination of two features of the PAT signal, amplitude attenuation and pulse rate increase. Validating the algorithm on a group of 96 patients that included normal subjects, patients with sleep apnea, and patients with periodic leg movements during sleep revealed a high correlation of 0.88 between manually scored ASDA-defined arousals and the automatically derived PAT autonomic arousal index. We believe that utilizing a combination of the features of the above-described algorithms to analyze a PAT record will allow us to distinguish between sympathetic activation due to respiratory events or due to other causes. Thus, in cases where the result of an RDI selective algorithm is consistent with or higher than that of the autonomic arousal algorithm, it can be reliably used to provide sleep apnea diagnosis. In cases where the PAT reveals a high arousal index but a low RDI, additional disorders such as periodic limb movements or upper airway resistance syndrome should be suspected and further explored. It is yet to be determined, however, if specific features of the digital response may be further analyzed to provide additional useful information about the underlying cause of the sympathetic activation.
A relatively high consistency was found between PRDI in the laboratory and at home, as well as between at-home studies, compared to previous studies that reported rather large night-to-night variability of RDI.19 21 It is possible that the consistency found in this study results from the relatively high incidence of patients with severe apnea re-studied at home. We conclude from these results that the WP100 showed a high level of reproducibility, although this may have been related to the specific population studied.
The ability of the PAT system to detect sleep-related respiratory disturbances could be affected by peripheral vasculopathy or neuropathy, and/or autonomic nervous system dysfunction, through the disruption of the normal response of the peripheral arteries to sympathetic activations. Nevertheless, the presence of such conditions should only be applied as exclusion criteria in extremely severe cases but not for example in the common diabetic patient. Obviously, patients who are treated with -blocker medications, those who underwent bilateral sympathectomy, and others with Raynaud disease or acrocyanosis are not candidates for the WP100 device. In patients with severe finger deformity (eg, rheumatoid arthritis), the adequate applying of the PAT probe could be a technical problem. In fact, none of the subjects in the current study were excluded on the basis of these conditions, and only two subjects were excluded because of treatment with -blocking agents. Therefore, the use of the device was not in practice limited by these pathologies, and the vast majority of the subjects with suspected OSAS would be eligible to use the WP100 device.
This study has several potential limitations. First, the study population consisted of patients with snoring/sleep apnea syndrome and healthy volunteers. One could argue that the accuracy of PAT in recognizing respiratory disturbances might be different in other patient populations (such as insomniacs). However, some of our patients complained of difficulties falling and maintaining sleep along with the hypersomnolence, and we did not notice a decreased accuracy of the WP100 in recognizing events in these individuals. Nevertheless, expanding this study to other patient populations will add more information regarding its applicability. In addition, we have evaluated the PAT for apneas and hypopneas but not for increased upper airway resistance syndrome. Respiratory effort related arousals are well recognized, and indeed it would be reasonable to be able to recognize them with this device, as these are also associated with sympathetic activation. However, this was beyond the scope of this study and is currently being investigated in a separate study.
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Conclusion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Footnotes |
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Drs. Bar, Dvir, Sheffy, and Schnall are employees of Itamar Medical Ltd. Dr. Pillar and Professor Lavie are consultants in that firm.
Supported in part by a grant from Itamar Medical Ltd., Caesarea, Israel.
Received for publication December 28, 2001. Accepted for publication September 25, 2002.
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References |
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adreno-receptor mediated constriction of the digital vascular bed [abstract]. Sleep 2001;24(Suppl),133R
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