Continuous Noninvasive Measurement of Pulsus Paradoxus Complements Medical Decision Making in Assessment of Acute Asthma Severity: Comment

Continuous Noninvasive Measurement of Pulsus Paradoxus Complements Medical Decision Making in Assessment of Acute Asthma Severity: CommentThe discordance between the observed and derived respiratory rate is the focus of continued investigation. Lack of a “gold standard” in this aspect of the study limits further data analysis. Our research assistants were instructed to count the respiratory rate over 30 s and multiply by 2. This method may not have been sufficient, and direct observation for 1 min may be necessary. Respiratory rates collected by clinicians in triage and monitoring systems are frequently in error as suggested by direct observation. The phenomenon of respiratory rate collected by a pulse oximeter has been reported on previously. A PP monitor, if commercialized, could also display respiratory rate since the rate of the counted maxima and minima SBP peaks is equivalent to the respiratory rate. Rejected SBP peaks plagued by noise could still be counted, ensuring respiratory rate accuracy.
It is well-known that respirophasic changes in BP and perfusion are detectable in both pulse oximeters and plethysmographs. These devices are affected by the vagaries of perfusion, which are influenced by local peripheral extremity factors such as ambient temperature. Despite this, there are numerous reports of detecting PP qualitatively by pulse oximetry and of detecting respiratory obstruction quantitatively by pulse oximetry. Identifying the signature of PP in the presence of other confounding signals becomes an important task and is technically challenging. Pulse volume changes induced by respiratory events that are measured as the AUC may correlate with varying respiratory impedance. However, this study was limited by the lack of an external PP measure. Respiratory events generally follow a predictable pattern in relation to heart rate, which would help to negate interfering perfusion signals induced by local vascular events. Changes in plethysmographic volume, which reflect PP, can be quantitated by either a change in peak amplitude or area-under-the-curve calculations. Signal-processing schemes will require a look-up table correlating respirophasic pulse volume changes with known degrees of PP, measured in units of millimeters of Hg. In the present study, another commercially available plethysmograph was used, which demonstrated results that were similar to those in Figure 4 (data not shown). The scaling of the analog output of other instruments may vary, leading us to predict that plethysmograph amplitude varies by 1% for each millimeter of Hg of PP.

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