Oximetry plethysmography also showed PP-like phenomena, which correspond to the BP measured PP events (Fig 4, top, A). A linear regression model describes a transfer function, which relates AT-PP in units of millimeters of Hg to a decrease in plethys-mographic amplitude (Fig 4, bottom, B). The slope of this relationship is roughly 0.01 V/mm Hg, where for each 1-mm Hg change in AT-PP, the oximeter plethysmograph peak amplitude would decrease by 0.01 V. The measurement of PP, embedded and AT in a continuous noninvasive BP recorder (ie, AT-PP), discriminated asthmatic adult patients who had been admitted to the hospital/relapsed from asthmatic adult patients who had been discharged from the hospital and was a well-tolerated procedure. The optimized AT-PP threshold for hospital admission was > 11.3 mm Hg following standardized treatment. in detail
This value is near 11 mm Hg, which discriminated patients who had been admitted to the hospital/relapsed from patients who had been discharged from the hospital in a pediatric acute asthma study population. Both observed thresholds also compare favorably to the first NAEPP asthma guidelines, which recommended hospital admission at a PP of 12 mm Hg. The subsequent NAEPP guidelines continued to recommend PP measurement but have dropped actionable PP thresholds. Initial AT-PP measurements prior to standardized treatment failed to predict disposition in the present study. By contrast, our previous pediatric study showed that the groups of patients who had been admitted to the hospital/relapsed and had been discharged from the hospital were identifiable before treatment began. This discrepancy may be a manifestation of the loss of reserve effort in respiratory dyscrasias in children. Presenting asthmatic children who had tired (but still generated a high PP) needed more time to convalesce and thus required hospital admission.
Sensitivity and specificity after standardized therapy in determining correct disposition were higher overall for the treating physicians than for the AT-PP measure, reconfirming the treating physician as a “gold standard” in asthma management studies. However, overlapping errors were limited to two patients, suggesting that their combination could be of clinical and economic value. There were five patients admitted to the hospital with normal AT-PP measures who were considered to have been unnecessarily admitted to the hospital on a subsequent medical record audit, and there were two released patients who relapsed and were determined to have had high AT-PP values.
Figure 4. Top, A: representative PP data from a BP monitor (FINAPRES; Ohmeda) and oximetry plethysmograph that were recorded simultaneously. Arrows indicate maxima and minima SBP values induced by a —20-mm Hg inspiratory pressure, which was identified by the PP algorithm. Arrowheads denote corresponding plethysmograph waveforms, which also indicate the presence of PP. In both panels A and B, PP was induced in a healthy subject by inspiration through a fixed resistance, while mouth pressure was monitored. Bottom, B: correlation of variable degrees of induced PP measured by a BP monitor, with changes in the plethysmographic waveforms from an oximeter shown (O). NIBP = noninvasive BP monitoring.