Continuous Noninvasive Measurement of Pulsus Paradoxus Complements Medical Decision Making in Assessment of Acute Asthma Severity: Induced PP in Healthy Volunteer
PP was induced in a healthy adult using an established technique by having him breathe through a fixed resistance connected to a two-way nonrebreathing valve (Hans Rudolph; Kansas City, MO) that was attached to a manometer (OEM Medical; Marshalltown, IA). Airflow resistance occurred during inspiration, whereas expiration was unimpeded. The reference subject’s BP and oximetry plethysmograph findings were recorded continuously in the sitting position while he sequentially generated inspiratory mouth pressures from — 5 to — 20 mm Hg in 5-mm Hg increments. The subject controlled the generated mouth pressures by observing manometer readings. The respiratory rate was 20 breaths/min. Continuous BP was recorded noninvasively (FINAPRES; Ohmeda; Madison, WI). This device approximates invasive arterial BP monitoring as well as the mounted arterial tonometer (NCAT; Nellcor) and has been used previously by our group and others.
Data from the BP monitoring device (FINAPRES; Ohmeda) was digitized by an analog-to-digital converter (MP-100; Biopac Systems; Santa Barbara, CA), which created a text file that could be analyzed by the AT-PP monitoring algorithm described previously. Pulse plethysmography findings were obtained from a pulse oximeter (model 395; Nellcor) that was specially configured to separately record plethysmograph signals from the visible red and infrared photodiodes. Data transfer from the oximeter was accomplished digitally in real time through its analog signal output. This aspect of the study was also approved by the institutional review board. Data from continuous BP monitoring of the study subject were analyzed for PP by the dedicated AT-PP algorithm previously illustrated. Text files from the oximeter plethysmograph were analyzed (MP-100 software; Biopac Systems). A change in inspiratory and expiratory plethysmographic pulse amplitude caused by PP was calculated for at least 10 respirations in each induced PP data file, and the mean ± SD was calculated. Correlation of the percentage change in plethysmograph amplitude against the AT-PP for the same respirations was performed, and a linear regression model was constructed across the increasing degrees of negative inspiratory pressure and AT-PP.