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(Chest. 2000;118:348-352.)
© 2000 American College of Chest Physicians

Analysis of Physician Ability in the Measurement of Pulsus Paradoxus by Sphygmomanometry*

Gregory D. Jay, MD, PhD; Kazuya Onuma, MD; Robert Davis, MD; Mei-Hsui Chen, MS; Anthony Mansell, MD and Dale Steele, MD

* From the Departments of Medicine (Drs. Jay and Davis) and Pediatrics (Drs. Mansell and Steele), Brown University School of Medicine, Providence, RI; the Center for Statistical Sciences (Mr. Chen), Brown University, Providence, RI; and the Department of Molecular Medicine (Dr. Onuma), Tohoku University, Japan.

Correspondence to: Gregory D. Jay, MD, PhD, Department of Emergency Medicine, Rhode Island Hospital, 593 Eddy St, Providence, RI 02903; e-mail: gregory_jay_MD{at}brown.edu


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Context: Measurement of pulsus paradoxus (PP) is one of several measures previously advocated in the National Heart, Lung, and Blood Institute asthma management guidelines: a pulsus of > 12 mm Hg warranted hospital admission. It is one of only a few measures that is not effort dependent and therefore important in the evaluation of patients with asthma.

Objective: Determination of physician accuracy in measuring PP.

Design: A model of induced PP in a trained healthy subject without respiratory disease was constructed with a fixed inspiratory resistance with measurement of inspiratory air pressure and beat-to-beat BP noninvasively.

Setting: Laboratory.

Participants: Attending physicians from emergency medicine and critical care disciplines who served as consecutive examiners of the trained reference subject generating known PP.

Interventions: A total of 19 attending physicians were assessed for ability in measuring PP by sphygmomanometry and by palpation. The reference subject generated 4° of PP sequentially, with each examiner blinded to the value of negative inspiratory pressure and PP. Examiners first assessed PP qualitatively by palpation, followed by its measurement within 2 min.

Main outcome measure: Proximity of physician-measured PP (PPm) to true PP (PPt).

Results: At inspiratory pressures of - 10, - 15, - 20, and - 25 mm Hg, PPt was 13.7, 16.2, 19.1, and 20.7 mm Hg, respectively (F = 14.8, p < 0.0001; analysis of variance [ANOVA]). At the same pressures, PPm was 13.1, 17.5, 17.7, and 18.0 mm Hg (p > 0.10; ANOVA). Linear regression of PPm against PPt for each examiner revealed a slope (SE) of 0.53 (0.23), and not a 1:1 relationship.

Conclusions: Past and present guidelines do not account for the challenges in measuring PP, especially in tachypneic patients. Sphygmomanometric determination of PP should be augmented by new aids developed through technological innovation.

Key Words: asthma • BP • physical examination • pulsus paradoxus


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Inspiratory decline of the arterial pulse was first described in asthma by Floyer in 1717.1 The exaggerated decrease in BP was first termed pulsus paradoxus (PP) by Kussmaul in 1873.2 PP is defined as an inspiratory fall in systolic arterial BP >=10 mm Hg. This phenomenon is observed in many clinical conditions, including severe asthma,3 4 5 tension pneumothorax,6 cardiac tamponade,7 COPD,8 croup,9 and massive pulmonary embolism.10 PP has been researched as an objective measure of severity in asthma.11 12 It is considered an index of airway obstruction that has been recommended by authoritative practice guidelines.13 14 A clear advantage of PP as an asthma severity index is that PP is a noneffort-dependent measure, in contrast to peak expiratory flow rate and FEV1. The manual measurement of PP with a sphygmomanometer is technically demanding. Neither the interobserver variability of PP determination by sphygmomanometry15 nor its inherent accuracy has been researched. This is a relevant flaw in the medical literature, since the initial National Heart, Lung, and Blood Institute (NHLBI) guidelines for the diagnosis and management of asthma specified that patients with a PP > 12 mm Hg deserve hospital admission.13 Present guidelines14 recommend PP measurement and do not identify PP thresholds.

In this study, we determined if attending physicians can accurately measure PP in an idealized setting both quantitatively (by sphygmomanometry) and qualitatively (by palpation). The examiners were from emergency medicine and critical care disciplines, and served as research subjects. The noninvasive finger BP monitor (FINAPRES; Ohmeda; Englewood, CO) provided the criterion standard for comparison. This device has proven to be a clinically accurate and precise method for tracking changes in BP.16


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Study Design
Physicians’ ability to accurately measure experimentally induced PP was assessed in a single reference subject. Attending-level physicians from emergency and critical care disciplines served as examiners who were self-selected based on subjective ability to measure PP. The study was conducted in a quiet laboratory setting, and the examiners were blinded to degree of PP. The reference subject (R.D.) was trained to generate incremental and predetermined degrees of PP by self-measured inspiratory pressure.

Subject and Technique for Induction of PP
PP was induced in a healthy adult by his breathing through a fixed resistance connected to a two-way nonrebreathing valve (Hans Rudolph; Kansas City, MO) attached to a manometer (OEM Medical; Marshalltown, IA). Airflow resistance occurred during inspiration, whereas expiration was unimpeded. The reference subject’s BP and respiratory plethysmograph were recorded in the sitting position while he sequentially generated inspiratory mouth pressures from - 10 to - 25 mm Hg in -5 mm Hg increments. The reference subject controlled the generated mouth pressures by observing continuous manometer readings. Inspiration was initiated every 5 s (12 breaths/min) and accompanied by auditory cues indicating inspiration, thus allowing the examiners to focus on the ergonomic demands of measuring PP with a syphgmomanometer. The examiner could hear these cues despite the presence of a stethoscope. The reference subject was unable to randomize the degrees of PP that were instead incrementally increased. Examiners were blinded to this information.

Noninvasive BP Measurements
The FINAPRES device was used to noninvasively acquire a continuous BP reading. This device approximates invasive arterial BP monitoring and has been validated in previous study.17 Data from the FINAPRES was digitized by an analog-to-digital converter (model MP100; Biopac Systems; Santa Barbara, CA) connected to a computer (Macintosh Power PC 6100; Macintosh; Cupertino, CA).

Determination of Respiratory Phase
Chest wall movements were recorded using a piezoelectric respiration transducer (Crystal Trace; Pro-Tech Service; Woodenville, WA) with a stretchable hook-and-loop-fastener strap placed around the chest at the nipple level. A differential amplifier (model DA100A; Biopac Systems) was used for signal amplification. The amplified respiratory signal was digitized by the same analog-to-digital converter used above.

PP Measurement by Physicians
While the BP was monitored on the left middle finger of the reference subject by the FINAPRES device, an examiner measured BP with a syphgmomanometer at the right brachial artery for induced PP at each inspiratory pressure. Examiners first determined if any PP was present via palpation with a yes/no format, followed by PP estimation via syphgmomanometry within 2 min for each of four induced values of PP in separate sessions. Examiners compared observed systolic pressure between inspiration and expiration, and subtracted the two values. Examiners were told to identify a PP value that appeared reproducible after at least three attempts. This was defined as PP by us to all examiners prior to measurement. Examiners used the same syphgmomanometer and were allowed to familiarize themselves with its operation. The examiners were queried in regards to their knowledge of PP and their ability to accurately measure PP. Prior to data collection, both the FINAPRES device and syphgmomanometer were used to confirm that no difference in systolic BP existed between extremities.

True PP Determination
PP was defined as the lowest inspiratory systolic BP subtracted from the peak expiratory systolic pressure. Mean and SD of continuous PP measurements in 2 min were calculated for each induced PP for each examiner.

Statistical Analysis
Data of induced PP were classified by inspiratory pressure and evaluated by box plot analysis and analysis of variance (ANOVA). Under the assumption that the repeated measures within each examiner were exchangeable, separate analyses of repeated measure data were done by linear regression and logistic regression for the examiners’ value of measured PP (PPm) and the qualitative assessment of PP (palpation). A probability of palpation was defined as the number of correct identifications of PP by palpation divided by the total number of examiners for a given true PP (PPt). The value of PPt served as the predictive variable.

Computations were done using the statistical software packages (SAS; SAS Institute; Cary, NC; and S-PLUS; MathSoft; Cambridge, MA). The subroutine within SAS was PROC MIXED. Power analysis revealed that a minimum of 18 examiners were needed in order to achieve 80% power in determining the relationship between PPm and PPt.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
A total of 19 examiners were used in this study; all were board certified in their respective disciplines of emergency medicine (n = 12), pediatric emergency medicine (n = 5), and pulmonary medicine/intensivists (n = 2). The combined level of experience for examiners was 9.2 ± 7.4 SD years since the end of postgraduate medical training, with two of the authors (D.S. and A.M.) serving as examiners. Across all examiners, the test subject generated values (mean ± SD) of PPt as follows: 13.7 ± 2.2, 16.2 ± 2.0, 19.1 ± 2.4, and 20.7 ± 2.8 mm Hg (F = 14.83; p < 0.0001; ANOVA) for inspiratory pressures of - 10, - 15, - 20, and - 25 mm Hg, respectively (Fig 1 ). For the same inspiratory pressures, PPm values were 13.1 ± 6.5, 17.5 ± 9.6, 17.7 ± 9.9, and 18.0 ± 11.3 mm Hg (p > 0.10; ANOVA; Fig 1 and Table 1 ). The results for t tests with Bonferroni correction across the inspiratory pressures indicate that p < 0.01 for PPt (for every test) and p > 0.10 for PPm (for every test). This indicates that as inspiratory pressure became more negative, PPt increased, but PPm is inconclusive. Subgroup analyses by specialty training did not demonstrate differing results (data not shown).



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  		Figure 1.. Box plot analysis of PPm and PPt against increasing inspiratory pressure in the reference subject for all examiners.

 

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  		Table 1.. Summary of PPm and PPt as Inspiratory Pressure Decreased*

 
In order to evaluate whether PPm is equivalent to PPt at all levels of inspiratory pressure, we used PPt as the predictive variable in a linear regression analysis for repeated-measure PPm. We found a slope (SE) of 0.53 (0.23) with intercept (SE) of 7.37 (4.40; Fig 2 ). The box plots of slope and intercept revealed a wide variation among examiners (Fig 2 , inset). Hence, we reject the hypothesis that a slope of 1.0 existed between PPm and PPt. Alternatively, by combining the data across all examiners and viewing PPm as independent data points, subsequent linear regression revealed a slope of 0.4 (data not shown).



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  		Figure 2.. Linear regression of PPm against PPt showing the relationship that best fits all examiners combined (solid line) in contrast to the idealized relationship between PPm and PPt (dotted line). Inset: Box plot analysis of regression line slope and y intercept for all examiners.

 
The results of a logistic regression analysis of palpation for PP is shown in Figure 3 , with slope (SE) of 0.10 (0.06). The hypothesis that an increase in PPt will improve the ability of palpation is rejected. Examiners inconsistently palpated the higher degrees of PPt, despite correctly doing so at lower values.



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  		Figure 3.. Regression of probability of palpation against PPt superimposed on the raw data for all examiners. The relationship (solid line) that best fits all examiners is not significantly different from zero.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
A limitation of our study is that the examining physicians did not practice their skill in determining PP prior to measurement. However, asking emergency physicians and critical care specialists who feel comfortable measuring PP to do so in an ad hoc manner is in keeping with clinical reality. Physicians are unable to "practice" this element of physical examination unless provided with some objective criterion that presently does not exist. There were no attempted measurements of physiologic PP of 2 to 5 mm Hg to determine what, if any, false-positive measurement occurred across the examiners. This was necessarily omitted, since the examiners would have recognized that one of the degrees of PP did not require exertion. This would have potentially biased the examiners. It is also possible that the palpation of PP may have biased the subsequent sphygmomanometric measurement of PP. Finally, values of induced PPt were not constant during each of the 2-min measurement sessions, and we are not comparing beat-to-beat PPt and PPm. Measuring PPm requires several cardiac cycles, and can only therefore be compared to an average of PPt. Fortunately, the variability of the reference subject was not large, and PPt significantly differed across the inspired pressures. The FINAPRES device has been reported to overestimate finger systolic pressure,18 but since two such measures are being subtracted, this error would not be a factor.

The present results indicate that measuring PP under idealized conditions, which included quiet surroundings, slow respiratory rate, and auditory cues of respiratory phase, may be problematic. Physicians have already recognized that PP determination by sphygmomanometry is technically demanding and time consuming. However, the literature does not contain a systematic study of physicians’ ability to measure PP accurately. Assessing for PP is a useful adjunct to physical examination among patients with asthma or in consideration of life-threatening processes, such as cardiac tamponade. Currently, there are no aids in determining PP quantitatively and noninvasively.

The results are both relevant and timely. The NHLBI has revised expert guidelines for the evaluation of patients presenting with acute asthma.14 The recommendation by the expert panel of PP as a severity measure is based on a solid body of evidence3 5 11 that correlates the physical sign with indices of expiratory flow obstruction, such as FEV1. However, the major mechanical consequence of acute asthma is now thought to be increased inspiratory muscle elastic loading as a result of dynamic lung hyperinflation.19 20 PP is a measure of inspiratory impedance as it affects pleural pressure, left ventricular stroke volume, and right ventricular output.21 Therefore, measurement of PP is pertinent to respiratory distress,22 inspiratory muscle fatigue, air leak, and other mechanical consequences of dynamic hyperinflation during acute asthma.

The present findings are perhaps most germane to the assessment of childhood asthma. Children < 5 years old are rarely able to perform peak expiratory flow rate and spirometry. PP, as an index of severity that does not require active cooperation, could be particularly applicable to acute asthma in infants and children. Unfortunately, high respiratory rates (sometimes > 60 breaths/min) in these settings are likely to confound measurements of PP further, producing even more variability than we found at a respiratory rate of 12 breaths/min. Previous NHLBI guidelines recommended patient admission for PP >= 12 mm Hg. The validity of this threshold was previously investigated among children presenting with acute asthma and treated by pediatric emergency medicine specialists blinded to all severity measures.12 Admitted and successfully discharged patients presented to the emergency department with a PP > 11 mm Hg and < 11 mm Hg, respectively. Despite the fact that the NHLBI guideline PP admission threshold was not validated in an outpatient setting and relies on manual PP determination, 12 mm Hg is a good approximation to 11 mm Hg,12 a value arrived at using the arterial plethysmographic techniques illustrated in the present report.

The present results indicate that PP measured qualitatively by palpation may also be unreliable. A number of the physicians examined detected a palpable PP near the mean PPt of 16.2 mm Hg, but failed to measure its amplitude successfully by BP cuff. These same physicians were inconsistent in detecting PP as PPt increased. Pulse oximeters equipped with a visual plethysmographic display could be used as aids23 24 to confirm the presence of a palpable PP. This commonly available equipment is perhaps underutilized.

Assessment for PP is part of a thorough physical examination and should not necessarily be abandoned. Traditional teaching in US medical schools and some postgraduate training programs includes the expectation that physicians have the ability to measure PP with a BP cuff. Perhaps the key to this test is not to be physiologically precise, but rather to detect PP of sufficient magnitude that would influence some type of clinical decision making. The physician examiners in this study demonstrated a trend in accurately detecting PP qualitatively by palpation as PPt increased. These data and the subjective clinical experiences of the investigators (which inspired the study) suggest that simply detecting the presence of a paradoxical pulse by palpation could be more clinically meaningful than a quantitative measure that is likely erroneous. We also recommend that patients suspected of having a paradoxical pulse be studied by other noninvasive means, such as Doppler echocardiography, which may reveal variations between mitral and tricuspid flow. Appropriate future studies could be the determination of the receiver operator curve characteristic in palpating for an action level of PPt or the minimum PP detectable by optical plethysmography. Possible limitations not accounted for in the study design include our determination of PPt and PPm on different extremities. No difference in systolic pressure between extremities was confirmed at baseline, but whether a difference in PP exists between extremities is unknown.


   Acknowledgements
 
We wish to thank Constantine Gatsonis, PhD, for his input.


   Footnotes
 
Abbreviations: ANOVA = analysis of variance; NHLBI = National Heart, Lung, and Blood Institute; PP = pulsus paradoxus; PPm = measured PP; PPt = true PP

Received for publication September 13, 1999. Accepted for publication February 18, 2000.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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  2. Kussmaul, A (1873) Uber schwielige mediatino-pericarditis und den paradoxen puls. Berl Klin Wochenschr 10,433-435
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J. Rayner, F. Trespalacios, J. Machan, V. Potluri, G. Brown, L. M. Quattrucci, and G. D. Jay
Continuous noninvasive measurement of pulsus paradoxus complements medical decision making in assessment of acute asthma severity.
Chest, September 1, 2006; 130(3): 754 - 765.
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