Elsevier

Heart & Lung

Volume 30, Issue 1, January–February 2001, Pages 26-38
Heart & Lung

Issues in Cardiac Care
Comparison of pulmonary artery and central venous pressure waveform measurements via digital and graphic measurement methods☆,☆☆

https://doi.org/10.1067/mhl.2001.112504Get rights and content

Abstract

Background: Techniques to measure pulmonary artery (PA) pressure waveforms include digital measurement, graphic measurement, and freeze-cursor measurement. Previous studies reported the inaccuracy of digital and freeze-cursor measurements. However, many of the previous studies were small and did not thoroughly examine the circumstances of when digital measurements might be inaccurate. Objectives: To compare digital measurements and graphic measurements of PA and central venous pressure (CVP) waveforms in patients with a variety of respiratory patterns, and to compare digital measurements and graphic measurements of CVPs in patients with abnormal or right ventricular waveforms. Methods: A total of 928 patients were enrolled in this study. Waveforms from the PA and CVP were collected from each patient. The monitor pressure value (digital measurement) printed on the recorded waveform was compared with the pressure value obtained by a graphic strip recording and measured by one of the primary investigators (graphic measurement). Results: Digital measurements were found to be inaccurate in measuring waveforms in all respiratory categories and in measuring right ventricular waveforms. PA diastolic values and CVP values were the most inaccurately measured waveforms. Digital errors of more than 4 mm Hg were common. Conclusion: There were instances in which the monitor’s digital measurement was substantially different from the graphically measured value. This difference has the potential to mislead interpretation of clinical situations. The monitor’s ability to occasionally give digital measurement values similar to the graphic measurements may lead to a false sense of security in clinicians. Because the accuracy of the monitor is inconsistent, the bedside clinician should interpret waveforms through use of a graphic recording rather than rely on the digital measurement on the monitor. (Heart Lung® 2001;30:26-38.)

Section snippets

Current issues in measuring pressure values

The bedside clinician, (eg, nurse) can obtain pressure measurements through 1 of 3 Methods: (1) reading waveform values directly from the monitor (digital measurement); (2) measuring the waveform directly from a printed copy of the wave (graphic measurement); and (3) using the freeze-cursor function on the bedside monitor. Based on the research that suggests that digital measurements may be inaccurate, the best options are the second and third methods.

Bedside monitor programs use filtering

Study questions

To consider the issue of the accuracy of bedside monitoring, the following questions are addressed.

  • 1.

    What are the differences between those PA and CVP pressure waveform measurements obtained through digital measurement and those obtained through graphic measurement?

  • 2.

    Does respiratory rate or the mode of mechanical ventilation affect the accuracy of PA and CVP waveform measurements obtained through digital or graphic measurements?

  • 3.

    Are there differences between the CVP waveform measurements obtained

Methods

The study was conducted at 3 community and 2 university hospitals between 1995 and 1998. The Human Subjects Committee at Washington University approved an expedited review of the study. Data collection involved obtaining a monitor strip from any patient with a PA catheter in place. Neither subjects’ names nor any other identifying characteristics were collected as part of the study.

Sample description

During the measurement period, 928 patients were enrolled in the study and 965 measurements were obtained. All patients were from adult critical care units with surgical, cardiac-surgical, cardiac-medical, or general-medical admitting diagnoses. The total sample size and the sample size of each of the respiratory categories for each manufacturer are presented in Table I.

. Total sample size

CompanyTotal sample sizeSB < 20 breaths/minSB > 20/minVB onlyVB + SB < 20/minVB + SB > 20/minAbnormal and RV

Discussion

The answers to the 3 research questions are consistent with previous research that indicates that digital display measurements have limited accuracy. In several circumstances, the differences between digital and graphic measurement were of clinical importance. For at least 1 value (PAS, PAD, or CVP) in each respiratory category, a statistically significant difference between the digital measurement and graphic measurement values was present (Table II). The Wilcoxon matched pairs signed-rank

Conclusion

Use of graphic recordings as the gold standard must continue despite improvements in digital engineering. Clearly, errors occur with all monitors’ digital measurements in all respiratory and abnormal waveform patterns. Monitor program design excludes correct measurement of waveforms such as RV or abnormal venous waves. Some of the digital measurement errors are not clinically important. However, when the errors are clinically important, the only protection for the patient is the bedside

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☆

Supported by the St Louis chapter of AACN, Marquette Electronics, and Mennen Medical.

☆☆

Reprint requests: Thomas S. Ahrens, DNS, RN, CCRN, CS, Nursing Service Barnes-Jewish Hospital, One Barnes Hospital Plaza, St Louis, MO 63110.

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