Original Contribution

Noninvasive continuous or intermittent Blood pressure and heart rate patient monitoring in the ED^?,??,?

Richard M. Nowak MD ^a,?, Ayan Sen MD ^a, Audwin J. Garcia MD ^a, Heidi Wilkie^a,

James J. Yang PhD ^b, Michael R. Nowak ^a, Michele L. Moyer BSN ^a

^aDepartment of Emergency Medicine, Henry Ford Health, System, Detroit, MI 48202, USA

^bDepartment of Epidemiology and Statistics, Henry Ford Health, System, Detroit, MI 48202, USA

Received 17 January 2011; revised 11 May 2011; accepted 11 May 2011

Abstract

Objective: Continuous invasive Blood pressure and heart rate (HR) monitoring in the emergency department (ED) is valuable in managing critically ill patients. Novel noninvasive finger cuff technology allows this same uninterrupted monitoring for almost any individual. This exploratory study compares ED noninvasive continuous to intermittent measurements of these variables.

Methods: A variety of acutely ill ED patients (n = 40) with broad ranges of BP and HR underwent simultaneous monitoring using interval standard automated ED devices and continuous finger cuff technology (Nexfin; Bmeye, Amsterdam, The Netherlands) over a period of 2 hours. At baseline and at 15-minute intervals, simultaneous measurements for BP and HR were recorded and compared.

Results: There were 25 men and 15 women enrolled with a mean age of 62.2 years (SD, 12.6). Eighteen had acute dyspnea, 11 with probable stroke syndrome, 3 with suspected sepsis, and 8 with a systolic BP greater than 180 or less than 100 mm Hg. Blood pressure and HR values (n = 344) simultaneously measured by automated ED equipment and the Nexfin finger cuff device were compared. The Pearson Correlation coefficients were high, and the Bland-Altman plots showed good agreement between the 2 determinations.

Conclusion: Continuous BP and HR monitoring measured by the Nexfin finger cuff device in this trial showed reasonable agreement when compared with the intermittent values obtained by automated ED equipment. However, theoretically, noninvasive and continuous monitoring of the BP and HR might better reflect underlying hemodynamics than these same measurements obtained intermittently and, thus, could be important in patient management. More study is needed to determine the optimal method of monitoring these parameters.

(C) 2011

^? This study was funded in part by a $5000 nonrestricted grant from

Bmeye (Amsterdam, The Netherlands). All of the listed authors have no

conflicts of interest to disclose.

^?? This trial was registered on the Clinicaltrials.gov Website with the Identifier: NCT00851214. It was first registered on February 24, 2009 and

closed out on February 17, 2010.

^? Results of the study were presented at the 2009 ACEP and 2010 SAEM meetings.

* Corresponding author.

E-mail address: [email protected] (R.M. Nowak).

Introduction

Feeling the pulse and determining heart rate (HR) has been done for centuries. The clinical usefulness of monitoring the blood pressure (BP) using the Riva-Rocci sphygmomanometer was initially described in 1903 by Dr Harvey Cushing in patients undergoing surgical procedures under general anesthesia [1]. Since then, the clinical value of

0735-6757/$ - see front matter (C) 2011 doi:10.1016/j.ajem.2011.05.014

this monitoring has been widely recognized; and methods using intra-arterial access have been developed so that continuous evaluation of these Hemodynamic variables is available for use in the critically ill patients [2,3]. Automated devices (using mostly oscillometric methods, BP cuffs, and attached electrocardiographic [ECG] electrodes) have been more recently developed to measure brachial BP and HR and are widely used in the monitoring of many ill or injured ED patients. However, these devices only measure the BP intermittently (ranging from every minute to once every 120 minutes); and there is no real-time visualization of the actual pulse waveform.

It may be important for the ED clinician to be able to noninvasively and continuously monitor the BP and HR on a beat-to-beat basis in some patients that are being assessed and treated. Currently, there are few options available for this and include bioreactance (eg, NICOM, Cheetah Medical, Portland, Oregon) [4,5], impedance cardiography [6,7], and the use of novel finger cuff technology (Nexfin; Bmeye, Amsterdam, The Netherlands). The Nexfin device is currently cleared for hemodynamic monitoring in adults by both the Food and Drug Administration and the European Union. It has been compared with Brachial cuff and Arterial line measurements in other medical arenas and has been shown to be consistently accurate within a 10% range [8-10]. When the Nexfin device has been compared with the BP and HR measurements obtained by intra-arterial monitoring in children undergoing cardiac surgery, the correlations be- tween the results have been even higher [11]. However, its use has never been compared with the standard BP and HR values obtained using the automated brachial cuff devices and precordial electrode-based monitoring systEMS used in most EDs within the United States. Emergency department patients can experience wide fluctuations in these hemody- namic parameters (with or without therapy); and so, a comparison of noninvasive continuous to intermittent BP and HR monitoring is warranted.

This study compares the values obtained by the standard intermittent automated ED equipment for systolic BP (SBP), Diastolic BP (DBP), mean arterial BP (MAP), and HR to those simultaneously measured by the Nexfin finger cuff device over a 2-hour period in a variety of acutely ill patients presenting to the ED. Scatter (Pearson correlation co- efficients) and Bland-Altman plots are used to show the comparisons between these concurrent measurements.

Methods

The Nexfin device

The Nexfin determines finger arterial BP with the volume clamp method as proposed by Penaz [12]. An optical plethysmograph mounted in an Inflatable cuff measures arterial blood volume, and the cuff pressure is controlled so

that arterial blood volume is kept constant. This can be accomplished only by a cuff pressure counter pulsating arterial pressure throughout the cardiac cycle, that is, cuff pressure equals arterial pressure. To calibrate the measure- ments, the arterial walls should be “unloaded” so that transmural pressure is zero. The unloaded state can be determined from the physical criteria as described by Wesseling et al [13]. The calibrated finger arterial BP wave shape is transformed into a Brachial artery pressure wave shape using a generalized transfer function [14] and a level correction method compensating for the pressure drop in the peripheral arteries [15]. The volume clamp method obviously needs adequate blood flow present in the fingers. When the peripheral finger arteries are severely constricted, measuring may become difficult or even impossible. In this case, Nexfin issues a warning when no plethysmogram is detected.

The position of the hand with respect to the heart is measured with the heart reference system (HRS); and thus, Nexfin readings are automatically corrected for any hydrostatic pressure difference. These methods together assure that the displayed BP levels are comparable with brachial artery pressures.

Study design

This was a prospective, observational, exploratory, convenience study of adult (N18 years old) acutely ill patients presenting to the main campus ED at Henry Ford Hospital in Detroit, MI, and who were triaged to the resuscitation room or as a level 1 severity of illness. The trial was approved by the Henry Ford Hospital Institutional Review Board, and informed consent was obtained from all patients before enrollment. Patient inclusion criteria included those with acute shortness of breath thought to be caused by exacerbation of congestive heart failure (CHF) and/or chronic obstructive lung disease (COPD), probable acute stroke syndromes, and suspicion of sepsis and patients with a presenting abnormal BP (SBP N180 or b100 mm Hg). As the intent was to gather hemodynamic information as soon as possible after ED presentation, the initial Clinical impression of the treating physician was used to evaluate patients for possible study enrollment. The final physician diagnosis, after the patient completed their evaluation and treatment, was recorded and often differed from the initial impression. Patients were excluded if they were in cardiopulmonary arrest, documented to have a ST-segment elevation myocar- dial infarction (as these individuals are immediately transferred to the cardiac catheterization laboratory), were pregnant, or if they exhibited excessive agitation that might make the study monitoring difficult to accomplish.

Methods of measurements

A clinical research assistant, trained in the use of both technologies, applied a standard wall mounted automated

Fig. 1 One of the investigators (RMN) demonstrating how patients were simultaneously monitored by the 2 devices. On his right is the Nexfin machine and monitoring screen, the wrist and finger cuff apparatus, and the HRS attached to his clothing at the level of the brachial artery. On his left is a standard brachial BP cuff attached to an automated BP device and the wall-mounted monitoring screen. Lastly, there are 5 electrodes attached to the chest for continuous ECG and HR monitoring displayed on the wall-mounted screen.

brachial cuff device to one of the upper extremities. In addition, 5 electrodes were attached to the patient for lead 2 cardiac monitoring. The devices used included a Hewlett Packard, Santa Clara, CA, USA or Agilent-PT Care Systems NBP module M1008B, Palo Alto, CA, USA or a Philips IntelliVue-MP50 NBP module PNM 8003A, Andover, MA, USA hooked up to a wall-mounted monitor screen and an appropriately sized Philips, DataScope, or Drager brachial BP cuff. Subsequently, the right size finger cuff (small, medium, or large) was placed on the opposite extremity second, third, or fourth digit (the cuff could be changed to an alternate finger on the same hand if the patient felt significant digital discomfort during the study); and this was connected to the Nexfin device. The HRS was attached to the patient’s gown at the level of the brachial artery to ensure that the comparison BP values were both measured at the same arterial level even if the patient moved his/her hand in an upward or downward motion (Fig. 1).

At baseline and every 15 minutes, for a total of 2 hours, the SBP, DBP, MAP, and HR were simultaneously measured by the standard ED automated system and the Nexfin device. The standard automated BP system requires the evaluation of a very variable number of individual cardiac contractions (up to 56 beats in this study) to make a single BP determination (the reported SBP and DBP are recorded from different beats). The Nexfin device, however, measures BP on a beat- to-beat basis and reports an average numerical reading over 5 cardiac contractions, which is updated on its screen every

2 seconds. The continuous BP recording is displayed as an arterial waveform similar to that seen when using invasive arterial monitoring (Fig. 2). The research assistant entered a marker in the Nexfin recording when the automated standard system was initiated and also when it first reported a numerical BP value. All of the individual beat-to-beat Nexfin BP measurements over this same period were averaged by the device, and these averages are the values that are reported as the comparison BP. The investigators felt that this was the optimal way to compare the BP values obtained by both systems. The pulse rates were simultaneously recorded at the 15-minute time intervals both from the lead 2 ECG monitor and the averaged HR measured by Nexfin over the time that it took the automated ED device to measure a BP.

The results of the Nexfin monitoring were blinded to the treating ED nurses and physicians, and patients were managed as clinically indicated. All subjects were encour- aged to indicate to the clinical research assistant if they experienced any discomfort with the use of either system.

Statistical analysis

Pearson product-moment correlation coefficients were used to measure the correlation between the BP and HR measurements of the standard ED automated device and those obtained by the Nexfin. To visually evaluate the consistency between the 2 methods, we created scatter and Bland-Altman plots for the SBP, DBP, MAP, and HR values.

Fig. 2 The Nexfin screen displaying the noninvasively obtained continuously derived brachial BP tracings and other calculated hemodynamic variables.

For the Bland-Altman plots, the differences in measurements were defined as the standard automated ED values minus those of the Nexfin device. Each Bland-Altman plot mean value and the 95% range of the differences were also calculated and shown.

Table 2 The final ED diagnosis of study patients

Results

A total of 40 patients were enrolled in the trial over a 4- month period in 2009. There was a single trained research

Table 1 Overview of the HR and BP variables measured by each device

assistant assigned to this study on a part-time basis resulting in the small number of patients enrolled. No patient was excluded because of the inability of either device to measure the BP and pulse. There were 25 men and 15 women, and the

Final ED diagnosis	n
Atrial fibrillation with rapid ventricular response	1
Acute myocardial infarction	1
acute asthma	1
Bacteremia	2
Acute Bell palsy	1
Acute CHF	6
acute cholecystitis	1
Exacerbation of COPD	4
Musculoskeletal chest pain	1
Dehydration	1
Hypertensive emergency	1
Hypertensive urgency	3
Hypotension, not specified	1
Acute hypoxia, not specified	1
acute intracranial hemorrhage	1
Pneumonia	2
Acute renal failure	1
Acute subarachnoid hemorrhage	1
Acute stroke	5
Syncope	1
gait disturbance, not specified	1
Acute weakness, not specified	3

Variable	N	Mean	SD	Median	Minimum	Maximum
DBP	344	77.19	22.94	73.00	29.00	151.00
DBP	344	78.43	19.97	73.00	35.00	157.00
Nexfin
SBP	344	143.56	36.88	138.00	69.00	230.00
SBP	344	142.69	42.70	141.00	48.00	263.00
Nexfin
MAP	344	99.31	25.32	95.50	47.00	169.33
MAP	341	101.08	27.01	98.00	40.00	195.00
Nexfin
HR	344	83.90	22.48	84.00	46.00	161.00
HR	344	83.81	22.48	84.00	43.00	159.00
Nexfin
All BP were expressed in millimeters of mercury; and HR, in beats per minute. N equals the number of comparison measurements.

A B

DBP

Nexfin DBP (mmHg)

40 60 80 100 120 140 160

Automated DBP - Nexfin DBP (mmHg)

0

50

40 60 80 100 120 140 160

-100

-50

Automated DBP (mmHg)

Pearson correlation coefficient 0.75, P < .0001

40 60 80 100 120

Automated DBP + Nexfin DBP/2 (mmHg)

Mean bias = -1.24, upper limit of agreement = 29.27, lower limit of agreement = -31.74

Fig. 3 Diastolic BP measurement comparisons. A, Scatter plot. B, Bland-Altman plot (mean and 95% confidence interval shown).

mean patient age was 62.2 years (SD 12.6 with a range of 36.4 to 88.9 years). There were 18 patients with acute shortness of breath (CHF or COPD), 11 with probable stroke, 3 with suspected sepsis, and 8 with a presenting SBP greater than 180 or less than 100 mm Hg studied. The overall recorded (n = 344 or 341) descriptions of the 4 hemodynamic variables measured by each device are displayed in Table 1 and show a wide range of values. The wide spectrum of the final ED diagnosis made within our initial 4 broad categories is shown in Table 2. Only 2 patients eventually received invasive hemodynamic monitoring indicating the overall noncritically ill status of our patient population.

The scatter and Bland-Altman plots for comparison of DBP measurements are shown in Fig. 3, for SBP in Fig. 4, for HR in Fig. 5, and for MAP in Fig. 6. Although there

are generally high correlations with the BP values, there are a few outliers seen on the scatter plots. For example, the BP readings for 1 patient at the 60-minute comparison time point were 143/41 mm Hg with the automated device and 193/140 mm Hg for the Nexfin-derived value. All the other 8 comparison time point values for this patient for the SBP and DBP were very close. In this case, we suspect that the patient at the 60-minute time point had grabbed the stretcher side rail or somehow otherwise increased the external pressure directly on the finger cuff, which is known to cause falsely increased BP values. To what extent this affected other comparison values that were beyond the 95% confidence intervals is not known at this time, although we believe that this probably happened infrequently.

A B

SBP

Nexfin SBP (mmHg)

200

250

Automated SBP - Nexfin SBP (mmHg)

50

100

50 100 150 200 250

50

100

150

-50

0

Automated SBP (mmHg)

Pearson correlation coefficient 0.83, P < .0001

100 150 200

Automated SBP + Nexfin SBP/2 (mmHg)

Mean bias = 0.87, upper limit of agreement = 47.54, lower limit of agreement = -45.80

Fig. 4 Systolic BP measurement comparisons. A, Scatter plot. B, Bland-Altman plot (mean and 95% confidence interval shown).

A B

Heart Rate

Nexfin HR (beats per minute)

80 100 120 140 160

Automated HR - Nexfin HR (beats per minute)

10 20

40 60 80 100 120 140 160

40

60

-40 -30 -20 -10 0

Automated HR (beats per minute)

Pearson Correlation Coefficient .97, P < .0001

60 80 100 120 140 160

Automated HR + Nexfin HR/2 (beats per minute)

Mean bias = 0.09, upper limit of agreement = 11.38, lower limit of agreement = -11.21

Fig. 5 Heart rate measurement comparisons. A, Scatter plot. B, Bland-Altman plot (mean and 95% confidence interval shown).

In all patients enrolled, there was enough digital artery perfusion to allow an arterial waveform to be constructed. Less than 10% of patients over the 2-hour monitoring period required a change of the finger cuff to an alternate digit secondary to some mild discomfort from the continuous cuff application.

Limitations

This trial was relatively small with 40 patients enrolled and included a variety of different acute Disease states. However, in terms of the number of individual BP and HR comparisons, it was similar to other published studies comparing hemodynamic measurements by different devices [8]. In addition, it was a convenience sample and not a consecutive patient study. In addition, because informed consent was required, very critically ill patients were not able

to be enrolled and thus studied. At the present time, it is not clear what the criterion standard is for noninvasively measuring the BP in the ED and, thus, what comparisons should be made with the Nexfin data.

If the study had enrolled more patients, we may have recruited some with severe peripheral vascular disease, hypotension, or severe Peripheral vasoconstriction (with or without the use of vasopressors) where the finger cuff may not have detected any arterial pulsations. The frequency of this problem in the ED patient population remains to be determined. However, if this were to occur, the Nexfin device would indicate the lack of pulsation detection and would not report any hemodynamic data. The automated devices were not calibrated before the study. In addition, the BP and HR measurements were made at the same time but in different upper extremities. It is possible that some patients may have had previously unknown differences in

A B

MAP

Nexfin MAP (mmHg)

150

200

Automated MAP - Nexfin MAP (mmHg)

50

50 100 150 200

50

100

-50

0

Automated MAP (mmHg)

Pearson Correlation Coefficient 0.81, P < .0001

60 80 100 120 140 160

Automated MAP + Nexfin MAP/2 (mmHg)

Mean bias = -2.05, upper limit of agreement = 29.74, lower limit of agreement = -33.83

Fig. 6 Mean arterial BP measurement comparisons. A, Scatter plot. B, Bland-Altman plot (mean and 95% confidence interval shown).

the BP in each upper extremity, although the frequency of this is unknown.

As with all technologies operator, knowledge of the idiosyncrasies of any device is important in the use of that equipment. For any future trials, the effects of external pressure on the finger cuff itself on the hemodynamic readings will need to be better monitored. There may be a need to change the finger cuff to an alternative digit in some patients (b10 % of cases) because of some physical discomfort during the 2 hours of continuous hemodynamic monitoring.

A standard Bland-Altman analysis, without correction for repeated values, was used to simplify statistical calculations. Consequently, the true limits of agreement are likely to be marginally wider than those described in this article.

Discussion

This study is the first to report comparisons of intermittent BP and HR measurements obtained by standard automated devices to the same values obtained from the Nexfin continuous finger cuff monitoring in a variety of different patients presenting to the ED. The BP and HR measurements had large ranges, and the final ED diagnoses made were very broad. There were high correlations seen between the measurements, suggesting that this novel Nexfin monitoring tool may have a place in the hemodynamic assessment and management of many different acutely ill ED patients. The best correlation was noted between the HR measurements, which was not unexpected, as the 5 electrode system monitors instantaneous HR (usually a proprietary machine averaged number of previous beats); and this was compared with the Nexfin-averaged HR over the time that it took the automated devices to measure the BP. There may be some difference in the averaging process of the HR with each system, but this made little differences in the comparison measurements.

The authors noted significant variability of the BP and HR in nonsedated and/or nonventilated/acutely ill ED patients who were continuously monitored using finger cuff technology. Most ED physicians have limited understanding of the dynamic nature of the underlying hemodynamics of any ED patient unless an arterial line is placed. The beat-to-beat BP and HR changes seen may represent the resilience of the human body and its autonomic balances, which can often be affected by the underlying illness, injury, medications given, malnutrition, and the emotional state of an individual. Clinical studies concerning HR variability have been instrumental in delineating these changing responses [16-18]. Perhaps, through use of the Nexfin device, we may extend our knowledge and understanding of not only the HR variability but also the BP variability of ED patients and its effects on clinical outcomes.

Although there were some differences between the BP measurements, it is not clear which recordings were most

accurate given the underlying variability in BP that was seen. The standard brachial cuff reported BP, whether by automated or Manual methods, is taken from 2 different heart beats (one for systolic and one for diastolic BP) separated by a varying amount of time. In our study, the BP recorded by Nexfin was the beat-to-beat one averaged over the same time interval that the automated device required before reporting a BP measurement. The continuously averaged values displayed by Nexfin would seem to be a better reflection of the real underlying BP and HR and their trending over time.

At the present time, unless an intra-arterial line is placed, we do not know what the actual continuous BP profile of any patient is during their evaluation and therapeutic management in the ED. This may be especially important in the patients whom we have studied who do not generally get invasive hemodynamic monitoring: possible acute CHF, suspected stroke or sepsis, and those with significant presenting BP abnormalities. In these patents, it is not clear how often BP and HR should even be measured and/or recorded and so the decision for the frequency of measurement is often left to nursing/physician staff preferences. This may be problematic as significant but unrecognized BP and HR changes may be occurring that could affect patient outcomes. One report has suggested that wide fluctuations of BP in the first 3 hours of the ED stay in patients with acute ischemic stroke are associated with an increased risk of death at 90 days [19]. A systematic review of the literature shows that dynamic changes of arterial waveform-derived variables (Pulse pressure and SBP variation) during mechanical ventilation are highly accurate in predicting volume responsiveness in critically ill patients with accuracy greater than that of traditional static indices central venous pressure [20]. Further studies are warranted using continuous BP and HR monitoring in the ED to further determine its effect on patient management and outcomes.

Continuous noninvasive BP and HR allow the recording of physiologic data without the requirement to recycle an automated device and wait for the measurements to be made. If a patient requires very frequent automated BP determina- tions, this can often be uncomfortable to the patient because of the recycling process. The use of the Nexfin finger cuff in our study was associated with some minor digital discomfort in a few patients and that was alleviated by simply putting the cuff on an alternative finger.

The Nexfin device also has the capability to summarize the entire period of beat-to-beat monitoring on a single screen (or printed out on a single page) so that complete ED stay BP and HR trending can be available to the treating physician as well as for the inpatient doctors. This may help us gain an understanding of the role of early continuous assessments on ED patient management and risk stratifica- tions. From a patient safety perspective, these continuous measurements may enable the reduction in harm secondary to missed periods of Abnormal vital signs.

There is currently underway a multinational, multicenter registry (PREMIUM) enrolling similarly acutely ill patients and recording continuous noninvasive Nexfin hemodynamic measurements over the initial 4 hours in the ED to better characterize the presenting and changing hemodynamic profiles of these patients and to determine their relationships to clinical outcomes.

This study of acutely ill ED patients showed that the Nexfin continuous finger cuff monitoring measurements of BP and HR do generally correlate with the same intermittent values obtained by the currently used automat- ed devices. However, the most informative and meaningful method of measuring the BP and HR may be the continuous method because of the way the values are obtained, displayed, and summarized over time. Noninva- sive continuous physiologic monitoring of these parame- ters, thus, could have important ramifications regarding patient assessments, therapeutic plans, outcomes, and safety and requires further ED study.

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