.

.

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Ambulatory Impedance Cardiography A Systematic Review Monica J. E. Parry

4

b Background: Standard noninvasive impedance cardiography has been used to examine the cardiovascular responses of individuals to a wide range of stimuli in critical care and laboratory settings. It has been shown to be a reliable alternative to invasive thermodilution techniques and an acceptable alternative to the use of a pulmonary artery catheter. Ambulatory impedance cardiography provides a similar assessment of cardiac function to standard noninvasive impedance cardiography, but it does so while individuals engage in activities of daily living. It offers portability and the option of managing complex patients in outpatient settings. b Objective: To critically examine through a literature analysis the validity, reliability, and sensitivity of ambulatory impedance cardiography for the assessment of cardiac performance during activities of daily living. b Methods: The Cochrane Database of Systematic Reviews (CDSR), The Cochrane Database of Methodology Reviews (CDMR), The Cochrane Central Register of Controlled Trials (CENTRAL), Database of Abstracts of Reviews of Effects (DARE), National Health Service Economic Evaluation Database (NHS EED), Health Technology Assessment (HTA), and The Cochrane Methodology Register (CMR; 1966Y2005); MEDLINE (1950Y2005); and CINAHL (1982Y2005) were searched using the following terms: ambulatory cardiac performance, impedance cardiac performance, AIM cardiac performance monitor, thoracic electrical bio-impedance, impedance cardiography, ambulatory impedance monitor, bio-impedance technology, ambulatory impedance cardiography, bio-electric impedance; also included were reference lists of retrieved articles. Studies were selected if they used an ambulatory impedance monitor to examine one or more of the following cardiovascular responses: pre-ejection period (PEP), left ventricular ejection time (LVET), stroke volume (SV), or a combination of these. b Results: Studies have been predominantly descriptive and have been focused on a young, male population with a

Judith McFetridge-Durdle

normal body mass index (BMI; 25Y29 kg/m2). Inconsistencies in determining specific markers of cardiac function (e.g., PEP and SV) across studies necessitated that results be reported by outcome for each study separately. b Discussion: Ambulatory impedance monitors are valid and reliable instruments used for the physiologic measurement of cardiac performance. Sensitivity is established utilizing within-individual measurements of relative change. This is especially important in light of an aging population and technical advances in healthcare. Further research is warranted using nursing interventions that focus on an older, female population who have a BMI greater than 30 kg/m2. Availability of noninvasive ambulatory measures of cardiac function has the potential to improve care for a variety of patient populations, including those with hypertension, heart failure, pain, anxiety, and depressive symptoms. b Key Words: ambulatory monitoring & impedance cardiography

I

mpedance cardiography (ICG) is a noninvasive technology that provides information regarding hemodynamic and fluid status (Lasater & Von Rueden, 2003). Noninvasive assessments of cardiac performance using bioelectrical impedance were first used in 1966 (Kubicek, Karnegis, Patterson, Witsoe, & Mattson, 1966) when impedance measurements within the thorax were used to estimate cardiac output (CO). Impedance changes are generated by fluctuations in blood volume and flow Monica J. E. Parry, RN, PhD(C), ACNP, CCN(C), is Advanced Practice Nurse/Acute Care Nurse Practitioner, Cardiac Surgery, Kingston General Hospital, Kingston, Ontario, Canada; Doctoral Candidate, Faculty of Nursing, University of Toronto; and Strategic Training Fellow in the FUTURE Program for Cardiovascular Nurse Scientists. Judith McFetridge-Durdle, PhD, RN, is Associate Professor, School of Nursing, Dalhousie University, Halifax, Nova Scotia, Canada and Key Mentor in the FUTURE Program for Cardiovascular Nurse Scientists.

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283

284 Ambulatory Impedance Cardiography velocity in the ascending aorta during systole and diastole. Impedance to electrical current decreases during systole due to increased blood volume and flow velocity, and increases during diastole as flow is reduced. Pulsatile impedance changes reflect ascending aortic flow and left ventricular function. Baseline thoracic impedance (Z0), pulsatile impedance/time changes (dZ/dt), and electrocardiography (ECG) are used to calculate various measures of cardiac function. Electrocardiographic and impedancegenerated hemodynamic waveforms are depicted in Figure 1, and impedance-generated hemodynamic parameters and definitions are listed in Table 1. Impedance cardiography is a reliable and valid noninvasive technique for measuring various indices of cardiovascular function in critical care environments and laboratory settings (McFetridge & Sherwood, 1999; Shoemaker et al., 1996, 1998, 2001). It has been shown to be a reliable alternative to invasive thermodilution techniques and an acceptable alternative to the standard use of a pulmonary artery catheter in a variety of populations (Shoemaker et al., 1996, 1998, 2001; Van De Water, Miller, Vogel, Mount, & Dalton, 2003), including critically injured obese (r = .85, p G .0001) and nonobese patients (r = .82, p G .0001; Brown, Martin et al., 2005), patients with atherosclerotic and rigid thoracic aortas ages 55Y70 years (r = .87) and over 70 years (r = .80; Brown, Shoemaker, Wo, Chan, & Demetriades, 2005), patients admitted to the emergency department with cerebrovascular accidents (Velmahos, Wo, Demetriades, Bishop, & Shoemaker, 1998), and hospitalized patients with advanced, decompensated chronic heart failure (Albert, Hail, Li, & Young, 2004). Impedance cardiography has been used also to evaluate the hemodynamic adjustments underlying blood pressure (BP) responses during mental stress. Overall, evidence has shown that systemic vascular resistance (SVR) control of BP during mental stress is a marker of cardiovascular disease risk (Light & Sherwood, 1989; Sherwood & Turner, 1995). Exaggerated SVR response during mental stress has been associated with myocardial ischemia (Blumenthal et al., 1995). Moreover, SVR responses to stress are blunted during the high estrogen phase of the menstrual cycle (McFetridge & Sherwood, 2000). These laboratory findings suggest that stress alters hemodynamics and influ-

FIGURE 1. Electrocardiographic and impedance-generated hemodynamic waveforms.

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q

TABLE 1. Impedance-Generated Hemodynamic Parameters and Definitions Hemodynamic Variable

Parameter

Definition

Thoracic fluid status Left ventricular function

Z0 = thoracic impedance CO = cardiac output

Baseline fluid status in chest Amount of blood ejected from the left ventricle in 1 minute Cardiac output divided by body surface area Amount of blood ejected with each beat Amount of resistance that the heart must pump against

Preload Afterload

Contractility

CI = cardiac index SV = stroke volume SVR = systemic vascular resistance dZ/dt = impedance changes over time PEP = pre-ejection period LVET = left ventricular ejection time

Reflects the force of ventricular contraction

Time from ventricular depolarization to ventricular ejection Period of time over which blood is ejected from the left ventricle

ences the balance between myocardial oxygen demand and supply. There are a number of commercially available impedance cardiographs but the Minnesota Impedance Cardiograph Model 304B (Instrumentation for Medicine, Greenwich, CT) is the most widely validated standard and has been extensively used in research applications (McFetridge & Sherwood, 1999). It has been used in populations including (a) normotensive males with and without a family history of hypertension (Hamer, Jones, & Boutcher, 2006); (b) female alcoholics with transitory hypertension after early abstinence (Bernardy, King, & Lovallo, 2003); (c) lonely and nonlonely undergraduate students (Cacioppo et al., 2002); (d) individuals with paraplegia (Raymond, Davis, & van der Plas, 2002); and (e) men with mild, stable heart failure (Davis et al., 2006). The Minnesota Impedance Cardiograph Model 304B has also been used to (a) investigate the effect of sleep on cardiac activity (Carrington, Walsh, Stambas, Kleiman, & Trinder, 2003), (b) determine racial differences in vascular reactivity (Kelsey, Alpert, Patterson, & Barnard, 2000), and (c) examine the effects of smoking and oral contraceptive use on the hemodynamic responses to stress in women (Straneva, Hinderliter, Wells, Lenahan, & Girdler, 2000). This model uses a 4-mA constant current source with a 100-kHz oscillator frequency and includes a display of Z0. A tetrapolar band electrode configuration has been adopted in most studies using the

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impedance technique for monitoring cardiovascular hemodynamics. Two electrodes initiate a high-frequency excitation current while the impedance is measured across an inner two-voltage electrode. The upper and lower voltage electrodes are placed around the base of the neck and around the thorax at the level of the xiphisternal junction (McFetridge & Sherwood, 1999). The outer current electrodes are placed at a minimum distance of 3 cm from the voltage electrodes (Figure 2). Impedance cardiography is ideally suited to nursing studies because it poses minimal risk and is cost-effective (McFetridge & Sherwood, 1999). There is a small risk of local skin irritation associated with electrode application. Applying an over-the-counter nonallergenic protective substance to the skin before applying the electrodes reduces this risk. With the introduction of the ambulatory impedance monitor, it should be possible to assess cardiovascular hemodynamics relatively unobtrusively during a wide range of procedures, in response to a variety of interventions, and in both static and active environments. Ambulatory impedance monitors offer portability, so that assessments of cardiovascular hemodynamics are not required to be in a laboratory or critical care setting. This technology is particularly exciting when the possibilities of assessing cardiac function, including CO, stroke volume (SV), pre-ejection periods (PEP), and left ventricular ejection times (LVET) during activities of daily living (ADLs) are considered. It may be possible to evaluate cardiac performance and, subsequently, manage patients in outpatient settings rather than admitting patients to the hospital for assessment of their hemodynamic parameters. Objective The purpose of this review is to determine whether ambulatory impedance monitors are valid and reliable instruments, with adequate sensitivity to detect change in cardiac performance, for use in nursing research and practice.

Methods Criteria for Considering Studies There was no restriction to the type of study or description of participant considered for this review. However, to establish the power of an ambulatory impedance monitor to assess various cardiovascular responses, it was necessary to include in the review studies that compared an ambulatory impedance monitor to a reference standard. It was also necessary to restrict the instrumentation, or the type of an ambulatory impedance monitor, to one that examined one or more of the following cardiovascular outcomes: PEP, LVET, and SV. Studies were not included if they did not use an ambulatory impedance monitor for the assessment of cardiac performance. Search Strategy The Cochrane Library (1966Y2005), including the Cochrane Database of Systematic Reviews (CDSR), The Cochrane Database of Methodology Reviews (CDMR), The Cochrane Central Register of Controlled Trials (CENTRAL), Database of Abstracts of Reviews of Effects (DARE), National Health Service Economic Evaluation Database (NHS EED), Health Technology Assessment (HTA), and The Cochrane Methodology Register (CMR); MEDLINE (1950Y2005); and CINAHL (1982Y2005) were searched using the following terms: ambulatory cardiac performance, impedance cardiac performance, AIM cardiac performance monitor, thoracic electrical bio-impedance, impedance cardiography, ambulatory impedance monitor, bio-impedance technology, ambulatory impedance cardiography, and bio-electric impedance (Deville, Bezemer, & Bouter, 2000; Farbey, 1993; Greenhalgh, 1997; Haynes, Wilczynski, McKibbon, Walker, & Sinclair, 1994; Higgins & Green, 2005). MEDLINE was searched using a combination of text words and Medical Subject Headings (MeSH; 1950Y2005; Lowe & Barnett, 1994). Using the MeSH database, the following MeSH headings were established: cardiography,

FIGURE 2. Typical band electrode placement for the Minnesota Impedance Cardiograph Model 304B (Instrumentation for Medicine, Greenwich, CT) [reference standard].

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286 Ambulatory Impedance Cardiography impedance and monitoring, ambulatory. The MeSH headings were combined and the scope of the search was restricted to the major topic headings, including impedance plethysmography and transthoracic impedance. Lastly, the reference lists of all retrieved papers were examined. Included studies were published in English. Dissertations and conference proceedings were excluded from the review. Methods of the Review All publications identified were evaluated to ensure they met the inclusion criteria. Standard methods were used to collect data and assess the methodological quality of the studies (Higgins & Green, 2005; The Evidence-Based Medicine Working Group, 2002). Due to the clinical diversity of the studies included in this review, a metaanalysis was not done. Inferences about the results of the studies are based on a critical appraisal, conducted by the authors, and not on meta-analytic techniques. Description of Studies Seven studies were identified in the search (Barnes, Johnson, & Treiber, 2004; Bishop, Pek, & Ngau, 2005; Hawkley, Burleson, Bernston, & Cacioppo, 2003; Riese et al., 2003; Sherwood, Hughes, & McFetridge, 2003; Sherwood, McFetridge, & Hutcheson, 1998; Vrijkotte, van Doornen, & de Geus, 2004). Reference lists of these studies revealed another four studies for examination (Boomsma et al., 2000; Kizakevich et al., 2000; Nakonezny et al., 2001; Vrijkotte, van Doornen, & de Geus, 2000). When examined, two studies were excluded: the results were not yet available in one study (Boomsma et al., 2000), and it was difficult to determine the type of impedance monitor used in the second study (Kizakevich et al., 2000). Of the nine studies meeting the inclusion criteria, in two the validity, reliability, and sensitivity of an ambulatory impedance cardiograph had been evaluated (Sherwood et al., 1998, Nakonezny et al., 2001) with the most widely validated standard, the Minnesota 304B. One study was a randomized controlled trial comparing the impact of daily health education on lowering BP in African-American adolescents (Barnes et al., 2004). In one study, the feasibility of large-scale ensemble averaging of ambulatory impedance cardiograms was examined using the Vrije Universiteit-Ambulatory Monitoring System (VU-AMS; Riese et al., 2003). The remaining five studies examined were the relationship of ethnicity (Bishop et al., 2005; Sherwood et al., 2003), sex and trait anger (Bishop et al., 2005), work stress (Vrijkotte et al., 2000, 2004), and loneliness (Hawkley et al., 2003) on a number of indices of cardiac function.

Results Instruments Three ambulatory impedance monitors were used to examine cardiovascular indices: the AIM-8 (Bio-impedance Technology, Chapel Hill, NC), the VU-AMS (Version 4.6, VU-FPP, Amsterdam), and the AZCG (World Wide Medical Instruments, Dallas, TX).

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Ambulatory Impedance Monitor The AIM-8 is a microcomputerbased bioelectric impedance monitor and signal processing system designed to assess a number of indices of cardiac function in a 24-hour ambulatory environment. It consists of a 3  4  1.5 in. plastic enclosure that contains a credit-card-sized bioelectric impedance cardiograph, a credit-card-sized internal computer, and a 9-V battery power source. The AIM generates an 80-kHz, 2-mA constant sine wave alternating current. The AIM computer section ensemble averages, analyzes, and stores ECG, dZ/dt, and Z0 waveforms and the computed cardiac function indices during each measurement sequence. The AIM can function in a continuous or continuous-manual mode and is capable of being activated by a cuff-pressure sensor initiation of each ambulatory BP measurement. A tetrapolar combination of spot and band electrodes was developed for use with the AIM system. The recording electrodes are band electrodes placed around the base of the neck and around the thorax at the tip of the xiphoid process, identical to that for the Minnesota reference configuration (Sherwood et al., 1998). Three spot electrodes are used as current electrodes, one applied behind the right ear (over the base of the mastoid process), and the other two on the lower right and left rib cage 6 cm below the lower recording band electrode. The two impedance current spot electrodes (right ear and lower rib cage) are sources for the ECG signal, approximating a lead II configuration. The AIM is worn on a belt around the waist during assessment of normal daily activities. Processing of the impedance signals is accomplished using either COPWORKS or COP-WIN (Bio-impedance Technology), which permit ensemble averaging of impedance waveforms to filter noise and respiratory artifacts. Vrije Universiteit-Ambulatory Monitoring System The VUAMS is an ambulatory monitoring device designed to record ECG and ICG recordings from six spot electrodes. Two electrodes on the back send a high-frequency current and two measuring electrodes on the chest pick up the voltage drop over the thorax. The lower current electrode on the back is placed at least 3 cm below the horizontal plane of the lower measuring electrode on the chest, placed at the xiphoid process. The upper current electrode on the back is placed 3 cm above the horizontal plane of the upper measuring electrode on the chest. The upper chestmeasuring electrode is placed at the jugular notch of the sternum between the collar bones. Two additional chest electrodes are placed, one on each side of the lower chest. Thoracic impedance is assessed against a constant current of 50 kHz, 0.35mA (Riese et al., 2003). The VU-AMS does not record the full ECG but instead uses a continuous time series of R-wave-to-R-wave intervals derived from a three-lead ECG (Vrijkotte et al., 2000). The R-wave is used as an approximation of the onset of the electromechanical systole (EMS) and the PEP scoring is made relative to the R-wave (Riese et al., 2003). Vagal tone is assessed using a root mean square of successive differences (RMSSD) in these interbeat differences (Vrijkotte et al., 2000). A fixed QYR interval is often added to this abbreviated PEP to allow easy comparison with the usual Q-wave-based PEP (Riese et al.). Ensemble averaging involves digitalizing the ECG and the ICG over all beats

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in a preset period of time (e.g., 60 sec) with respect to the peak of the ECG R-wave, then averaging by summing the digitalized samples for each signal and dividing by the number of synchronized beats (Riese et al.). This procedure is used to reduce the impact of single-beat fluctuations in the impedance signal related to respiration and thoracic movements (Riese et al.).

vides analogue and digital conversion of signals. Digitalized signals are stored on a 20-MB flash card (Nakonezny et al.). Programming during set-up, signal monitoring, and uploading of data are accomplished using standard communication software through digital input/output connectors and a serial interface to a microcomputer system. Participants A total of 215 individuals were evaluated with the AIM (Barnes et al., 2004; Bishop et al., 2005; Sherwood et al., 1998, 2003), 197 with the VU-AMS (Riese et al., 2003; Vrijkotte et al., 2000, 2004), and 157 with the AZCG (Hawkley et al., 2003; Nakonezny et al., 2001). The subjects included in this systematic review were predominantly young, male adults with a normal BMI (25Y29 kg/m2; Table 2).

Ambulatory Impedance Cardiograph The AZCG (World

Wide Medical Instruments) is an ambulatory monitor designed for noninvasive acquisition of physiological data during daily activities. It is a 4.5  9.5  16 cm device weighing 400 g with batteries. The analogue subsystem is composed of a three-lead ECG and a four-lead electrical impedance system, which produces a constant current source of 50 kHz, 2 mA (Nakonezny et al., 2001). The acquired analogue impedance signals are filtered, amplified, and differentiated to produce signals for Z0, $Z, and dZ/dt. The ECG and ICG each employ a digitally controlled, sampled-signal rebalance method for waveform stability. The digital subsystem pro-

Validity, Reliability, and Sensitivity Inconsistency in determining PEP and SV across studies necessitated that results be reported by outcome for each study separately. q

TABLE 2. Characteristics of Study Participants by Ambulatory Monitoring System Sample Size (n)

Gender

Mean Age (years)

Ethnicity

BMI (kg/m2)

AIM Sherwood, McFetridge, & Hutcheson (1998)

11

Male (n = 5) Female (n = 6)

25*

27

Sherwood, Hughes, & McFetridge (2003)

20

35*

Barnes, Johnson, & Treiber (2004)

35

Male (n = 9) Female (n = 11) Male (n = 21) Female (n = 14) Male (n = 74) Female (n = 75)

White (n = 6) African-American (n = 4) Asian (n = 1) African-American (n = 10) White (n = 10) African-American (n = 35)

Study

Bishop, Pek, & Ngau (2005)

149

VU-AMS Vrijkotte, van Doornen, & de Geus (2000) Riese et al. (2003)

109 21

Vrijkotte, van Doornen, & de Geus (2004)

67

AZCG Nakonezny et al. (2001)

Hawkley et al. (2003)

10 12 135

Male (n = 109) Male (n = 7) Female (n = 14) Male (n = 67)

Male (n = 10) Male (n = 6) Female (n = 6) Male (n = 68) Female (n = 67)

16.2 T 1.4

25 30

21.5*

Chinese (n = 51) Malays (n = 51) Indian (n = 47)

N/A

47.2 T 5.3 29 T 5.14

Dutch (n = 109) Dutch (n = 21)

25 22

47.1 T 5.2

Dutch (n = 67)

25

24.7 T 2.0 20 T 0.52

N/A N/A

24 N/A

19.2 T 1.0

White (n = 112) African-American (n = 9) Asian (n = 10) Other (n = 4)

G27

Notes. AIM = Ambulatory Impedance Monitor; VU-AMS = Vrije Universiteit-Ambulatory Monitoring System; AZCG = New Ambulatory Impedance Cardiograph; BMI = body mass index; N/A = data not available. *SD not recorded.

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288 Ambulatory Impedance Cardiography Validity Validity is the degree to which an instrument

measures what it is supposed to measure (Gassert, 1990; Hill, 1988; Portney & Watkins, 1993). Mean values for four indices of cardiac function measured while subjects were sitting and standing with the AIM compared to the Minnesota 304B (standard) and the AZCG compared to the ZCG-Minnesota 304B (standard) monitoring systems are depicted in Figure 3. Cardiovascular responses to sitting and standing are similar across all groups. No data are available for posture and monitor effects for the VU-AMS. Pearson’s r values for the correlations between the AIM and the Minnesota 304B (standard) cardiac function indices measured while the subjects were sitting and standing ranged from .87 to .96 (Sherwood et al., 1998). Pearson’s r values for the correlations between the AZCG and ZCGMinnesota 304B (standard) cardiac function indices again measured while the subjects were sitting and standing ranged from .68 to .98 (Nakonezny et al., 2001). Two ambulatory impedance systems (AIM, AZCG) correlate well with the standard Minnesota 304B. No data are available comparing the third ambulatory impedance system (VU-AMS) to a standard. Reliability Reliability and validity are related intricately. Reliability is the consistency, accuracy, and precision of a measure (Hill, 1988). With repeated measures of cardiac indices, the less variability the AIM shows, the higher its reliability. Reliability also reflects accuracy, such that a reliable instrument gives the true score and minimizes error. Sherwood et al. (2003) assessed the hemodynamic variations in ambulatory BP (ABP) measurements in AfricanAmericans during ADLs. Impedance cardiographic assess-

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ments of CO were synchronized to each ABP measurement using the AIM. They found CO to be a significant predictor of systolic BP in Whites [Z(1, 429) = 3.38, p = .0007] but not in African-Americans (p = .61), whereas SVR was a significant predictor of systolic BP in African-Americans [Z(1, 393) = 3.37, p = .0007] but not in Whites (p = .46). Their observations provide preliminary evidence that individual differences in hemodynamic patterns of BP regulation observed in a laboratory environment are reproducible with ambulatory impedance measures taken during ADLs. The AIM was also evaluated in African-American adolescents (n = 35) to determine the reproducibility of daytime and nighttime ambulatory bioimpedance-derived measures of hemodynamic function (Barnes et al., 2004). Reliability may be defined as the degree of consistency with which an instrument measures a physiological variable (Gassert, 1990). It is considered to be synonymous with the accuracy of the instrument. Across 2 months, Barnes et al. (2004) found heart rate (HR; r = .81) to be highly repeatable and SV (r = .54), CO (r = .56), PEP (r = .59), and LVET (r = .74) to be moderately repeatable. Stroke volume (ml/min) is often determined as per the equation derived by Kubicek et al. (1966): SV = D(L/ Z0)2(LVET)(dZ/dt)max, where D (; cm) is set to a constant value of 135; L (cm) is the distance between recording electrodes; Z0 (;) is the basal thoracic impedance related to air, blood, and tissue levels; and LVET (msec) is the left ventricular ejection time measured from the B-point to the X-point of the dZ/dt waveform. Using this derivation, and the AZCG, Hawkley et al. (2003) reported that loneliness predicted a lower CO (l per min = HR  SV) during a normal day. They found that differences observed in a

FIGURE 3. Mean values for cardiovascular indices in subjects during sitting and standing postures measured using the AIM monitor compared to the Minnesota 304B (standard) and the AZCG compared to the ZCG-Minnesota 304B (standard). HR = heart rate; SV = stroke volume; PEP = preejection period; LVET = left ventricular ejection time; Si = sitting; St = standing. , AIM; , Minnesota; , AZCG; , ZCG.

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laboratory setting generalized to everyday life. Lonely participants exhibited a higher total peripheral resistance (TPR; dyne sec cmj5, a measure compared to SVR) and a lower CO than nonlonely participants across the various situations and social contexts in which they were measured during a normal day. The reliability of a measurement to detect response changes over time is dependent on the reproducibility of the method (Vinet, Nottin, Lecoq, Guenon, & Obert, 2001). Using the VU-AMS, Riese et al. (2003) found large individual differences in absolute PEP. They concluded that measuring individual differences in sympathetic activation by using absolute PEP confounds sympathetic drive with individual differences in adrenoceptor functioning. For example, a shorter PEP indicated improved contractility/ sympathetic activation and a longer PEP indicated diminished contractility; and increased afterload caused lengthening of PEP and increased preload caused shortening of PEP (Obrist, Light, James, & Strogatz, 1987). Impedance cardiography usually estimates PEP as the time interval between the ECGYQ wave (onset of EMS) and the B-point in the ICG, which is the start of the rapid upslope of dZ/dt to its maximum value (Sherwood et al., 1990). The PEP is defined as the period between the onset of EMS and the onset of left ventricular ejection at the opening of the aortic valve. However, because the VU-AMS does not record a full ECG, PEP scoring must be made relative to the Rwave, which is used as an approximation of the onset of the EMS. The precision and accuracy of an instrument also reflects the reliability of the measure. To improve the accuracy, precision, and reliability, the VU-AMS ambulatory PEP should be used mainly in a within-subjects design (Riese et al., 2003). Within-subjects changes in PEP across the day can easily be determined via various reactivity measuresVlying, sitting, and standing. Sensitivity For each index of cardiac function, the response to standing was computed as the difference between

standing and sitting values, expressed as a percent change from the sitting value (Sherwood et al., 1998). The mean postural responses measured using the AIM compared to the Minnesota 304B and the AZCG compared to the ZCG-Minnesota 304B are depicted in Figure 4. The AIM and AZCG monitoring systems indicate similar postural responses for HR, SV, PEP, and LVET. This would suggest consistency, accuracy, sensitivity, and enhanced reliability.

Discussion Standard ICG is a reliable and valid noninvasive technique for measuring various indices of cardiovascular function in critical care environments and laboratory settings (McFetridge & Sherwood, 1999; Shoemaker et al., 1996, 1998, 2001). It has been shown to be a reliable alternative to invasive thermodilution techniques and an acceptable alternative to the standard use of a pulmonary artery catheter in a variety of populations (Shoemaker et al., 1996, 1998, 2001; Van De Water et al., 2003), including critically injured patients who are obese (Brown, Martin, et al., 2005), elderly patients with atherosclerotic and rigid thoracic aortas (Brown, Shoemaker, et al., 2005), patients admitted to the emergency department with cerebrovascular accidents (Velmahos et al., 1998), and hospitalized patients with advanced decompensated chronic heart failure (Albert et al., 2004). Reliable measurement of cardiac performance and hemodynamic responses during ADLs are extremely important in detecting changes that may be imposed by intervention studies. The validity and reliability of two ambulatory impedance cardiographs (AIM, AZCG) were tested against the reference standard Minnesota 304B during sitting and standing. The devices were compared in healthy subjects. Both the AIM and the AZCG tracked changes across conditions closely with this reference standard and appeared to provide valid and reliable estimates of

FIGURE 4. Responses to standing, expressed as percent change for standingj sitting values (means) for HR, SV, PEP, and LVET recorded using the AIM, Minnesota model 304B (standard), AZCG, and ZCG-Minnesota 304B (standard). HR = heart rate; SV = stroke volume; PEP = pre-ejection period; LVET = left ventricular ejection time. , AIM; , Minnesota; , AZCG; , ZCG.

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290 Ambulatory Impedance Cardiography cardiac function (HR, SV, PEP, and LVET). Moreover, both ambulatory impedance systems estimate PEP as the time interval between the ECG-Q wave (onset of EMS) and the B-point in the ICG, which is the start of the rapid upslope of dZ/dt to its maximum value. These results support those of Riese et al. (2003) and Sherwood et al. (1990), who recommend that within-individual measurements of relative change are more valid than absolute values. No data were found describing the validity of the VU-AMS to a reference standard. In repeated measures of a characteristic, the less variability an instrument shows, the higher its reliability (Hill, 1988). Barnes et al. (2004) found HR to be highly repeatable and SV, CO, PEP, and LVET to be moderately repeatable across 2 months. However, they did not control for postural changes, physical activity levels, or affective states in their data analysis, explaining some of the variability in reproducibility of some of the bioimpedance measures. In addition, a number of methodologic factors can impact reproducibility, such as consistency of electrode placement during instrumentation, minimization of electrode resistance by thorough preparation of the placement area (shaving, cleansing, skin prep), and consistency in waveform editing. Efforts should be made to address these issues when using ambulatory impedance in outpatient settings. Information from this systematic review is important for nurses in light of the complexities of the patient populations and the technical advances in healthcare. The clinical utility of ambulatory impedance for the noninvasive monitoring of cardiovascular responses of individuals to various nursing interventions in outpatient settings is immense. Nurses are ideally positioned to incorporate ambulatory ICG into research and practice settings, with a variety of patient populations. As the population ages, caring for individuals with multiple comorbid conditions will become a part of everyday practice. Ambulatory ICG provides a similar assessment of cardiac function to standard noninvasive ICG, but it does so while individuals engage in ADLs. It offers portability and the option of managing complex patients in outpatient settings. Further research is warranted comparing ambulatory ICG to the reference standard Minnesota 304B in older, female populations who have a BMI greater than 30 kg/m2. All studies included in this systematic review were descriptive. Evaluating the within-individual measurements of relative change in cardiovascular responses of individuals to various nursing interventions has the potential to improve care for a variety of patient populations including those with hypertension, heart failure, pain, anxiety, and depressive symptoms. Because ambulatory ICG permits the examination of hemodynamic responses to stress during ADLs, it may help identify individuals at risk for future cardiovascular events. q

Accepted for publication April 6, 2006. Corresponding author: Monica J. E. Parry, RN, PhD(C), ACNP, CCN(C), Cardiac Surgery, Kingston General Hospital, 76 Stuart Street, Kingston, Ontario, Canada (e-mail: [email protected]).

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