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European Journal of Heart Failure 2003 5(3):253-259; doi:10.1016/S1388-9842(02)00250-7
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© 2002 European Society of Cardiology

Central hemodynamic responses during serial exercise tests in heart failure patients using implantable hemodynamic monitors

Å. Ohlssona, D. Steinhausb, B. Kjellströmc, L. Rydend,* and T. Bennettc

a Southern Hospital Stockholm, Sweden
b St. Luke's Hospital Kansas, MO, USA
c Medtronic Inc Minneapolis, MN, USA
d Department of Cardiology, Karolinska Hospital S-171 76 Stockholm, Sweden

* Corresponding author. Tel.: +46-8-51773161; fax: +46-8-311044. E-mail address: lars.ryden{at}ks.se


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Introduction: Exercise testing is commonly used in patients with congestive heart failure for diagnostic and prognostic purposes. Such testing may be even more valuable if invasive hemodynamics are acquired. However, this will make the test more complex and expensive and only provides information from isolated moments. We studied serial exercise tests in heart failure patients with implanted hemodynamic monitors allowing recording of central hemodynamics.

Methods: Twenty-one NYHA Class II–III heart failure patients underwent maximal exercise tests and submaximal bike or 6-min hall walk tests to quantify their hemodynamic responses and to study the feasibility of conducting exercise tests in patients with such devices.

Results: Patients were followed for 2–3 years with serial exercise tests. During maximal tests (n=70), heart rate increased by 52±19 bpm while SvO2 decreased by 35±10% saturation units. RV systolic and diastolic pressure increased 29±11 and 11±6 mmHg, respectively, while pulmonary artery diastolic pressure increased 21±8 mmHg. Submaximal bike (n=196) and hall walk tests (n=172) resulted in SvO2 changes of 80 and 91% of the maximal tests, while RV pressures ranged from 72 to 79% of maximal responses.

Conclusions: An added potential value of implantable hemodynamic monitors in heart failure patients may be to quantitatively determine the true hemodynamic profile during standard non-invasive clinical exercise tests and to compare that to hemodynamic effects of regular exercise during daily living. It would be of interest to study whether such information could improve the ability to predict changes in a patient's clinical condition and to improve tailoring patient management.

Key Words: Exercise testing • Hemodynamic monitoring • Heart failure

Received May 2, 2002; Revised July 15, 2002; Accepted September 25, 2002


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Patients with heart failure constitute an increasingly large group [1,2]. The management and treatment of these patients are often guided by the medical history, physical examination, chest X-ray and echocardiography. As a complement to these methods, exercise tolerance testing is sometimes used to assess functional impairment, especially in the chronic phase of the disease. During standard exercise test procedures, only non-invasive parameters are obtained, such as heart rate (HR), blood pressure, maximal workload and the relative grade of dyspnea and fatigue. There are indications that there may be a discrepancy between these acute, noninvasive parameters and invasively obtained data [3–5]. In particular, access to central hemodynamic information during exercise may be useful when managing complicated patients [6–8]. Until now such data could only be obtained via catheterization and accordingly only at relatively infrequent time points. This process is too complicated for serial patient evaluations in clinical practice.

We developed and evaluated an implantable hemodynamic monitoring (IHM) system, designed similar to an ordinary pacemaker system, with two sensor-equipped leads positioned in the right ventricle [9]. This system generates long-term ambulatory records of several hemodynamic variables. An advantage with this system is its capability to present long-term hemodynamic trends during activities of daily living. This study was conducted to test the hypothesis that long-term ambulatory monitoring would improve understanding of the hemodynamic burden of various types of exercise tests and the relation between the tests and the hemodynamic burden of daily living.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. The monitoring system
The IHM system (Model 10440; Medtronic, Inc.) has been more extensively described in a previous paper [9]. In brief, it consists of two sensor-equipped leads connected to a monitor and memory device. The external design is similar to an ordinary pacemaker system, with the leads positioned in the right ventricle and the IHM device implanted in the subcutaneous tissue below the left or right clavicle. The implant procedure for the IHM takes advantage of the well established implantation techniques commonly used for standard pacemaker implants.

A continuous measurement of right ventricular pressure (RVP) is obtained from one of the sensors (Model 4328A; Medtronic, Inc.), and mixed venous oxygen saturation (SvO2) from the other (Model 4327; Medtronic, Inc.). Hemodynamic variables are measured beat-by-beat, averaged and stored in the IHM's memory. Data can be stored at different resolution levels ranging from high resolution, where data are averaged over two second periods, to low resolution, where data are averaged over approximately 60-min periods. With low resolution, the memory is capable of storing data over 4-week periods. Stored data are retrieved via radio frequency telemetry to a PC-computer for processing and printout. In this study, the patients visited the outpatient clinic every 2–4 weeks during the duration of the study, for data retrieval.

Measured variables chosen for presentation in this paper are: HR, right ventricular systolic (RVSP) and diastolic pressure (RVDP), an estimation of the pulmonary artery diastolic pressure (ePAD; Refs. [10–12]) and SvO2. In addition to the variables reported here, the IHM system also measures and records maximum right ventricular dP/dt, maximum negative dP/dt, pre-ejection interval, and total systolic time interval.

2.2. Patient characteristics
The IHM system was implanted in 21 patients at three centers (Table 1) for this study. The patients were in NYHA Class II–III. Their mean left ventricular ejection fraction was 24±9% at the time of implant. The patients were followed for a mean of 25±14 months (range: 1–39). Fifteen of the patients completed the initial 12-months of follow-up; 3 patients died and three were transplanted. Thirteen patients completed an additional 12-months of follow-up (total of 24-months), during which two additional patient deaths occurred. Seven patients completed 3 years of follow-up with one additional death during the third year (Table 2).


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  		Table 1 IHM-1 patient demographics (numbers if not indicated otherwise)

 


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  		Table 2 Follow-up schedules and completed exercise tests

 
The overall intention for this IHM study was to conduct a pilot evaluation of the feasibility of chronic implantable monitoring including long-term performance and reliability of the system and these results were previously published [9]. The ability of the patients to conduct exercise was one of the patient selection criteria and exercise testing was an approved subset of the protocol for all patients (Table 2). However, the detailed exercise results have not been previously reported.

This investigation conforms with the principles outlined in the declaration of Helsinki and the protocol was approved by the ethics committee of each participating hospital. All patients gave their written informed consent prior to enrollment. Studies at hospitals in the US were conducted under US FDA Investigation Device Exemption No. G950062.

2.3. Study protocol
Standardized, serial exercise tests were performed under predefined, reproducible conditions, as described below, to evaluate the RVP, the ePAD and the SvO2 responses during hemodynamic stress.

A maximal, symptom limited bicycle exercise test in upright position was performed just after implant and was repeated after 2, 6 and 12-months of follow-up (n=70). Data from the IHM systems’ sensors were collected and transmitted by radio frequency link to the PC-Programmer. The PC-Programmer stored all data for subsequent processing.

Approximately every 4–6 weeks after implant, submaximal bicycle exercise tests (n=196) or standardized 6-min hall walk tests (n=172) were performed. The submaximal bike test was performed over 6-min using a fixed load that was 30–40% of the patient's maximal load during the baseline maximal exercise test. Alternatively, some patients exercised for 6-min at two pre-selected workloads: 3-min at 10–25 W and 3-min at 25–50 W. The 6-min walk test was performed at a speed determined by each patient individually with standardized instructions and encouragement from an investigator [13].

During all three types of exercise test, hemodynamic data from the IHM system were stored in the high resolution setting (beat-by-beat measurements averaged over 2–8 s data storage intervals). Illustrations of the printed trends from each type of exercise test in one patient are shown in Fig. 1.


Figure 1
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  		Fig. 1 Time plots of the continuous, high resolution (2 s) data acquired and stored by the IHM. Each type of exercise test (maximal exercise test, 6-min walk test and submaximal bike test) are shown from one heart failure patient in this study. Top plot is HR, second plot is SvO2 measured in the right ventricle, third plot is RVSP and RVDP and bottom plot is ePAD. Shaded areas show the variability of the samples, while the solid lines are filtered data using a 10 s moving average.

 
As reported in a prior paper [9], pressures recorded by the IHM system systematically underestimated values measured simultaneously by a Swan–Ganz catheter. Thus, all values reported here were corrected for this –5.5 mmHg offset.

2.4. Statistics
The results from all tests were evaluated individually and then pooled to describe overall averages. Analysis of variance was used to verify the changes, and t-tests, corrected for repeated comparisons, were used to determine statistical significance of the responses; P values <0.05 were considered significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1. Heart rate response
Prior to the start of the maximal exercise test, with the patients in the resting supine position, HRs were 78±15 bpm increasing by 52±19 bpm during maximal exercise on the bicycle ergometer (Table 3). The HR of 130±22 bpm reached during peak exercise averaged 82±12% of the predicted maximum HR for healthy, age-matched subjects (Predicted HR=220-Age) [14]. Individual rate responses ranged from 64% of the predicted maximum to as high as 105%. The former patient, as well as four other patients who failed to achieve >70% of their age-predicted maximum were likely exhibiting chronotropic incompetence due to concomitant sinus node disease or to blunted autonomic reflex control of HR which is known to occur in heart failure [15]. HR increased 34±17 bpm during the 6-min hall walks, which represents 66% of the HR response during maximal exercise. Similarly, HR increased 33±16 bpm during submaximal bike tests (64% of the peak response).


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  		Table 3 Hemodynamic Data derived from the IHM (Mean±S.D.)

 
3.2. Mixed venous oxygen response
During supine rest prior to the maximal exercise tests the SvO2 averaged 65±8%. This relatively low resting SvO2 level is reasonably explained by the underlying cardiac disease. The SvO2 decreased to 35±21% during maximal exercise, (approximately 35% saturation units). Resting levels of SvO2 prior to 6-min hall walks and submaximal bike tests were comparable to the levels before maximal exercise (Table 3). Interestingly, the decreases in SvO2 during the 6-min hall walk tests were approximately 91% of the responses seen during maximal exercise. The increased oxygen utilization and decrease in SvO2 during 6-min of submaximal bike exercise was approximately 80% of the response noted during maximal exercise.

3.3. Right ventricular systolic pressure response
The patients’ RVSP averaged 41±13 mmHg prior to the maximal exercise (Table 3). The RVSP increased markedly to 70±15 mmHg during peak exercise. Individual RVSP responses ranged from a low response of +7 mmHg in one patient to as much as +62 mmHg in another. Resting RVSP prior to the 6-min walk tests was comparable to those recorded before maximal exercise. An increase of 22±8 mmHg in RVSP was noted during the 6-min walk test, which was 77% of the average peak response observed during maximal exercise where the increase was 29 mmHg. During submaximal bike exercise similar RVSP values were achieved as during the 6-min hall walks. RVSP increased from 42±12 at rest, to a peak at 63±13 mmHg at the peak of the test. This change of 21±6 mmHg was approximately 72% of the peak response seen during maximal exercise.

3.4. Right ventricular diastolic pressure response
A relatively normal level of RVDP with a mean of 6±4 mmHg was seen at rest during the supine control period before exercise. At the peak of exercise, RVDP increased to a mean value of 17±7 mmHg. Individual RVDP responses ranged from a low response of –1 mmHg in one patient to as much as +38 mmHg in another. Resting RVDP prior to the 6-min walk tests and bike tests were comparable to those recorded before maximal exercise (Table 3). The 6-min walk and submaximal bike tests induced increases of 9±4 and 8±4 mmHg in RVDP, respectively. Thus, the RVDP responses during these two working conditions were approximately 74–79% of the average peak response during maximal exercise.

3.5. Pulmonary artery diastolic pressure response
Estimated pulmonary artery diastolic pressure averaged 20±6 mmHg during the supine rest period prior to maximal exercise. The ePAD increased markedly during the maximal exercise test to a peak value of 41±10 mmHg, with individual responses ranging from +5 to +54 mmHg across patients. Again, resting ePADs prior to both types of submaximal tests were comparable to those before maximal exercise (Table 3). The response to the 6-min walk test was 77% of the average peak response observed during the maximal exercise test while the bike test response was 70%.

Complications from device implants and long-term performance of this technology, including SvO2 accuracy during exercise, have all been reported previously in more detail [9]. In brief, complications were typical to those seen with similar devices, like standard pacemakers. The reproducibility of the sensor measurements over time were within ranges anticipated by the study design. SvO2s during submaximal and maximal exercise were compared to simultaneous reference SvO2 samples. Long-term, there were significant bio-interface factors (blood clots and/or tissue interactions) confounding approximately 40% of the SvO2 sensors so more development is needed.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
This study demonstrates the feasibility of long-term hemodynamic monitoring of heart failure patients. Moreover, it revealed some interesting observations regarding the hemodynamic impact of various types of exercise tests. It also gave some insights into the relation between the hemodynamic level of daily living and defined levels of exercise. In fact the utility of exercise testing has been debated [16,17]. This may partly be due to the lack of more detailed information of the hemodynamic workload during exercise.

As congestive heart failure patients experience clinical decompensation or disease progression, SvO2 will drop in proportion to their depressed cardiac output [18,19]. Changes in SvO2 from resting levels of approximately 70% down to 60% or lower are clinically relevant. However, during ambulatory monitoring, SvO2 may decrease even more. Interestingly, we found SvO2 decreases of approximately 30% even at submaximal exercise levels. This indicates that submaximal exercise and daily activities may be a considerable burden for this patient category, reflecting a very limited capacity for increasing cardiac output. Repeated observations may consequently serve the purpose of identifying spontaneous fluctuations in the disease state. The ability to conduct work at a higher SvO2 may then be an early sign of improvement in this patient category.

Patients with congestive heart failure have elevated RVP and pulmonary artery pressures. Some CHF patients can have pulmonary venous or pulmonary artery hypertension producing even higher right heart pressures. Nathan et al. and Gibbs et al. [6–8] reported on significant increases in pulmonary artery pressures in ambulatory heart failure patients. The measurement systems used in their studies did, however, limit follow-up to only a few days or at most a week. The present IHM system allowed reliable recording of resting and ambulatory RVP and pulmonary artery pressures over more than 2 years. Additionally, the repeated, serial exercise tests conducted over the follow-up period resulted in reproducible recording of pressures. This held true even as pressures increased by more than 30 mmHg (above baseline) during maximal exercise tests (Table 3; Figs. 1 and 2).


Figure 2
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  		Fig. 2 Hemodynamic responses during maximal exercise (n=70), 6-min walk tests (n=172) and submaximal bike tests (n=196). Data are Mean±S.D. from tests pooled over 25±14-months of follow-up in 21 patients. HR, Heart Rate; SvO2, Mixed venous oxygen saturation; RVSP, right ventricular systolic pressure; RVDP, right ventricular diastolic pressure; ePAD, estimated pulmonary artery diastolic pressure; RVPP, right ventricular pulse pressure; Changes in SvO2 are negative values (decreases) while all other variables are positive (increases) as illustrated.

 
Left ventricular pressure and oxygen consumption are sensitive parameters for evaluation of heart failure [21]. Invasively obtained hemodynamic information should add useful information when measured during hemodynamic stress [6–8]. However, it has not been feasible to routinely catheterize heart failure patients during tests despite the potential usefulness. It is even less practical to consider repeated catheterizations to track central hemodynamic responses over time. The data presented in this study illustrate a simple method to obtain such data with an implantable monitoring system. Potentially, the serial comparisons over time may be a more valuable method to evaluate the individual patient's status than tests conducted intermittently and without such detailed hemodynamic information.

The IHM system measured changes during exercise in RVDP (an estimate of RV filling pressure) and ePAD (an estimate of LV filling pressure) in addition to RVSP and HR. Notably, ePAD increased markedly, by 105, 80 and 70% for maximal bike tests, 6-min hall walk tests and submaximal bike tests, respectively. This may provide additional evidence of the inability of the left ventricle to respond adequately and provide the needed cardiac output.

Exercise tests used to evaluate the exercise tolerance of heart failure patients are traditionally continued to the maximal level of exertion the patient can perform. These maximal levels are compared over time and the recorded information is used as an indicator of the patient's functional status at the time of the test as well as a predictor of the patient's prognosis [20]. It has been suggested that exercise tests at submaximal levels generate sufficient hemodynamic stress for assessment of functional impairment in patients with chronic heart failure [10]. Our results may support the trend toward use of more submaximal exercise tests in heart failure patients since we observed changes in central hemodynamic variables during submaximal exercise that were nearly 70–90% of the changes seen during maximal stress tests. Submaximal tests are relatively easy to perform compared to maximal tests. Maximal tests require the supervision of a physician, are time consuming, and require specially trained medical personnel to conduct. In contrast, a 6-min hall walk test may be supervised by a nurse, and can be performed more repeatedly over time and the monitored parameters are subsequently directly available. Such repeat testing may become more practical as more heart failure clinics emerge. Typically, these clinics see their patients frequently and have nurses assess the patient's status and therapeutic regimen [22,23]. Thus, when standardized patient instructions are used to remove subjective bias and variability, these tests are quite reproducible and likely reflect the patient's exercise capability at the time. Looked upon in another way observations from monitoring of daily activity, may, when compared to data from submaximal exercise tests, give a good estimate on how heavy daily activity actually is for a patient. Comparisons over time may also serve as an indicator on deterioration or improvement.

Since it remains unclear whether any artificial clinical test truly mimics activities of daily life, the ambulatory data available from the implantable device could provide even better indications of the patient's exercise limitations. The IHM device reported here also measures and stores physical activity levels and this measure can be used to quantify the level of activity a patient is conducting on a day-to-day basis. Thus, periods when a patient's activity levels are zero may allow day-to-day comparison's of hemodynamic values under resting, baseline conditions. Additionally, the activity signal can allow correlation of the magnitude of the hemodynamic changes occurring throughout the day to the level of physical activities.

The possibility readily exists to combine assessment of responses during activities of daily living and structured exercise stress. Patients with implanted hemodynamic monitors could be instructed to perform a predefined activity systematically each day. Activities may be selected which are a part of the patient's ordinary life such as routine walking or walking up and down stairs. This could support patient assessment without more costly in-clinic stress tests. Furthermore, data retrieved from the implantable monitor at the time of clinic visits may, in the future, be retrieved trans-telephonically from the patient's home.

Heart failure patients enrolled in the device feasibility study reported here, did not have their care plans or therapies modified by the data available from the IHM, because the monitors’ accuracy and performance were still investigational. Thus, whether or not actual patient care will be positively impacted by the use of such devices remains speculative. Subsequent studies are under way to further evaluate the safety and utility of such devices.

In conclusion, these data indicate that continuous hemodynamic monitoring providing information on filling pressures and mixed venous oxygen content is valuable for the understanding of hemodynamics during daily activities. Devices allowing such observations may be of great value in the study of patients with severe heart failure subjected to various types of treatments whose effects require careful observation beyond physical signs and symptoms. Moreover, such systems could certainly be of great interest during the development and testing of new pharmacological and technical tools to improve heart failure treatment. Whether these assumptions become true remains to be demonstrated by additional, more outcome oriented, clinical trials.


   Acknowledgements
 
We gratefully acknowledge the clinical support of Spencer Kubo, MD, Linda Nelson, RN, and Debbie Cardinal, RN. This study was supported, in part, by grants from the Swedish Heart–Lung Foundation.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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F. Braunschweig, C. Linde, P. B. Adamson, A. Magalski, E. Erdmann, B. Kjellstrom, and T. Bennett
Continuous central haemodynamic measurements during the six-minute walk test and daily life in patients with chronic heart failure
Eur J Heart Fail, June 1, 2009; 11(6): 594 - 601.
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