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European Journal of Heart Failure 2004 6(5):627-634; doi:10.1016/j.ejheart.2003.12.021
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© 2004 European Society of Cardiology

Six years follow-up of an implanted SvO2 sensor in the right ventricle*

Barbro Kjellströma,*, Cecilia Lindeb, Tom Bennetta, Åke Ohlssonc and Lars Rydenb

a Heart Failure Management, Medtronic Inc. MS CW210, Minneapolis, MN 55432, USA
b Karolinska Hospital Stockholm, Sweden
c Southern Hospital Stockholm, Sweden

* Corresponding author. Tel.: +1-673-514-8219; Fax: +1-763-514-8347. E-mail address: barbro.kjellstrom{at}medtronic.com


   Abstract
Top
 Notes
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
Introduction.: Mixed venous oxygen saturation (SvO2) is a standard invasive measure used in the management of congestive heart failure patients. The reliability of a long-term SvO2 sensor remains unproven.

Methods.: Nine patients (NYHA Class I/II, n=2/7) were implanted with a dual chamber pacemaker modified to utilize a right ventricular SvO2 lead (Medtronic Inc., Models 8007/4327A IPG/Lead). Invasive studies compared sensor SvO2 to reference (Optical Swan–Ganz catheter) at 0, 3 and 9 months. Symptom limited tests (Bikemax) with metabolic assessment and arterial oxygen saturation measurements performed 1–7 days, 3.5 and 9.5 months post-implant allowed for cardiac output calculations. Long-term sensor performance was confirmed by submaximal tests, Bikesubm in years 1–3, and Walkin-place every 6 months for the duration of follow-up.

Results.: Sensor SvO2 readings were stable over time when compared to the Swan–Ganz Catheter. Non-invasive CO measured during Bikemax was in normal ranges for this patient population, 3.7±0.9 l/min at rest and 8.4±2.2 l/min at peak-exercise. Resting SvO2 values from Bikesubm and Walkin-place did not change significantly over time (P0.1 vs. 1 year) and neither did the change from rest to peak exercise during Bikesubm (P0.05 vs. 1 year) or Walkin-place (P0.05 vs. 4 year).

Conclusion.: While limited in size, this small pilot study suggests that long-term monitoring of SvO2 by implanted devices may be feasible. The clinical value remains to be proven in future studies.

Key Words: Abbreviations • CO, cardiac output • PA, pulmonary artery • RV, right ventricle • SaO2, arterial oxygen saturation • SvO2, mixed venous oxygen saturation • S.D., standard deviation • VO2, oxygen uptake

Received June 26, 2003; Revised October 16, 2003; Accepted December 10, 2003


   1. Introduction
Top
 Notes
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
1.1. Mixed venous oxygen saturation
Mixed venous oxygen saturation (SvO2) measured in the pulmonary artery is an average of the venous oxygen saturations of the body. It reflects the balance between oxygen supply and demand and might be used for diagnostic decisions, therapeutic guidance, prognostic prediction and, in combination with oxygen uptake and arterial oxygen saturation to determine cardiac output according to Fick. The introduction of the Swan–Ganz pulmonary artery (PA) catheter with fiberoptic oximetry simplified the measurement of mixed venous oxygen saturation and started ongoing discussion of the value of recording SvO2 in the clinical setting. Early studies showed that SvO2 changes, reflected changes in cardiac output (CO; Refs. [1,2]). This was, however, not confirmed in subsequent studies [3–5]. Nevertheless, it may be speculated that SvO2 changes could guide therapy and be a reliable diagnostic tool, particularly in combination with other diagnostic methods [6].

SvO2 measurements from the right ventricle (RV) accurately reflect measurements derived in the PA [7,8]. Initially right ventricular venous oxygen saturation was evaluated as a method for rate adaptive cardiac pacing [9–12]. It was demonstrated that the SvO2 was a physiologically acceptable sensor. Lack of long-term sensor stability was, however, a drawback causing this concept to be abandoned. Ohlsson et al. [13,14], in their attempts to construct a hemodynamic monitoring system, implanted a permanent sensor with the capability to measure both SvO2 and pressures in the right ventricle. This allowed continuous monitoring of hemodynamic parameters in patients with cardio-pulmonary disease and easily accessible, ‘non-invasive’ CO measurements both at rest and during exercise at clinic visits [15]. Although the short term accuracy and stability of an implantable SvO2 sensor have already been proven in previous studies [7–15], the long-term stability has not been demonstrated.

The objective of the present study was to confirm short and long-term stability of a modified pacemaker lead allowing measurement of SvO2 from the right ventricle.


   2. Methods
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 5. Conclusion
 References
 
2.1. Device description
The OxyElite (Model 8007, Medtronic Inc., Minneapolis, USA) was a dual chamber (DDD), unipolar, rate responsive pacemaker modified to measure SvO2. Rate-response was provided by an activity sensor, a piezoelectric crystal mounted on the inside of the pacemaker can. The ventricular oxygen sensing lead was a polyurethane permanent pacing lead with a porous platinum steroid tip (Model 4327A, Medtronic Inc., Minneapolis, USA). The oxygen sensor portion of the lead was a sealed capsule containing red and infrared emitting diodes, a photo detector and an integrated circuit located approximately 3 cm from the lead tip. Oxygen saturation was detected as a ratio of red/infrared light reflection, as described earlier [9]. Standard implanting technique was used and the ventricular lead tip placed in the RV apex.

At each follow-up, pacemaker parameters and SvO2 histograms stored in the device memory were retrieved by interrogation using a pacemaker programmer (Model 9760, Medtronic Inc., Minneapolis, USA). The oxygen saturation histogram feature in the OxyElite continuously measured and stored the SvO2 values at 4-s intervals during ambulatory conditions and sorted the measurements into eight SvO2 histogram bins. The bins ranged from <49% up to >77% in steps of 4%. When the programmer telemetry head was placed over the pacemaker, real time SvO2 could be read on the programmer screen and stored, this was used to record SvO2 levels during the exercise tests required in the study.

Since this study was intended to evaluate the sensor performance independently from pacing, no data on the pacemaker performance is reported.

2.2. Patient selection
Following informed written consent, patients were included if they had a traditional indication for pacemaker implantation. If the patients were not already on anticoagulants or aspirin, such treatment was recommended to start at time of implant. This investigation conformed to the principals outlined in the Declaration of Helsinki and the local ethics committee approved the protocol.

2.3. Study protocol-first year
2.3.1. Oxygen sensor performance
Mixed venous oxygen saturation recorded by means of an optical Swan–Ganz catheter (Opticath, Oximetrix system, Abbot Laboratories, Chicago, USA) was used as a reference at the time of implantation and 3 and 9 months thereafter to verify the ventricular oxygen sensor performance. Measurements were made simultaneously by the OxyElite, the Swan–Ganz catheter and by blood samples drawn from the pulmonary artery. At implant, recordings were only made at rest, while at the 3- and 9-month follow-up, SvO2 measurements were obtained at rest, after 5-min of supine sub-maximal bicycle exercise and following recovery after exercise.

2.3.2. Symptom limited bike test and cardiac output measurements
A symptom limited maximal bicycle ergometry test was performed within the first week after implantation and repeated after 3.5 and 9.5 months. Simultaneous metabolic assessment (CPX, MedGraphics, St Paul, USA) and pulse oximetry SaO2 (Ohmeda, Louisville, USA) measurements allowed calculation of cardiac output (CO) according to a modified Fick principle previously validated by Ohlsson et al. [15].

2.4. Study protocol-long-term
2.4.1. Oxygen sensor stability
To demonstrate the stability of the SvO2 response over time, sub-maximal bicycle exercise tests (5 min at 60% of the previously determined maximal workload) were performed every 2–4 weeks during the first year of follow up and subsequently every 6 months for another 2–2.5 years. During the first year, SvO2 was always recorded continuously throughout exercise. Thereafter, SvO2 was recorded at supine rest before exercise, sitting on the bicycle, at peak exercise and following recovery after exercise.

After 3–3.5 years of follow up the sub-maximal bicycle tests were replaced by a ‘walking-in-place’ procedure performed at each 6-month follow-up visit for the remaining time of follow-up. This test was uncontrolled exercise that included SvO2 measurements at supine rest and after 30–60 s of light exercise. The actual level of exercise may have varied at different visits. Decreases in oxygen saturation during exercise were calculated as the difference from supine rest prior to exercise to the peak response at the end of the exercise.

2.4.2. Chronic, ambulatory oxygen sensor data
The data from the oxygen histograms retrieved from the device memory were compared over time to study trends in ambulatory oxygen saturation in each individual patient. Time spent at various oxygen saturations could be calculated as a proportion of the total time by comparing the number of counts in each bin with the total number of counts recorded since last follow-up. During the first year the patients were followed every 2–4 weeks and thereafter every 6 months. The histograms during the first year were merged to equalize the evaluation period (6-month periods for all patients except #3 and #5, for whom the periods were 2 months).

2.4.3. Statistical methods
Data are reported as mean±standard deviation (S.D.). Student's paired t-test was used to determine differences between measured variables. Differences were considered significant at P<0.05. The correlation between sensor SvO2 values and those obtained from the Opticath and blood samples were evaluated according to a linear regression model.


   3. Results
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 5. Conclusion
 References
 
3.1. Patient characteristics
Nine consecutive patients (mean age 69±14 years) were implanted with the OxyElite DDD pacemaker during 1994. The indications for pacing are presented in Table 1 together with other pertinent patient characteristics.


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  		Table 1 Patient characteristics

 
All pacemaker implantations were uneventful. The oxygen sensor lead was implanted with the distal tip in the RV apex. The pacing thresholds and electrograms from the ventricular lead were in the standard range. There were no pacemaker related adverse events during the full course of the study.

One patient (#3) was previously diagnosed with pulmonary sarcoidosis and was thus expected to have low SvO2 levels compared to normal (65–75%), this patient also developed chronotropic incompetence during the course of the study. To enable optimal pacemaker programming heart rate histograms were therefore favored over the SvO2 data following the 6-month follow-up. Another patient (#7) had a failing oxygen sensor response after 6 weeks and the function did not return during the remaining period of follow up. One patient (#4) died 8 months after pacemaker implant due to myocardial infarction. Two additional patients died after 1.9 (#5) and 4.3 (#6) years of follow-up and one patient (#2) expressed a wish to withdraw from all medical therapies and discontinued follow-ups after 4.3 years. The four remaining patients completed 6 years of follow-up.

3.2. Oxygen sensor performance
A total of 24 invasive tests from the nine patients were evaluated to verify oxygen sensor performance during the first year of follow-up. The SvO2 readings from the implanted oxygen sensors showed high correlations with those from the blood samples (Fig. 1A) and from the Swan–Ganz Opticath SvO2 (Fig. 1B). Based on the serial invasive data, resting SvO2 readings from the implanted system were comparable at each catheterization during the first year of follow up (Table 2). The SvO2 measured with the implanted oxygen sensor was slightly lower (2.5–5.5%) compared to the levels derived from blood and Opticath.


Figure 1
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  		Fig. 1 Panel (A): the correlation between SvO2 measurements by the implanted SvO2 sensor and pulmonary artery blood SvO2. Panel (B): the correlation between SvO2 measurements by the implanted SvO2 sensor and the Swan–Ganz Opticath. Values are from rest and exercise performed at implant (only rest), 3 and 9 months after implant. Both methods demonstrate a good correlation, although a greater variability was observed during exercise (lower SvO2 values).

 


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  		Table 2 SvO2 measured simultaneously by the OxyElite, the Swan–Ganz opticath and blood samples drawn in the pulmonary artery

 
3.3. Symptom limited bicycle exercise test and cardiac output measurement
All patients performed the first post-implant test within 1 week and seven patients performed the 3.5- and 9.5-month tests, resulting in a total of 23 symptom limited bicycle tests. One patient (#6) showed signs of silent ischemia (ST segment depression >4.5 mm) at peak workload. For safety reasons the 3.5 months bike test was omitted in this patient.

The SvO2 measured during supine rest was stable over time and the SvO2 sensor showed a reproducible response to posture changes and exercise in all patients.

Mean resting CO was 4.0±1.3 l/min, 3.3±0.8 l/min and 3.8±0.7 l/min post-implant and at 3.5 and 9.5 months, respectively. The corresponding values were 8.1±2.1, 8.4±2.6 and 8.8±2.0 l/min at peak exercise. These values were in the normal range for all patients.

3.4. Oxygen sensor stability during long-term follow-up
Mixed venous oxygen saturations measured at rest prior to sub-maximal exercise tests and ‘walking-in-place’ tests and the changes during the tests are summarized in Table 3. Recordings from the test performed closest to the annual follow-up were averaged. The resting SvO2 values were stable over the 6 years of follow-up (P>0.1 vs. the 1 year follow-up) and the response to the exercise showed a good reproducibility. Eighty-eight submaximal bicycle tests and 23 ‘walking-in-place’ tests were performed during the study. Fig. 2 illustrates the resting values together with changes in SvO2 during the submaximal bicycle tests for individual patients.


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  		Table 3 Resting values and changes in SvO2 (%) during sub-maximal bicycle or walking-in-place tests over time presented as mean±S.D.

 


Figure 2
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  		Fig. 2 Individual SvO2 recordings made with the implanted SvO2 sensor from all the sub-maximal bike tests performed within the first 2.5 years from eight of the nine implanted patients. The graphs illustrate each test and individual mean±S.D. These tests were performed over a period of several years. Thus the patient's condition may vary as well as other conditions, e.g. weather and time of day for the test. This may explain the variability seen in SvO2 at rest and exercise.

 
3.5. Chronic, ambulatory oxygen sensor data during long-term follow-up
Six patients had oxygen saturation histogram data successfully collected during 1.5–6 years. Each patient had individual distribution patterns comparing SvO2 histograms over time. The SvO2 was distributed over the range of bins in all six patients and usually a majority of time was spent below an SvO2 of 70%. Examples of SvO2 histogram patterns from four individual patients are shown in Fig. 3. Panel A (patient #1) has stable oxygen saturation over time with a slight trend towards lower oxygen saturations at the 30-month follow-up. In Panel B, (patient #2) the changes in oxygen saturation levels correlated to changes in the patient's clinical condition. Between 6 and 12 months of follow-up the patient deteriorated with dyspnea and angina chest pain. An atrial flutter was discovered at the 8-months follow-up and the pacemaker was programmed from dual to single chamber pacing (VVI 70). Two weeks after the 12-months visit the patient underwent coronary bypass surgery and mitral valve replacement with serious post-operative complications. The patient spent the next 6 months in a nursing home for rehabilitation. Thereafter the patient's condition improved, however, with limited exercise capacity and remaining atrial flutter. Meanwhile the SvO2 histograms showed that the oxygen saturation was shifting towards more time spent at lower saturation levels. Panel C (patient #3) illustrates a patient diagnosed with sarcoidosis. The relatively high counts of SvO2 in the lower value range, <54%, could therefore be expected in this patient. Four months after implant the patient complained of more dyspnea and an exercise test revealed chronotropic incompetence. Subsequently, the pacemaker rate response feature was activated. The patient continued to experience dyspnea and at the 6-month visit the histogram was programmed to store heart rate to help optimize rate–response settings.


Figure 3
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  		Fig. 3 Distribution of oxygen saturation presented as percent of time spent at different oxygen saturation intervals in individual patients. Panel (A)=patient #1, Panel (B)=#2, Panel (C)=#3 and Panel (D)=#5. Each line represents either a 6 months follow-up period (Panel A and B) or a 2 months follow-up period (Panel C and D).

 
The SvO2 histogram trends shown in Panel D (patient #5) indicate a distribution with more than half of the time spent at SvO2 levels lower than 49%. Over time the upper range of the SvO2 distribution shifted from an initial 20% of the time spent at an SvO2 range of 64–68%, gradually shifting towards a SvO2 in the range of 60–63% after six months. The condition of this patient slowly deteriorated during the study with increasing claudication and heart failure as a possible reason for the changes in SvO2 over time.


   4. Discussion
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
5. Conclusion
 References
 
This is the first long-term study to demonstrate the reliability of the implantable SvO2 sensor in resting and exercise situations during 6 years of follow-up. Our first year results correspond to similar studies using an earlier version of the implantable SvO2 sensor [7–15]. Thus there was a good correlation compared to standard, invasive techniques during the first year with a small or no drift in the SvO2 values. The stability was sustained over the long-term follow-up.

In the present study resting SvO2 sensor derived values were somewhat lower than those from the optical Swan–Ganz values or blood samples. A prior report suggested similarity between SvO2 values derived from the distal PA and the RV outflow tract [7]. The present small discrepancy may depend on a theoretical possibility of a difference in SvO2 values from the PA and those obtained in the vicinity of the RV apical position, which, in the present study, was used for RV pacing. Sensor SvO2 values in the lowest range (<25%) varied more from values derived from the blood samples than values in the normal and subnormal ranges (Fig. 1). Fiberoptic techniques are generally known to be somewhat less reliable in the extreme lows. Moreover, the resting recordings were collected over a longer and more stable period while the lowest values were collected during a brief period of a few seconds during peak exercise. Accordingly the discrepancy between measurements may relate not only to deficiencies in the methods applied but also to physiological variability or restricted experimental conditions.

In contrast to previous experiences the present sensor did not show drift over long-term follow up [9–12]. The oxygen sensor used in the present study was slightly modified from previous models used in pacemaker studies, to improve measurement of reflected light and to be less susceptible to tissue overgrowth. Importantly, the long-term stability over as much as 6 years was obtained with the use of original factory-provided calibration factors. Thus no adjustment was made for the small initial offset and none of the sensors were recalibrated during the period of follow-up. The stability and reproducibility of the SvO2 sensor was tested non-invasively by repeated sub-maximal exercise testing approximately every 6 months, including four patients followed as long as 6 years. Resting SvO2 showed baseline stability and a high reproducibility over time. The change in SvO2 during sub-maximal exercise varied over time, however, the variations were all within the expected physiological range.

The modified oxygen sensor used in the present study is the same as the one used in a subsequent feasibility study of an implantable hemodynamic monitor [14]. Surprisingly and in contrast to our findings, and irrespectively of ASA or other anticoagulants, 40% of the SvO2 sensors in that study stopped responding within the first year without any apparent technical explanation. Whereas the present OxyElite study used one RV lead, the implantable hemodynamic monitor study used two, one for an SvO2 sensor and one for a pressure sensor. Although it cannot be ruled out it seems unlikely that this difference could explain the difference in sensor response.

The oxygen saturations collected while the patients were ambulatory showed interesting variations, both between individuals and within the individual patient. The devices’ histograms provided an indirect measurement of the activity levels and clinical condition over time and between out patients visits. In the 6 years of follow-up changes in the histogram patterns seemed to agree with changes in the patients’ medical conditions.

Despite the obvious limitations in this small patient material the pacemaker system used for the present study allowed for observations of long-term, sustained responsiveness of the SvO2 sensors. The clinical usefulness of an implantable SvO2 sensor remains to be established in a larger patient group.


   5. Conclusion
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusion
References
 
Measurement of mixed venous oxygen saturation with a sensor implanted in the right ventricle is feasible and safe. Oxygen saturation levels correlate well between the implanted sensor and SvO2 obtained with optical Swan–Ganz catheters or blood samples. Long-term stability determined by comparison of resting values and responses to submaximal exercise was reproducible over as long as 6 years of follow up.


   Notes
Top
 Notes
Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
{star} Supported by Medtronic, Bakken Research Center, Maastricht, The Netherlands and the Swedish Heart and Lung Foundation. Back


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

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  9. Lau C.P., Tai Y.T., Leung W.H., Leung S.K., Li J.P., Wong C.K., et al. Rate adaptive cardiac pacing using right ventricular venous oxygen saturation: quantification of chronotropic behavior during daily activities and maximal exercise. Pacing Clin Electrophysiol (1994) 17:2236–2246.[CrossRef][Medline]
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  12. Windecker S., Bubien R.S., Halperin L., Moore A., Kay G.N. Two-year experience with rate-modulated pacing controlled by mixed venous oxygen saturation. Pacing Clin Electrophysiol (1998) 21:1396–1404.[CrossRef][Medline]
  13. Ohlsson A., Bennett T., Otttenhoff F., Bitkover C., Kjellstrom B., Nordlander R., et al. Continuous ambulatory monitoring with an implantable system. The feasibility of a new technique. Eur Heart J (1998) 19:174–184.[Abstract/Free Full Text]
  14. Ohlsson A., Kubo S., Steinhaus D., Connelly D., Adler S., Bitkover C., et al. Continuous ambulatory monitoring of absolute right ventricular pressure and mixed venous oxygen saturation in patients with heart failure using an implantable hemodynamic monitor: results of a 1 year multicenter feasibility study. Eur Heart J (2001) 22:942–954.[Abstract/Free Full Text]
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