Continuous ambulatory hemodynamic monitoring with an implantable system : The feasibility of a new technique
Abstract: Continuous Ambulatory Hemodynamic Monitoring with an ImplantableSystem The Feasibility of a New Technique Åke Ohlsson Patients with chronic heart failure constitute an increasingly large group. Despiterecent therapeutic improvements, these patients have a poor prognosis. The diagnosisand the often complex pharma cological treatment are commonly evaluated and guidedby physical examinations and non-invasive measurements, although the relationshipbetween clinical signs and central hemodynamics is less obvious during the chronicphase of the disease. Access to invasive hemodynamic information should be of importance,but traditional catheterisations only give a snapshot of a supine patient. Long-termmonitoring should be useful but is difficult to achieve with the conventional catheterisationtechniques. The present study was performed to evaluate an implantable hemodynamicmonitoring system, capable of long-term measurements and storage of recorded data.Development of the system was based on available pacemaker technology and the externaldesign was similar to an ordinary pacing system. Sensors for continuous measurementsof oxygen saturation and pressure were incorporated in leads that were connectedto a monitor and memory unit contained within an ordinary pacemaker can. The sensor-carryingleads were positioned in the right ventricle, where mixed venous oxygen saturationcould accurately be measured compared to values obtained from the pulmonary artery.Diastolic pulmonary arterial pressure could be adequately estimated from the rightventricular pressure at the time of maximum dP/dt. These findings showed that importanthemodynamic parameters could be obtained from a right ventricular lead position avoidingthe pulmonary artery, which may impose a risk of dislodgement during long-term monitoring. A monitoring system with one oxygen sensor and one sensor for relative pressurevalues was implanted in five patients with severe cardio-pulmonary disease duringseven to 16 months. Another system in which the absolute pressures could be recordedwas implanted in 21 patients with severe heart failure. Acceptable recordings wereobtained for at least 12 months from 18 of these 26 systems without any serious monitor-relatedcomplications. The patients accepted the implanted monitors without concern. Recordings from the pressure sensors for absolute values were strongly correlatedto pressure obtained from a Swan Ganz catheter during repeated catheterisations.Directional and magnitudinal changes in pressure were adequately expressed by thesensor for relative values. The oxygen sensor failure rate was unacceptably highalthough the accuracy and long-term stability of the non-failing sensors were adequatecompared with oxygen saturation in blood, sampled from the pulmonary artery. Themonitoring system was capable of recording and storing hemodynamic parameters withvariable resolution, from low to high. During long-term recordings during daily lifethe recorded parameters adequately reflected hemodynamic changes related to everydayactivities. Based on oxygen saturation values from the implantable monitor, cardiacoutput could be "semi-invasively" obtained with a strong correlation withcardiac output determined during conventional catheterisations. Long-term ambulatoryhemodynamic monitoring was feasible and recorded data may be of value in the clinicalmanagement of heart failure. Key words: heartfailure, hemodynamic monitoring, implantable, oxygen saturation,pressure, ambulatory, right ventricle, dP/dt, long-term, sensor, cardiac output. ISBN 91-628-2602-6
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