Continuous ambulatory hemodynamic monitoring with an implantable system : the feasibility of a new technique

Abstract

Patients with chronic heart failure constitute an increasingly large group. Despite recent therapeutic improvements, these patients have a poor prognosis. The diagnosis and the often complex pharmacological treatment are commonly evaluated and guided by physical examinations and non-invasive measurements, although the relationship between clinical signs and central hemodynamics is less obvious during the chronic phase of the disease. Access to invasive hemodynamic information should be of importance, but traditional catheterisations only give a snapshot of a supine patient. Long-term monitoring should be useful but is difficult to achieve with the conventional catheterisation techniques. The present study was performed to evaluate an implantable hemodynamic monitoring system, capable of long-term measurements and storage of recorded data. Development of the system was based on available pacemaker technology and the external design was similar to an ordinary pacing system. Sensors for continuous measurements of oxygen saturation and pressure were incorporated in leads that were connected to a monitor and memory unit contained within an ordinary pacemaker can. The sensor-carrying leads were positioned in the right ventricle, where mixed venous oxygen saturation could accurately be measured compared to values obtained from the pulmonary artery. Diastolic pulmonary arterial pressure could be adequately estimated from the right ventricular pressure at the time of maximum dP/dt. These findings showed that important hemodynamic parameters could be obtained from a right ventricular lead position avoiding the 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 pressure values was implanted in five patients with severe cardio-pulmonary disease during seven to 16 months. Another system in which the absolute pressures could be recorded was implanted in 21 patients with severe heart failure. Acceptable recordings were obtained for at least 12 months from 18 of these 26 systems without any serious monitor-related complications. The patients accepted the implanted monitors without concern.

Recordings from the pressure sensors for absolute values were strongly correlated to pressure obtained from a Swan Ganz catheter during repeated catheterisations. Directional and magnitudinal changes in pressure were adequately expressed by the sensor for relative values. The oxygen sensor failure rate was unacceptably high although the accuracy and long-term stability of the non-failing sensors were adequate compared with oxygen saturation in blood, sampled from the pulmonary artery. The monitoring system was capable of recording and storing hemodynamic parameters with variable resolution, from low to high. During long-term recordings during daily life the recorded parameters adequately reflected hemodynamic changes related to everyday activities. Based on oxygen saturation values from the implantable monitor, cardiac output could be "semi-invasively" obtained with a strong correlation with cardiac output determined during conventional catheterisations. Long-term ambulatory hemodynamic monitoring was feasible and recorded data may be of value in the clinical management of heart failure.