Should a VVIR Pacemaker Increase the Heart Rate with Standing?

Effect of skin electrode location on radiofrequency ablation lesions: an in vivo and a three-dimensional finite element study

INTRODUCTION

OBJECTIVES

To assess the effect of skin electrode location on radiofrequency (RF) ablation lesion dimensions and energy requirements.

BACKGROUND

Little is known about the effects of skin electrode location on RF ablation lesion dimensions and efficiency.

METHODS AND RESULTS

Temperature-controlled ablation at 60 degrees C for 60 seconds was performed in six sheep. Paired lesions were created in the lateral, anterior, posterior, and septal walls of both the ventricles. For group 1 lesions, the skin electrode was positioned directly opposite the catheter tip (optimal). For group 2 lesions, we used either the standard posterior location or an anterior location if the posterior skin electrode location was used for group 1. Group 1 lesions were 5.8+/-0.8 mm deep and 9.3+/-1.9 mm wide, compared with 4.6+/-1.0 mm deep and 7.7+/-1.9 mm wide group 2 lesions (P < or = 0.001). Group 1 lesion dimensions also had less variability. A finite element model was used to simulate temperature-controlled ablation and to study the effect of skin electrode locations on lesion dimensions, ablation efficiency, and blood heating. The optimal location was 1.6 times more efficient, and the volume of blood heated to > or = 90 degrees C was 0.005 mm3 for optimal versus 2.2 mm3 for the nonoptimal location.

CONCLUSION

Optimal skin electrode placement: (1) creates deeper and larger lesions; (2) reduces lesion size variability; and (3) decreases blood heating.

Rate adaptive cardiac pacing using right ventricular venous oxygen saturation: quantification of chronotropic behavior during daily activities and maximal exercise

Central venous oxygen saturation (SvO2) closely reflects cardiac output and tissue oxygen consumption. In the absence of an adequate chronotropic response during exercise, SvO2 will decrease and the extent of desaturation may be used as a parameter for rate adaptive cardiac pacing. Eight patients with sinoatrial disease received a DDDR pacemaker capable of DDDR pacing by sensing either SvO2 or piezoelectric detected body movement. Both sensors were programmed to attain a rate of about 100 beats/min during walking, and with the lower and upper rates set at 50% and 90% of age predicted maximum, respectively. Chronotropic behavior of the two sensors were compared in the DDD mode with measurement of sensor responses, during everyday activities (walking, stair climbing, postural changes, and physiological stresses) and at each quartile of workload during a continuous treadmill exercise test. During walking at 2.5 mph, both sensors showed no significant difference in delay time (both react within 15 secs) or half-time (SvO2 = 36 +/- 12 sec and activity 24 +/- 3 sec; P = NS), although SvO2 driven pacing achieved 90% target rate response slower than activity sensing (124 +/- 16 sec vs 77 +/- 10 sec; P < 0.02). SvO2 pacing was associated with a more physiological rate response during walking upslope (68 +/- 12 beats/min vs 57 +/- 10 beats/min; P < 0.05), ascending stairs (59 +/- 10 beats/min vs 31 +/- 6 beats/min; P < 0.05), and standing (34 +/- 7 beats/min vs 9 +/- 2 beats/min; P < 0.05). The SvO2 sensor significantly overpaced in the first quartile of exercise (51.8 +/- 25.6% in excess of heart rate expected from workload), but the rate was within 20% of expected for the remainder of exercise. "Underpacing" was observed with the activity sensor at the higher workload. In conclusion, the SvO2 sensor demonstrated a more physiological response to activities of daily living compared with the activity sensor. Using a quantitative method, the speed of onset of rate response of the SvO2 sensor was comparable to activity sensing, and was more proportional in rate response. Significant overpacing occurs at the beginning of exercise during SvO2 driven pacing, which may be improved with the use of a curvilinear algorithm.

Adaptive rate pacing controlled by right ventricular preejection interval for severe refractory orthostatic hypotension

A 72-year-old African-American man with frequent recurrent syncope was found to have severe refractory orthostatic hypotension with concomitant supine hypertension. Pharmacotherapy was successful in controlling his supine hypertension but was unable to resolve his severe orthostatic hypotension. Temporary fixed rate tachypacing was only minimally effective in preventing syncope during upright tilt, while variable rate pacing based on degree of blood pressure fall was far superior. Following these observations, an adaptive rate pacing system controlled by right ventricular preejection interval was implanted (Precept DR Model 1200). The system adequately sensed the patient's fall in blood pressure when sitting or standing and augmented its rate accordingly, thus preventing syncope. While supine, the pacing rate fell to 60 ppm, thereby, avoiding an exacerbation of his concomitant supine hypertension. Over a 3-month follow-up period, he has had no further orthostatic or syncopal episodes. We conclude that adaptive rate pacing using right ventricular preejection interval may be an effective treatment for severe refractory orthostatic hypotension.