Rate adaptive cardiac rhythm management device using transthoracic impedance

Abstract

A cardiac rhythm management (CRM) device detects transthoracic impedance, extracts ventilation or other information, and adjusts a delivery rate of the CRM therapy accordingly. A four-phase sequence of alternating direction current pulse stimuli is periodically delivered to a patient's thorax. A transthoracic impedance signal is extracted using a weighted demodulation. Signal processing extracts ventilation information and removes cardiac stroke information using an adaptive lowpass filter. The adaptive filter cutoff frequency is based on the patient's heart rate; a higher cutoff frequency is provided for higher heart rates. Peak/valley detection indicates tidal volume, which is integrated to extract minute ventilation (MV). Short and long term averages are formed and compared to establish a MV indicated rate. Rate adjustment ignores MV information when a noise-measurement exceeds a threshold. An interference avoidance circuit delays delivery of the stimuli when telemetry pulses or other interfering signals are detected.

What is claimed is:  
     
         1 . A method of determining transthoracic impedance in a cardiac rhythm management device, the method comprising: 
 repeatedly delivering a multiple phase stimulus to a thorax region of a patient; and    demodulating more than one phase of the multiple phase stimuli to obtain sample points of a response signal including transthoracic impedance information.    
     
     
         2 . The method of  claim 1 , in which demodulating includes sampling a response to each phase of the multiple phase stimulus, and combining the sampled responses.  
     
     
         3 . The method of  claim 2 , in which sampling the response includes weighting the samples to obtain a filtering function.  
     
     
         4 . The method of  claim 1 , in which repeatedly delivering a multiple phase stimulus includes repeatedly delivering an at least four phase stimulus.  
     
     
         5 . The method of  claim 4 , in which demodulating includes sampling a response to each phase of the multiple phase stimulus, combining the sampled responses, and weighting second and third samples more than first and fourth samples responsive to the at least four phase stimulus.  
     
     
         6 . The method of  claim 5 , in which weighting includes weighting each of second and third samples approximately 3 times more than each of first and fourth samples responsive to the four phase stimulus.  
     
     
         7 . The method of  claim 1 , further comprising adjusting a rate of delivering cardiac rhythm management therapy based on ventilation information included in the transthoracic impedance information of a plurality of the sample points.  
     
     
         8 . The method of  claim 7 , further comprising: 
 demodulating a noise-response signal in the absence of the stimuli;    comparing the noise-response signal to a threshold value to determine whether the noise-response signal exceeds the threshold value; and    wherein the step of adjusting a rate of delivering cardiac rhythm management therapy includes delivering the cardiac rhythm management therapy independent of the ventilation information when the noise-response signal exceeds the threshold value.    
     
     
         9 . The method of  claim 1 , in which repeatedly delivering a multiple phase stimulus includes repeatedly delivering a multiple phase stimulus having alternating polarity phases.  
     
     
         10 . The method of  claim 9 , in which delivering a multiple phase stimulus includes repeatedly delivering a multiple phase stimulus having alternating direction current pulse phases.  
     
     
         11 . The method of  claim 1 , in which repeatedly delivering a multiple phase stimulus includes repeatedly delivering a square wave stimulus.  
     
     
         12 . The method of  claim 1 , in which repeatedly delivering a multiple phase stimulus includes repeatedly delivering a square wave including four alternating direction current pulses having an amplitude magnitude that is less than a tissue stimulation threshold.  
     
     
         13 . The method of  claim 1 , in which the amplitude magnitude is less than approximately 1 milliampere.  
     
     
         14 . The method of  claim 13 , in which the amplitude magnitude is approximately 320 microamperes.  
     
     
         15 . The method of  claim 1 , in which the duration of each phase of the multiple phase stimulus is approximately between 1 and 100 microseconds.  
     
     
         16 . The method of  claim 15 , in which the duration of each phase of the multiple phase stimulus is approximately 20 microseconds.  
     
     
         17 . The method of  claim 1 , in which repeatedly delivering a multiple phase stimulus includes waiting for a time period between successive deliveries of the multiple phase stimulus.  
     
     
         18 . The method of  claim 17 , in which the time period is less than approximately 55 milliseconds.  
     
     
         19 . The method of  claim 1 , further comprising: 
 detecting an interference signal; and    delaying delivery of the multiple phase stimulus when the interference signal is detected.    
     
     
         20 . The method of  claim 19 , in which detecting an interference signal includes detecting a telemetry signal, and delaying delivery of the multiple phase stimulus when the interference signal is detected includes delaying delivery of the multiple phase stimulus when the telemetry signal is detected.