Apparatus and method for monitoring pressure related changes in the extra-thoracic arterial circulatory system

Abstract

A method and apparatus for monitoring changes in the intra-thoracic pressure of a patient due to the patient's respiratory activity or volumetric changes in the extra-thoracic arterial circulatory system due to cardiac function based on the changes in pressure in the patient's extra-thoracic arterial circulatory system as measured by a plethysmography sensor, such as an photoplethysmograph. A frequency spectrum is generated for the plethysmograph signal and the frequencies of interest is isolated from the frequency spectrum by setting appropriate cutoff frequencies for the frequency spectrum. This isolated frequency is used to filter the plethysmograph signal to provide a signal indicative of the patient's respiratory activity or cardiac function.

What is claimed is: 
     
         1 . A non-invasive cardiac monitoring method comprising:
 passing light through a portion of a patient's extra-thoracic arterial circulation;   receiving light after having been passed through such a patient;   outputting a first signal based on the received light; and   producing a cardiac pressure signal as a measure of such a patient's cardiac function by isolating cardiac related pressure variations in the first signal.   
     
     
         2 . The method according to  claim 1 , wherein the cardiac pressure signal is determined substantially continuously. 
     
     
         3 . The method according to  claim 1 , further comprising outputting the cardiac pressure signal in a human perceivable format. 
     
     
         4 . The method according to  claim 1 , wherein producing a thoracic pressure signal comprises:
 determining frequency components of the first signal;   identifying a heart rate frequency component (f HR ) from the frequency components of the first signal; and   filtering the first signal based on the heart rate frequency component f HR  so as to isolate the heart rate frequency component f HR  from the first signal to produce the cardiac pressure signal.   
     
     
         5 . The method according to  claim 4 , wherein determining frequency components of the first signal includes analyzing the first signal with a Fourier transform that generates the frequency components of the first signal. 
     
     
         6 . The method according to  claim 4 , wherein producing a cardiac pressure signal further comprises removing any offset in the frequency components of the first signal. 
     
     
         7 . The method according to  claim 4 , further comprising determining a heart rate of such a patient via a heart rate monitor, and wherein identifying the heart rate frequency component f HR  from the frequency components of the first signal is accomplished based on the output of the heart rate monitor. 
     
     
         8 . The method according to  claim 1 , wherein producing a thoracic pressure signal comprises:
 determining frequency components of the first signal;   identifying a respiratory rate frequency component f RR  from the frequency components of the first signal; and   filtering the physiologic signal based on the respiratory rate frequency component f RR  so as to remove the respiratory rate frequency component f HR  from the physiologic signal, thereby isolating a heart rate frequency component f HR  of the first signal to produce the thoracic pressure signal.   
     
     
         9 . The method according to  claim 8 , further comprising determining a respiratory rate of such a patient via a respiratory rate monitor, and wherein identifying the respiratory rate frequency component f RR  from the frequency components of the first signal is accomplished based on the output of the respiratory monitor. 
     
     
         10 . The method according to  claim 1 , wherein producing a thoracic pressure signal comprises:
 determining frequency components of the first signal, including a respiratory rate frequency component f RR  and a heart rate frequency component f HR ;   identifying a respiratory rate frequency component f RR  from the frequency components of the first signal;   identifying a heart rate frequency component f HR  from the frequency components of the first signal; and   filtering the first signal based on the respiratory rate frequency component f RR  and the heart rate frequency component f HR  so as to isolate the heart rate frequency component f RR  from the first signal to produce the cardiac pressure signal.   
     
     
         11 . The method according to  claim 10 , wherein filtering the first signal includes:
 (1) setting a cutoff frequency (f cutoff ) as f HR −f smear  responsive to the respiratory rate frequency component f RR  being less than the heart rate frequency component f HR ,   (2) setting the cutoff frequency (f cutoff ) as f HR +f smear  responsive to the rate frequency component f RR  being greater than the heart rate frequency component f HR , where f smear  is a predetermined threshold frequency, and   (3) high pass filtering the first signal at the cutoff frequency f cutoff .   
     
     
         12 . The method according to  claim 10 , further comprising:
 determining a respiratory rate of such a patient via a respiratory rate monitor, and wherein identifying the respiratory rate frequency component f RR  from the frequency components of the first signal is accomplished based on the output of the respiratory rate monitor; and   determining a heart rate of such a patient based on a heart rate monitor, and wherein identifying the heart rate frequency component f HR  from the frequency components of the first signal is accomplished based on the output of the heart rate monitor.   
     
     
         13 . The system according to  claim 1 , further comprising identifying an occurrence of at least one of pulsus paradoxis, pulsus alternans, pulsus bisferiens, dicrotic pulse, anacrotic pulse, “waterhammer” pulse or a normal pulse based on the cardiac pressure signal.