Data acquisition instrumentation for heartbeat display in electrocardiogra

Burns, S. K.

Mark, R. G.

English

NASA Technical Reports Server

National Aeronautics and Space Administration 
Grant NGL 22-00 9-304 
QUARTERLY STATUS REPORT 
covering the period 
December 1, 1969 - February 28, 1970 
Submitted by: Stephen K. Burns 
Roger G. Mark 
March 16, 1970 
CASE F I L E  
C O P Y  
Massachusetts Institute of Technology 
Research Laboratory of Electronics 
Cambridge, Massachusetts 0213 9 
https://ntrs.nasa.gov/search.jsp?R=19700015884 2020-03-12T02:37:37+00:00Z
C r ,  . , r ~ . . ~ , ~ i c a ~ i o s  - . ?  3iophysics--Tne T e ~ p o ~ a l .  Stri:cture of Electrophysiological  Dnta 
IiCzrch 16, 1970 
A. i?esexreh Progress 
I,lost of our e f f o r t  has been focused on t he  e l e c t r i c a l  characteriza"c0n 
and dis2lay 02 ectopic  hear tbeats  appearing i n  t he  electrocardiogram. 
The grea te r  pa r t  of t h i s  e f f o r t  has Seen spent in developing data acqui- 
1. Analog ZnterSace 
%Xe desigsn of t he  analog i~pr t -ou 'cput  i n t e r f ace  f o r  our d i g i t a l  data 
processor has been completed. This in te r face  can accommodate 16 analog 
Inpu.t chamnels , 16 d i z i t a l  In;2tzt r; , 4. f u l l y  buffered analog outpwt c h a n n c l ~ ,  
cnd 16 d i g i t a l  ou.tpil;ts. Construction of t he  analog input-output c i r c u i t r y  
has becn caapleted and i s  o;perating nicely .  I.le an t ic ipa te  repackaging it 
i n t o  a companion cabinet  Jco the  Tektronix model 601 display oscil loscope.  
T'nis system w i l l  f i t  inJco a 1%-inch rack cabinet  and i s  highly portable.  
r;h ~ n e  following 14. I. 5l . 2ourth-ye a r  s'tudents i n  E l e c t r i c a l  Engineering 
a r e  pursuing t h e i r  Senior Tncsis within our laboratory under the  joinJL 
supervision of Professors Bunis and Piarli: 
Col les idis ,  Robert: A Sys'cern t o  Detect Ectopic Beartbeats 
@on:rad, Chester : Eevelopient of a Per~fusion System f o r  I so la ted  Organs 
Ce aslaan, Lawrence : Vectorcardiographic Characterization of Ectopic 
Sea-stbea-bs 
H~?ilierly, J a m s  : Detection and Recording of Ectopic Beartbea'bs 
Zaclcson, Jexlome : A Cardiac Idonitor with Audio Output 
J,qk-e::l?, 1Ccnnc.i;h: i3dl.:;c Iinvc: Vt:J.oci.i;y ns n Measure of Axberial 
Ulood Prc o susc 
Smith, DavicZ J.: Analysis of Cor t i ca l  Evoked Poten t ia l s  Recording 
o r  aepa t ic  Encephalopathy Subjects 
SJalters, Joseph: Signnl Preprocessing a s  an Aid t o  Real-Tine ECG 
Analysis 
C, 'The following 3:. 1. T. g ~ a d u a t e  students tn Xlec t r ica l  Engineering a r e  
p ~ ~ r s i l i i ~ g  "i;heir 14czster t l ~ c s i s  r e  search -c\rithin our la'oora-tom : 
Catiion, Bariy : Wavef oim aanipulaJ~lon using a real-time d i g i t a l  
da%a pfocessor. 
a r e s  i s  : A ' c a p  -recording system f o r  low-frequency data 
i ncoqosa t i ng  conlpensation f o r  speed-variations 
I i n  the  tape ' ~ r m s p o r t .  . I 
Jergens, Taul: T\!aveZo~~i~ acqil isi t ion,  storage and detect ion using 
sh i f t - r eg i s t e r  ~nemories .
Xress, David: I n  Vivo !\ConLtoring of I n t e r s t i t i a l  Fluid  Elect rolytes .  
D. PL7bllcn-&ions : (copy a tJ~ached)  
D,;viC i;. 31-1 i ~ ~ , $ n ,  "PVC ik~i;ccior" Qiiarbcrly Progrcsa Report n'o. 96, 
Research Lribora-LOFJ oP E:lec.i;roi?icc, M. I. T., Carnbridge, Mass., 
January 15, 1 9 0 ,  pj?. 2b2-2&9. 



(XXVII. COMMUNICATIONS BIOPHYSICS) 
20 t \ \ THEORETICAL LIMIT \c' \ 
500 1000 2000 5000 10,000 20,000 
FREQUENCY (Hz)  
Fig. XXVII-6, P re s su re  ratios. 
The degree of match, however, is somewhat better than one might superficially infer 
from Fig. XXVII-4. Indeed, the transmission coefficient It1 is approximately -2 dB 
over the range 3 kHz < f < 5 kHz a s  shown in Fig. XXVII-5. These results a r e  t rans-  
lated into more familiar te rms  in Fig. XXVII-6. The ordinate in Fig. XXVII-6 is the 
ratio of the magnitude of the pressure at  the drum, I pm 1 , to the pressure,  I Ps  I , that 
would be measured just external to the concha if the concha were closed with a rigid 
plug. (Over most of the frequency range of interest,  the wavelength is sufficiently small  
compared with the radius of the head that I Ps I will be approximately twice the magni- 
tude of the free-field pressure for a wave coming directly at the ear ,  i. e . ,  8 = 4 =  IT/^.) 
The maximum possible pressure ratio is readily calculated under the assumption that 
a l l  of the available power is absorbed in the load. That is, 
2 2 2 
IpmI - R ~ + x ~  
Max -- 
\pS1 4RmRs ' 
where Rs i s  the rea l  part of the radiation impedance looking out from the concha. This 
ratio i s  plotted a s  the dashed curve labeled "Theoretical Limit" in Fig. XXVII-6. The 
actual pressure ratio (the solid curve of Fig. XXVII-6) differs from the theoretical limit 
by the factor 1 t 1 of Fig. XXVII-5. Thus in  the frequency region 3-5 kHz a better 
impedance match could increase the pressure ratio by only approximately 2 dB. At lower 
QPR No. 96 
(XXVII. COMMUNICATIONS BIOPHYSICS) 
frequencies, however, substantial improvement would be theoretically possible a t  least 
at any one frequency. (There a r e  limitations on the width of the frequency band over 
which a relatively good match between complex load impedances can be realized with a 
passive matching n e t ~ o r k . ~ '  The precise nature of these limitations has not been 
worked out for load impedances of the type corresponding to Z and Z but i t  is pos- 
a m ' 
sible that a better match at low frequencies could be achieved only at the cost of poorer 
performance in the 3-5 kHz region.) 
Figure XXVII-6 also compares the pressure ratio for the model of Fig. XXVII-3 with 
several measurements on rea l  ea r s  and replicas. (The measurements of Wiener and 
Ross have been reduced by 6 dB to reflect the difference between P and the free-field 
S 
pressure a s  discussed above.) No particular effort was made to adjust the parameters 
of the model to match these observations. In fact, a somewhat better match would prob- 
ably be obtained with a slightly longer canal. This would reduce the frequencies of both 
the 1/4 wavelength and 3/4 wavelength resonances near 3 kHz and 10 kHz. The concha 
i s  important both in enhancing the effect of the 3-kHz resonance and in providing the 
shoulder to extend the matched band to 5 kHz. 
W. M. Siebert 
References 
1. F.  M. Wiener and D. A. Ross, "The Pressure  Distribution in the Auditory Canal 
in a Progressive Sound Field," J. Acoust. Soc. Am. 18, 401 -408 (1946). 
-
2. E. A. G. Shaw and R. Teranishi, llSound Pressure  Generated in an External-Ear 
Replica and Real Human E a r s  by a Nearby Point Source," J. Acoust. Soc. Am. 44, 
240-249 (1968). 
3. J. Zwislocki, llAnalysis of Some Auditory  characteristic^,^ in Handbook of Mathe- 
matical Psychology, Vol. 111, R. D. Luce, R. R. Bush, and 33. Galanter (eds.) (John 
Wiley and Sons, Inc. , New York, 1965). 
4. H. W. Bode, Network Analysis and Feedback Amplifier Design (D. Van Nostrand 
7Co.,  Princeton, N. J . ,  1945). 
5. R. M. Fano, "Theoretical Limitations on the Broadband Matching of Arbitrary 
Impedances,If J. Franklin Inst. 249, 57-83; 139-154 (1950). 
-
B. PVC DETECTOR 
A premature ventricular contraction (PVC) is the spontaneous contraction of the ven- 
t r ic les  of the heart at an ear l ier  point in the cycle than would occur in normal cardiac 
rhythm. This is evidenced in the electrocardiogram (EKG) by abnormal wave shape of 
the QRS complex, the voltage associated with ventricular depolarization. Prematurity 
is not unique to PVCfs ,  but is also characteristic of superventricular arrhythmias. 
PVCfs a r e  known to be associated with drug toxicity and various cardiac diseases. 
The significance of PVCfs in the history of a cardiac patient i s ,  at present, a topic of 
QPR No. 96 
(XXVII. COMMUNICATIONS BIOPHYSICS) 
research. The usual practice of examining a short s t r ip  of EKG record is inadequate 
when the frequency of PVC occurrence is low, a s  is ordinarily the case in the unhospi- 
talized patient. Since the time required to examine records extensive enough for sound 
statistical data would be prohibitive, the utility of an instrument for automatic PVC 
detection i s  clear.  
The instrument described in this report was designed to fulfill this purpose. In par-  
ticular, the feasibility of a scheme for diagnosing abnormal QRS wave shape has been 
investigated; the detection of prematurity is relatively simple and has previously been 
implemented with reasonable success. The principal design cr i ter ia  for the PVC detec- 
t o r  described here were minimization of hardware and simplicity of operation. These 
a r e  important i f  widespread data a r e  to be accumulated. 
The characteristic of QRS wave shape that was utilized was duration; more specif- 
ically, the width between baseline crossings of those components of the EKG which meet 
an amplitude criterion. Cardiac conduction in the case of a PVC i s  abnormal, and the 
resulting QRS i s  generally prolonged. Furthermore, the normal QRS duration is rel-  
atively constant for a given patient (unlike, for example, cycle length), and so there i s  
no need for automatic updating of this parameter by the instrument. This considerably 
simplifies the hardware that i s  required, a s  well a s  the reliability of the device. 
1. Circuit Description 
A block diagram and illustrative waveforms for the PVC detector appear in 
Fig. XXVII-7. The action of the instrument i s  to  indicate a P V C  whenever an EKG com- 
ponent of either polarity with sufficient amplitude exceeds a preset width. 
The EKG signal is buffered by the input amplifier, which provides gain adjustment 
and rudimentary filtering. The principal purpose of the filtering is to attenuate low- 
frequency baseline shifts in the input signal. The distinction between positive and neg- 
ative polarity is eliminated in the next stage, which i s  an absolute-value amplifier o r  
full-wave rectifier. The full-wave rectified waveform is high-passed to remove the DC 
component and shift the baseline slightly below zero. 
The resultant waveform i s  fed into a pair of comparators. The f i rs t  of these, the 
llR-detector,ll determines whether an EKG component is of sufficient amplitude to be pro- 
cessed. The "zero-crossing detector," on the other hand, produces a high voltage when- 
ever  the waveform lies below a fixed threshold close to zero. This voltage is used to  
reset  a linear ramp, the amplitude of which is therefore proportional to the interval 
since the latest zero crossing of the high-passed, rectified EKG signal (Fig. XXVII-8). 
The "width-detector" i s  a third comparator which produces a negative voltage when- 
ever  the ramp waveform exceeds a preset threshold. At this time, an output pulse indi- 
cating the occurrence of a PVC i s  produced, provided that the other input to the gate i s  
QPR No. 96 243 
QPR No.  96 
(XXVII. COMMUNICATIONS BIOPHYSICS) 
Fig. XXVII-8. Waveform of ramp generator. The output of the ramp 
generator is shown above the EKG signal used as  an input 
to the PVC detector. Notice that the full width of the first 
PVC does not regis ter  because it dips below the threshold 
of the zero-crossing detector. 
The uppermost t race is the event-marker of the chart 
recorder used for this record, activated by the detector 's  
output pulse generator through a relay. Both pulses a r e  
actually triggered by the f i rs t  ramp voltage for the corre-  
sponding PVC, but the delay of the event-marker actuation 
makes this unclear. 
low. Under normal operation, this condition prevails i f  the S-R flip-flop has been rese t  
by the R-detector since the latest zero crossing. Thus, an EKG component must meet 
two conditions to produce an output pulse: i t  must exceed a preset width and a preset 
amplitude. 
The width and amplitude thresholds a r e  set by the operator, along with the gain of the 
input stage. The instrument 's controls were designed to simplify the experimental pro- 
cedure a s  much a s  possible, and avoid the need for auxiliary monitoring equipment. The 
outputs of the R-detector and gated width detector a r e  wired to lamps mounted on the 
control panel. Observation of the R-detector output allows the operator to adjust the 
gain by observing the point where the R-detector (at maximum threshold) is just acti- 
vated once each cardiac cycle. This ensures efficient use of the dynamic range of the 
system, and avoids the necessity of separate adjustment of the zero-crossing threshold. 
Once the gain has been set  in this fashion, the R-detector threshold can be lowered to 
process more than one component of each QRS complex. The width-detector threshold is 
QPR No. 96 
2 
t t l  +tcitiitiiiii1iti%ilT~ - - --=3:Hf =C& z- 25 m m l s  - - - 
Q P R  No.  96 
Fig. XXVII-9. Samples of EKG records used in testing the PVC detector. 
Detector output i s  shown above each EKG. Strips were 
recorded at 5 and 25 mm/sec. 
1. From record No. 1: Detection of a PVC i s  shown in 
the presence of noise. 
2. From record No. 3: Two types of PVC a r e  shown. 
The f i rs t  type was consistently missed by the instru- 
ment, while the second was consistently detected. 
3. From record No. 3: S l o w  e r recording than above, 
showing the high incidence of PVCf s .  Most a r e  of the 
second type cited above. Also shown i s  the successful 
detection of two PVCfs in the presence of baseline 
shift. 
4. From record No. 5: Shown a re  three PVCfs correctly 
detected, one in the presence of considerable noise. 
5. From record No. 12: This i s  the PVC that could not 
be detected by the instrument. Shown a re  one false 
positive and one false negative. The false positives 
occurred only when the width threshold was lowered in 
an unsuccessful effort to detect the PVCfs. 
QPR No. 96 
Table XXVII-1. Summary of tes t  results. 
Key to column headings 
True neg. : Normal beats so adjudged by the instrument. 
True pos. : PVC's correctly diagnosed by the instrument. 
False neg. : PVC's not detected by the instrument. 
False pos. : Output pulses of the instrument without an actual PVC on the EKG. 
PVC's missed: The ratio of (false neg.)/(true pos. + false neg.), expressed a s  a 
percentage. In other words, the fraction of PVC's missed by the 
instrument. 
False alarms:  The ratio of (false pos.)/(true pos. t false pos.), expressed a s  a 
percentage. In other words, the fraction of a larms (output pulses) 
which a re  false. "r 
Conf. limit: The statistical upper limit of the t rue failure rate,  asser ted with 
90% confidence. Based on a sample corresponding t o  the denomi- 
nator t e rm for the two classes  of failure cited above. Not com- 
puted for  sample sizes G2 or for  actual failure rates  >50%. 
* 
Actually nodal premature beats with widened QRS. 
?A considerably more optimistic figure would be the ratio of (false ~ o s . ) / ( t r u e  
neg. t false pos.), o r  the fraction of normal beats incorrectly adjudged PVC's.  
This would not be a s  meaningful a s  the figure cited, which essentially answers 
the question, "When the fire bell rings, what a r e  the chances that there really 
is a f i re  ? I' 
QPR No. 96 
(XXVII. COMMUNICATIONS BIOPHYSICS) 
then adjusted by finding the setting where every QRS just elicits an output pulse, a s  
observed through the second lamp. A switch is then thrown which sets  the threshold at 
1 .  5 times this setting. Such a procedure i s  usually adequate for distinguishing PVC's, 
but the threshold can be lowered from this point i f  required. 
2. Preliminary Results 
A ser ies  of prerecorded EKG records was used to evaluate performance of the PVC 
detector. These were selected for variability in the recognition problems presented, 
and not for being typical of EKG's expected in practice; hence, the results a r e  explora- 
tory in nature. Samples of the records a r e  shown in Fig. XXVII-9, and a summary is 
presented in Table XXVII- I .  
In most cases,  the failure to obtain statistically significant favorable results was 
attributable to small  sample size; 10 min of EKG was run for each record, but in  many 
the number of PVC's was small. The principal cause of genuine failures by the detector 
was baseline shift too sudden to be eliminated by the highpass filtering of the input stage. 
It is doubtful that more extensive filtering of a linear nature would remove this source 
of false alarm, since the frequency content of this particular artifact is s imilar  to that 
of a genuine PVC. 
On the other hand, the incidence of false negative responses by the instrument was 
acceptably low for all  but one record. The QRS for PVC's of this record was visibly 
abnormal, but it had no single component that could simultaneously meet the width and 
amplitude cr i ter ia  for detection. It i s  clear that the recognition scheme evaluated here 
was invalid for  an EKG record of this type (Fig. XXVII-9 (5)). 
The next phase of evaluation would be to tes t  the instrument's performance with EKG 
records more representative of the actual population. If results a r e  satisfactory, the 
simple addition of a digital counter should enable the detector to be used in acquiring 
data on cardiac arrhythmia. 
 h he research described in this report was part of a Master 's  thesis in  the Depart- 
ment of Electrical Engineering, M. I. T . ,  supervised by Professor S. K.  ~ u r n s . ]  
D. L. Sulman 
QPR No. 96 


Communications biophysics - The temporal structural of electrophysiological data, 16 March 1970 Quarterly status report, 1 Dec. 1969 - 28 Feb. 1970

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