Role of 'ECG' in the Diagnosis of Ischaemic Heart Disease
Item
- Title
- Role of 'ECG' in the Diagnosis of Ischaemic Heart Disease
- Date
- 1989
- extracted text
-
ROLE OF
IN THE DIAGNOSIS OF ISCHAEMIC
HEART DISEASE
' ECG'
There is a wide spread belief
amongst many doctors that cardio
logists and Physicians are overusing ECG and Stress Test. What
follows is the result of a series of discussions at both individual
and collective level by members of the Health Committee of the Lok
Vidnyan Sanghatana on this issue. To authenticate the views exp
ressed, two famous text books of cardio-vascular medicine have bderi
quoted liberally. This brief note has its obvious limitations; but
it is hoped that non-special!st doctors would benefit from it.
In spite of the development of more and more revealing,
accurate, non-invasive special investigations, to begin with, the
importance of history and clinical examination needs to be stressed.
" Although the development of these methods represents one of the
triumphs of modern medicine, their appropriate use is to supplement
but not to supplant a careful clinical examination, which remains
the cornerstone of the assessment of the patient with know or
suspected cardiovascular disease. There is a temptation in card
iology, as in many other areas of medicine, to carry out ejq>ensive,
uncomfortable, and occasionally even hazardous procedures to esta
blish a diagnosis when a'detailed and thoughtful history and physical
examination may be sufficient. Obviously, it is undesirable to
subject patients to the unnecessary risks and expenses ihherent in
many specialized tests when a diagnosis can be made on the basis of
an adequate clinical examination or when their management will not
be altered significantly as a result of these tests. (1)
The perspective that Wilson and his associates gave to the
electrocardiogram in 1933 in the clinical recognition of a myocardial
infarction, has been approvingly quoted by the most recent edition
(1990) of The Heart.'
'In general, we found the electrocardiogram
Heart,'
far more helpful in the diagnosis of coronary occlusion than the
physical findings, but of less value than the clinical history.' (2)
The physical examination is usually normal in a patient
with an evolving myocardial infarction. The physical examination
is useful primarily for excluding other explanations for the symp
toms or documenting any existing complication of the infarction.
Within the perspective quoted above, we would discuss some
questions that confront a non-specialist in cardiology (t a G.P. or
a gynaecologist or a surgeon etc.)
Question t Should all persons complaining of chest pain be subjected
to an ECG without any clinical evaluation ?
Answer : No.
Though ischaemic pain can pr.esdnt itself in a deceptive fashion,
a detailed history and clinical evaluation does give extremely
valuable pointers.
i) Nature of pain : Some of the characteristics of nonanqinal pain are : 11 Pain lasting less than five seconds or
greater than twenty to thirty minutes ( provided infarction
is excluded ); a pain aggravated or precipitated by one deep
breath; pain precipitated by a single movement of the trunk
or arm; pain relieved within a few seconds of lying horizontally
/Ctd
J
r
. 2.
( patients with angina rarely lie down to get relief );
pain relieved within a few seconds of one or two swallows
of food or water; pain localized to a very small area, e.g.,
an area & the size of the tip of a finger,-pain associated
with tenderness of chest wall (r unless the anginal pain is
referred to a site of previous-'-chest wall trauma.) • Diffe
rentiating the discomfort resulting from these nonbardiac
disorders from angina pectoris is usually possible*when the
quality of the pain and its dpr^tion-,,.. precipitating factors,
and associated symptoms are taken into consideration. Thus
the typical anginal episode usually begins gradually and
reaches maximum intensity over a .period of minutes before dissipating--usually as a result of cessation of the activity
that precipitated it. Noncororiary causes should be considered
in patients with sharp, stabbing,, or burning chest pain that
comes and goes in a matter of seconds, or with a dull conti
nuous ache in the chest. Similarly, changes in posture do
not usually affect the discomfort of myocardial ischemia,
and this maneuver helps to distinguish angina from peri
cardial disease or hiatus hernia. " (3)
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Angina_ Equivalents
There are : two symptoms .other., .than
angina pectoris which are sometimes signals of myocardial
ischaemia. These symptoms when'due to ischaemia are- called
angina equivalents. These symptoms*dyspnea and exhaustion,
are due to left ventricular dysfunction which is produced . .by
a global type of myocardial ischaemia. (4)
Myocardial infarction may also present with atypical
features : pain radiating to shoulders, arms, jaw on either
right side or both sides. There may be burning sensatipn in
the epigastrium which patient may report as 1 gas-1 rouble »■'
. but which would not be relieved by antacids. Thus there may
not be any pain in the chest and in 15-20 per cent of patients
of Acute Myocardial infarction, there is no pain ( silent
infarcts.) Thus overall clinical suspicioun based on detailed
history is more important than one or two typical symptoms.
ii) Accompanying factors :
Accompanying clinical features like breathlessness,
sweating, nausea vomiting, hiccoughs.palpitation, fall in BP
point towards myocardial infarction whereas absence of these
and presence of epigastric .tenderness, relief by antacids or
after belching point towards heartburn. Risk factors like
obesity, hypertension, diabetes ( though diabetics may get
silent ischaemia more often ), smoking, family history will
point towards cardiac origin, whereas frequent sighing, signs
of anxiety or depression point towards functional pain. When
in doubt, the doctor may err on the safer side and advise an
ECG, but routine ECG without any clinical evaluation of
chest-pain is to be deprecated.
The above are only some important points just to indi-.'-cate that clinical evaluation of chest-pain is quite impo-rtant. It is true that Clinical Picture can be misleading,
but so is the case with ECG.
, . p/-'--
" The resting electrocardiogram
UlUi, alUlUy -Ldlli _Lis
S IHJITlUClX
normal _L11
in OilL,
One LUU1U1
fourth
to one Jjalf of patients with chronic stable angina pectoris,
depending cn the incidence of previous-myocardial infarction
in the particular series of patients." (5)
I ;
.3.
In case of suspected Acute Myocardial Infarction, (AMI)
initial ECG many times turns out to be normal.
11 The
initial ECG is ‘diagnostic1 of acute infarction in slighthly
more than half the patients." (6) When the initial ECG turns
out to be normal, whether to keep the person under observation
and do serial ECG to detect or rule out AMI again depends on
the clinical picture.
Thus a detailed CLINICAL EVALUATION is necessary in
diagnosing the cause of chest-pain and ECG can supplement &
not supplant it.
Question s Should all persons above the age of 30 years/40 years
undergo ECG. even if there is no clinical suspicion
of IHD ?
'• ~i
Answer : The argument in favour of using ECG as a screening
test is based on the " surprise positive " findings
which help to detect IHD in asymptomatic individuals at
the -earliest. But it is forgotten that use of any test
.>, as a screening test for the detection of any disease
depends on the sensitivity and specificity of the test
and the prevalence of the disease in the population being
subjected to that test. Let us first be clear as to
what is sensitivity, specificity, and predictive vdlue
of a test.
True positive; Abnormal test result test in an
individual who has the disease.
False positive : Abnormal), test result in an indi
vidual who does net have the disease.
True negative ; Normal test result in an individual
who does not have the disease.
False negative : Normal test result in an indivi
dual who has the disease SENSITIVITY s Ability of a test to
recognise a high proportion of true positives.
______
True positives
x 100
True positives + false -negatives
SPECIFICITY : Ability of a test to give relatively few false
positives.
Ngoftt
—-- -------Tcue
,v X 100
True Negatives + False Positives
PREDICTIVE VALUE =
True Positive
True Positives+False Positives
The results of exercise tests have been compared with
coronary angiographic findings.
Symptom limited maximal
exercise tests or tests with achievement of 85% or above submaximal exercise grades and with multiple leads recording the
specificity of exercise test is about 90.95% while the sensi
tivity varies from 65 to 75%. (7)
.4.
The predictive value of any test increases with the
prevalence rate of the disease in the community. This is known
as BAYE's theorem.(8) The following table prepared on the basis
of this theorem gives the number of persons that would be corre-ctly and incorrectly diagnosed if 10,000 persons are investi
gated with a test whose sensitivity and specificity is 90% and
95% respectively. It will be seen that as the prevalence in
the population being tested goes down, the number of persons
with true positive result.gbes down and-those with false
positive result increases and at 1% prevalence, false +ve
results (495) surpass many times the true *ve results (90).
Thus the chances of calling a normal-person as a case of IHD
are more than five times the chances of correctly detecting a
case of IHD if routine ECG is carried out in a population
in which the prevalence of IHD is 1%.
RESULTS OF A TEST WITH 90% SENSITIVITY & 95% SPECIFICITY IN A ” POPULATION
6fi~ 10,
QQ~Q . (9”/" '
'
'
-----_ . I*.!-; - - J
t
'true
j i
Positive
'j
predictive
value
( 1/1*3 )
5.
+ve
True
-ve
FALSE*ve
FALSE
-ve
1
2
3
4
10 %
1 %
4500
900
90
4750
8550
9405
250 .
450
495
500
100
10
95 %
67 %
16 %
2 %
180
9310
490
20
27 %
i Prevalence
i rate of the
disease
(prior prob-ability.)
50 %
in Western countries/ the prevalence-of IHD-is 2 %
whereas in India, it is much less. Even if it is assumed to
be 2 %, falsp +ve results would far outnumber true -tve results.
Those who are incorrectly diagnosed as suffering from IHD
would be unnecessarily condemned for prolonged if not life-long
treatment for it. Presence of risk factors like diabetes,
hypertension, obesity, family history, smoking, etc.or presence
of symptoms suggestive of IHD increase the prior probability of
IHD. in such subset of the community, ECG has a high positive
predictive value and hence subjecting- such persons to ECG is
scientifically justified.
To conclude, though, more cases of IHD are now seen
between 30 to 40 years of age, and though persons
in their twenties have died of AMI, the overall
prevalence is too low to justify routine testing
with ECG of all individuals above 30 years of age,
because we are much more likely to be misled by
such routine testing. In some hospitals, all
patients admitted for any reason are subjected to
an ECG. This is patently unscientific and
exploitative.
Question: Do these statistics have any meaning when we deal
with individual patients ?
Answer: In medicine, any decision is necessarily based on some
statistical assumption. The fact that-the proponents
of 1 routine E.C.G. above 30 years of age1 do not advocate
such routine ECG in children or in adolescents, shows that
they have seme unstated cr even unconscious assumption about
the chances of detecting IHD through such screening. The above
paragraphs only point out that this statistical assumption is
incorrect and that for each patient of IHD correctly detected by
such screening, there would be 2.5 to 5.5 times more number of
normal persons who would be wrongly labelled as cases of IHD.
La
.5.
3 '
A 5
is the role of Stress Test ?
It may be argued that routine ECG is only to select
cases for future investigations, with say exercise-electro
cardiography (Stress Test.) However, the sensitivity of
Stress Test is low, and there are other problems also.
" In view of the relatively low sensitivity (70 to 85 per
cent of exercise stress electrocardiography p.1633), a
negative result does not rule out ischemic heart disease;
however it makes three vessel or left main disease much
less likely. Conversely, an adequate maximum exercise test- i
one achieving more than 85 per cent of predicted maximal
heart rate-is unlikely to miss three-vessel or left main
coronary artery disease. A major limitation of the sensi
tivity of the exdrcise electrocardiogram is that it cannot
be interpreted in many patients. This includes patients who
are incapable of reaching the level of exercise required, for
near-maximal effort (85 per cent or more of maximal predicted
heart rate), particularly those on betadrenoceptor blockers
or those who develop fatigue, leg cramps, or dyspnea, and
patients with abnormalities in the baseline electrocardiogram,
including those on digitalis. In such patients radionuclide
imaging may be very helpful." (10)
It has been estimated that with a 70 % sensitivity
and 85 % specificity of exercise Tolerance Test, and with the
assumption that prevalence of IHD in 4symptomatic adults
is 3 %, the probability of that individual with a positive
test has IHD is only 13 %l " Of 100 such positive tests, ,
87 of them would be false positive." (11) Since in India,
this prevalence is lower than 3 % the proportion of false
+ve exercise tolerance tests would be much more than this.
a •
Are there any hazards of stress-tost ?
A :
Stress-test is non-invasive but is not free of hazards.
The mortality rate is 10 per lac tests and morbidity rate
24 per lac. (12)
a s
What are the definite indications for stress-test ?
A :
Western Experts give the following indications for
Stress Test s
i) Aid in diagnosis of the cause of chest discomfort;
ii) Assess the prognosis of coronary heart disease;
iii) Assess the efficacy of therapy of coronary heart disease;
iv) Guide rehabilitation following myocardial infarction;
v) Aid in the development of an exercise prescription and
provide a safety check before a fitness program;
vi) Screen high-risk professionals;
vii) Assess a risk of development of clinical coronary
artery disease in asymptomatic persons. (13)
Out of these, the last one is controversial and we have argued
above as to why this is not a scientifically justified indi
cation. As for others, some of them need to be pruned to
Indian socio-economic reality. If the management of a patient
t
c
Anant Phadke, Lok Vidnyan Sanghatana - Peoples Science Movement,
Maharastra.
.6.
with myocardial infarction is not going to change substantially
due to the result of the stress test, the stress test would not
be useful. For example, a retired white collar worker leading
a stereotyped sedentary life does not need stress test after
an attack of AMI because his routine is anyway going to be of
low level activity. What is meant by fitness programme is to be
clarified. Many sedentary people begin a morning-walk of say
half an hour. Would they require a stress test before beginning
such a change in the routine ?
Angiography :
If the result of stress test is inconclusive, the patient
is many times asked to undergo coronary angio.graphy since it
gives a more direct and more reliable picture of the state of
coronary artries. However, it should be pointed out that, as
compared to the stress test, apart from being about 10 times
costly, coronary angiography is 14 times hazardous. In one large
series of data, overall mortality was 140 per lac tests, non-fatal
myocardial infarctions 70 per lac, Arrhythmias 560 per lac,
cerebral ischaemia 70 per lac, local vascular complications 570
per lac tests. (14)
In those patients who for financial or other
reasons are not likely to undergo any surgical intervention,
coronary angiography is redundant. But today in India, how
many of those who undergo coronary angiography are explained
the implications of coronary angiography so that they can make
an informed choice 2
Acute
Myocardial
Infarction
be admitted in
Should a person suspected of having
spite of a normal ECG 7
A
If clinically, the attending doctor feels that it is
a likely case of AMI, the patient should be admitted for
monitoring in spite of a normal initial ECG. However, if
ECG is normal even after 24 hours. Serum CK is also normal,
AMI can be ruled out. Two of the three indicators (clinical
picture, ECG, enzyme-levels) are required for diagnosis of
AMI.
AMI ?
Q
3
How often should
A
2
Patient of AMI should preferably be on a monitor
for first 48 hours to detect and treat instantly arrhythmias.
But later on, if the haemodynamic functions are stable, if
patient has no recurrence of pain, there is no advantage in
repeating ECG every day. In such cases, one ECG before
discharge is sufficient.
ECG
be repeated in a case of
References;
(1) Heart Diseases : A Textbook of (7)
Cardio-vascular medicine Ed.
Eugene Braunwald, W.B. Saund(8)
er1s Co.3rd Ed., 1988, p-1
(9)
(10)
(2) The Heart, Editor-in-chief J.
(11)
Willis Hurst, McGrew Hill Inf (12)
ormation Service Co.7th Edn.,
(13)
1990, p-1056.
(14)
(3) Heart Disease,op.cit.p-1316.
(4) Heart, op.cit. p-1051.
L
AMI
Q
(5) Heart Disease, op.cit.p-1321.
(6) Heart Disease, op.cit.p-205.
A
API Textbook of Medicine,
4th edn.,1986,p-397.
Heart Disease, pp.cit.p-1683
Heart Disease op.cit.p-1683*
Heart Disease,op.cit.p-1322
Heart Disease,op.cit.p-236
Heart Disease,op.cit.p-239
Heart Disease,op.cit.p-223
Heart Disease,op.cit.p-287
(*..modified)
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