A predictive instrument developed by Selker et al can be used to estimate the risk of cardiac arrest following myocardial infarction and to indicate how the risk would be modified by thrombolytic therapy.
Parameters:
(1) patient age
(2) systolic blood pressure
(3) electrocardiographic size of acute myocardial infarction
(4) sum of ST-segment elevation by chest pain onset
(5) corrected QT interval by chest pain onset
(6) time intervals 1 through 9 between ECG and cardiac arrest
(7) use of thrombolytic therapy in or before the time interval
|
Parameter |
Finding |
Value |
|
patient age |
< 35 years |
(35 - 53.6)^2 = 345.96 |
|
|
35 to 70 years |
((age) - 53.6)^2 |
|
|
> 70 years |
(70 - 53.6)^2 = 268.96 |
|
systolic blood pressure |
< 80 mm Hg |
80 |
|
|
80 to 100 mm Hg |
systolic blood pressure |
|
|
> 100 mm Hg |
100 |
|
corrected QT interval adjusted to time from onset of chest pain to ECG |
QTc < 0.44 |
0 |
|
|
QTc >= 0.44 and onset of chest pain to ECG <= 0.75 hours |
(QTc - 0.44) |
|
|
QTc >= 0.44 and onset of chest pain to ECG > 0.75 and < 1.25 hours |
(QTc - 0.44) * (5.5 - (6 * (hours from chest pain to ECG))) |
|
|
QTc >= 0.44 and onset of chest pain to ECG >= 1.25 hours |
(QTc - 0.44) * (-2) = = 0.88 - (2 * QTc) |
where:
• QTc = (QT interval) / SQRT(RR interval)
|
Electrocardiogram Size of Acute MI |
Value |
|
anterior MI |
(((2 * (number of leads in V1-V6 with ST-segment elevation)) + (amount of ST-segment elevation in leads V1-V6 over 1 mm) - 13.5)^2) |
|
no contiguous anterior ST segment elevation |
(((0.67 * (sum of ST segment elevations in leads II, aVF, III, I, aVL)) - 13.5 )^2) |
where:
• In contiguous leads with ST-wave elevations there must be at least 1 lead with an elevation >= 1 mm and 1 lead with an elevation >= 0.5 mm
• The 2 options are to (1) sum up all of the ST segment elevations, irregardless of size and direction , or (2) sum up only those >= 0.5 mm.
• The table on page 551 has summing of leads V1-V6 for an anterior MI; the example on page 554 says V1-V4.
• The footnote at the bottom of page 551 seems ambiguous but could mean that the maximum value for ECG size is set to 462.5, from ((35-13.5)^2) = 462.3
Sum of ST segment elevation in all leads
adjusted for time from chest pain onset to time of electrocardiogram
|
Time from Chest Pain Onset to ECG |
sum of ST segment elevation |
Value |
|
<= 0.25 hours |
<= 22 |
(sum of ST segment elevations) |
|
<= 0.25 |
> 22 |
22 |
|
> 0.25 and <= 0.75 |
<= (27 - (20 * (time from chest pain onset to ECG))) |
(sum of ST segment elevations) |
|
> 0.25 and <= 0.75 |
> (27 - (20 * (time from chest pain onset to ECG))) |
(27 - (20 * (time from chest pain onset to ECG)) |
|
> 0.75 and <= 1.25 |
<= (15 - (4 * (time from chest pain onset to ECG)) |
(sum of ST segment elevations) |
|
> 0.75 and <= 1.25 |
> (15 - (4 * (time from chest pain onset to ECG)) |
(15 - (4 * (time from chest pain onset to ECG)) |
|
> 1.25 and <= 2.0 |
<= (80 - (40 * (time from chest pain onset to ECG))) / 3 |
(sum of ST segment elevations) |
|
> 1.25 and <= 2.0 |
> (80 - (40 * (time from chest pain onset to ECG))) / 3 |
(80 - (40 * (time from chest pain onset to ECG))) / 3 |
|
> 2 hours |
|
0 |
where:
• It is unclear if an ST segment depression is ignored or a negative number or the absolute value is used. In the implementation the raw numbers are used.
|
Time Interval |
Hours from ECG to Cardiac Arrest |
Points |
|
Interval 1 |
> 0 and <= 0.5 |
1 |
|
|
> 0.5 |
0 |
|
Interval 2 |
> 0.5 and <= 0.75 |
1 |
|
|
<= 0.5 or > 0.75 |
0 |
|
Interval 3 |
> 0.75 and <= 1.25 |
1 |
|
|
<= 0.75 or > 1.25 |
0 |
|
Interval 4 |
> 1.25 and <= 1.75 |
1 |
|
|
<= 1.25 or > 1.75 |
0 |
|
Interval 5 |
> 1.75 and <= 2.5 |
1 |
|
|
<= 1.75 or > 2.5 |
0 |
|
Interval 6 |
> 2.5 and <= 4 |
1 |
|
|
<= 2.5 or > 4 |
0 |
|
Interval 7 |
> 4 and <= 8 |
1 |
|
|
<= 4 or > 8 |
0 |
|
Interval 8 |
> 8 and <= 24 |
1 |
|
|
<= 8 or > 24 |
0 |
|
Interval 9 |
> 24 and <= 48 |
1 |
|
|
<= 24 or > 48 |
0 |
|
Use of thrombolytic therapy |
Value |
|
thrombolytic therapy not given before or in the time interval between ECG and cardiac arrest |
0 |
|
thrombolytic therapy given before or in the time interval between ECG and cardiac arrest |
1 |
score for interval =
= (-0.00336 * (value for age)) - (0.1185 * (value for systolic blood pressure)) - (0.00505 * (value for ECG size of MI)) + (0.1026 * (value for sum of ST segment elevations)) + (11.8936 * (value for corrected QT interval by chest pain onset)) + (8.3110 * (value for time interval 1)) + (8.3865 * (value for time interval 2)) + (8.5451 * (value for time interval 3)) + (8.6451 * (value for time interval 4)) + (8.7791 * (value for time interval 5)) + (8.9471 * (value for time interval 6)) + (9.1565 * (value for time interval 7)) + (8.9045 * (value for time interval 8)) + (8.7714* (value for time interval 9)) - (0.5461 * (value for use of thrombolytic therapy in or before the interval)) - 2.0005
According to the example given on page 554, the calculation steps appears to start with calculating the survival for each interval.
survival for interval =
= 1 / (1 + EXP(score for that interval))
Then the cumulative survival for all intervals is calculated.
cumulative survival =
= PRODUCT(survival for each interval)
risk of death from cardiac arrest =
= 1 - (cumulative survival)
where:
• The instrument is labeled as predicting cardiac arrest. In theory not all cardiac arrests are fatal. But since this is based on not surviving, I interpret this as risk of death from cardiac arrest.
NOTE: In the other thrombolytic predictive instruments
risk of death =
= 1 / (1 + EXP((-1) * (score)))
Since
chance of survival =
= 1 - (risk of death)
chance of survival =
= 1 / (1 + EXP(score))
IMPLEMENTATION NOTE: This is a fairly complex algorithm. I have some uncertainty on how to add the ST segment changes. The results of spreadsheet need to be confirmed.
Specialty: Cardiology
ICD-10: ,
