8 - 20
BeneVision N1 Patient Monitor Operator’s Manual
8.8.2.2
Setting ST Point, ISO Point, and J Point
CAUTION
•
You need to adjust the ST points before starting monitoring, or if the patient's heart rate or ECG
morphology changes significantly, as this may affect the size of the QT interval and thus the
placement of the ST point. Artifactual ST segment depression or elevation may occur if the
isoelectric point or the ST point is incorrectly set.
•
Always make sure that the positions of ST points are appropriate for your patient.
To set ST point, ISO point, and J point, follow this procedure:
1.
Select the ST numeric area, ECG numeric area or ECG waveform area to enter the
ECG
menu.
2.
Select the
ST
tab
→
select the
Adjust
tab.
3.
Set
ST Point
.
The setting of
Auto Adjust
defines the method of adjusting the ISO point and J point.
Auto Adjust
is enabled by
default. In this case, positions of ISO point and J point are automatically adjusted accordingly. If you disable
when
Auto Adjust
, you need to manually adjust the position of ISO point and J point by selecting the arrows at
the right sides of
ISO
and
J
.
■
The ISO point (isoelectric) position is given relative to the R-wave peak. Position the ISO point in the middle
of the flattest part of the baseline (between the P and Q waves).
■
The J point position is given relative to the R-wave peak and helps locating the ST point. Position the J point
at the end of the QRS complex and the beginning of the ST segment.
■
The ST point is positioned a fixed distance from the J point. Move the J point to position the ST point at the
midpoint of the ST segment. Position the ST point relative to the J point at
J+60/80ms
,
J+40ms
,
J+60ms
or
J+80ms
. When
J+60/80ms
is selected, the ST point will be positioned 80 ms (heart rate 120 bpm or less) or
60 ms (heart rate more than 120 bpm) from the J point.
8.9
QT/QTc Interval Monitoring
The QT interval is defined as the time between the beginning of the Q-wave and the end of the T-wave. It
measures the total duration of ventricular depolarization (QRS duration) and repolarization (ST-T). QT interval
monitoring can assist in the detection of long QT syndrome.
The QT interval has an inverse relationship to heart rate. Faster heart rates shorten the QT interval and slower
heart rates prolong the QT interval. Therefore, several formulas can be used to correct the QT interval for heart
rate. The heart rate corrected QT interval is abbreviated as QTc.
QT/QTc interval monitoring is intended for adult, pediatric, and neonatal patients.
8.9.1
QT/QTc Monitoring Limitations
Some conditions may make it difficult to achieve reliable QT/QTc monitoring, for example:
■
R-wave amplitudes are too low
■
The presence of frequent ventricular ectopic beats
■
Unstable RR intervals
■
P-waves tending to encroach on the end of the previous T-wave at high heart rates
■
The T-wave is very flat or T-wave are not well defined
■
The end of the T-wave is difficult to delineate because of the presence of U-waves
■
QTc measurements are not stable
■
In the presence of noise, asystole, ventricular fibrillation, atrial fibrillation, and ECG lead off
For these cases you should select a lead with good T-wave amplitude and no visible flutter activity, and without
a predominant U-wave or P-wave.
Some conditions such as left or right bundle branch block or hypertrophy can lead to a widened QRS complex. If
a long QTc is observed you should verify it to ensure that it is not caused by QRS widening.
Because normal beats followed by ventricular beats are not included in the analysis, no QT measurement will be
generated in the presence of a bigeminy rhythm.
