Quality assessment of the recording
The need to check the quality of ABPM instrument readings
The need for quality control is due to disadvantages of blood pressure recording methods. These disadvantages of the auscultatory method are:
sensitivity to the accuracy of the microphones location relative to the artery and to the cuff turning on the arm during long period; requires direct contact of the microphone with the patient's skin; inefficiency in patients with severe auscultatory gap and some other changes of Korotkoff tone, with heart rhythm disturbances; low resistance to external noise (in the room, etc.). See also Auscultatory method.
Disadvantages of the oscillometric method are:
- relatively low resistance to heart rhythm disturbances;
- relatively low resistance to movement of the arm (see figure).
The figure shows that the biceps contractions and thickenings (1) captured by the cuff (2) mask the oscillations (3) of the artery. See also Oscillometric method.
Methods of quality control of ABPM readings
Quality control of ABPM readings can be realized in the following ways:
- visual inspection and manual culling of incorrect records using the software during analysis;
- automatic culling of incorrect records with followed cuff re-inflation and new signals recording during monitoring;
- binding to the R-wave of ECG, involved spectral signal recognition algorithms, etc;
- combination of the above-mentioned methods.
In practice it should be remembered that there are different approaches: some software does not indicate the automatically culled records in the analysis protocol; some software offers visual assessment the quality of signals and also calculates the validity of the total data or the percentage of satisfactory readings. In the first case the goal is speed and simplicity for the analysis; in the second case it is the reliability and thoroughness for the expert assessment.
Artifact sources in ECG monitoring
In combined ECG and blood pressure monitoring, artifact is loosely defined as electrical interference or, “noise,” that is recorded from sources other than the electronic signals of the heart. Artifact is a distortion of the signal being recorded. Artifact or extraneous electronic signals can obscure the very information cardiology professionals are trying to obtain. The two main types of artifact are:
There are a variety of sources for artifact in Holter recordings. Although the 60 cycle noise can be caused by skeletal muscle contractions and a moderate amount of wandering baseline may be due to respiration, much of the artifact encountered in Holter recordings can be reduced or eliminated. Female and pediatric patients must be treated the same as males to improve recording quality well enough to evaluate the subtle components of ECG such as P-wave, P-R interval, QRS duration and S-T segment for early detection of cardiac abnormalities.
Although the patient can cause artifact in the recording of an ECG, this may not be under the control of the cardiac technician performing the test. Skeletal muscle contractions are a common source of brief 60 cycle artifacts in any ECG. The electrical activity of skeletal muscle and that of the heart muscle are similar. Muscle noise due to movement can be reduced, but not eliminated. As this noise is generally of short duration, it is not our main concern. If the subject being monitored is planning prolonged, strenuous activity or is performing a stress test, lead location and skin preparation guidelines below are even more important. If the patient scratches at the electrodes, pulls on the lead wires or interferes with the recorder, artifacts will result and the test may be invalid. As patient cooperation is very important, it is essential to communicate the needs required for a successful test with the patient.
Some sources of 60 cycle noise are common mechanical faults such as broken lead wires, poor quality electrodes or electrodes that are not compatible with the snap lead. Because Event and Holter recorders do receive some physical abuse during use, loose or broken components need to be replaced. The snap lead connector and/or the patient cable connection to the recorder should be stable. The size of the electrode is not as important as long as they are Silver/Silver Chloride electrodes with plenty of adhesive to last 24-48 hours (or more for Event monitoring). If the recording is going to be of a longer duration, the patient should be supplied with extra electrodes and instructed on the best way to remove the old and replace the new ones. It is also important to check that the snap lead is compatible with the electrode so that it is not too loose and able to become disconnected. Assuming that the recorder, lead wires and electrodes are all compatible and in excellent condition, the following procedures will reduce artifact.