Lecture 22

Electrical safetyPhysiological effects of electricityMacroshock hazardMicroshock hazardElectrical safety codes and standard

Physiological effects of electricityThreshold of perceptionLet-go currentRespirator paralysis, pain and fatigueVentricular fibrillationSustained myocardial contractionBurns and physical injury

The threshold of perception is the minimum current that an individual can detect. It varies among individuals. The let-go current is the maximum current at which the subject can withdraw voluntarily from the current. The minimum threshold for the let-go current is 6ma. Higher current can cause involuntary contraction of respiratory muscles severe enough to cause asphyxiation if the current is not removed. This could happen at 18 to 22 mA. The ventricular fibrillation can happen if the current passes through the heart. It is the rapid, disorganized cardiac rhythm. The threshold is about 75mA to 400mA. If the current increases to 1 to 6A, the entire heart muscle contracts and stops beating. This is the sustained myocardial contraction. When the current exceed 10A, the resistive heat causes burns.

Threshold for Each Physiological Effect

Thresholds of Perception and Let-go Current

The thresholds of perception and let-go current are different among people and gender.

Duration/Body Weight on the Fibrillation Current and Shock Duration

The heavy the weight, the higher the threshold of ventricle fibrillation.

Let-go Current vs. Frequency

The let-go current has different threshold if the frequency of the current changes. In high frequency, the threshold increases as the frequency.

Points of Entry

The large externally applied current is called macroshocks. The magnitude of current needed to fibrillate the heart is far greater when the current is applied directly on the body surface than it would be applied directly to heart. If a device provides a conductive path to the heart, a very small current called microshocks can induce ventricular fibrillation.

Sources of Macroshock

If the chassis is not grounded, a fault connection can bring electric current to the surface of the chassis. If a person touches the chassis, he will get a macroshock. However, if the chassis is grounded, the current is directed to the ground and macroshock can be avoided.

Sources of Microshock

If there is a leak current from the power line to the chassis, the ground line can direct 99% of the current to the ground. However, in one case, if the ground is broken and the patient with a catheter touches the chassis, the leak current can go through the catheter and heart if the patient touches the chassis.

Electric Isolated Systems

The electric isolated system has two part, the power isolation and the signal isolation. In both cases, the circuits that connect to the body is electrically insulated from the rest of the circuit. The sensors and preamplifiers are isolated and the electrical signals have to be converted into other physical forms such as light to cross the isolation barrier. The diagram has shown the optical coupling of the signals. The power supply for the isolated unit can be battery or through isolated transformer.

Safety Codes and StandardsNFPA 99Standard for Health Care FacilitiesNational Electrical CodeArticle 517: Health Care FacilitiesAssociation for the Advancement of Medical Instrumentation (AAMI)Developed American National Standard on Safe Current Limits for Electromedical Apparatus

These are the codes and standard that a medical device must comply with.

The threshold of perception is the minimum current that an individual can detect. It varies among individuals. The let-go current is the maximum current at which the subject can withdraw voluntarily from the current. The minimum threshold for the let-go current is 6ma. Higher current can cause involuntary contraction of respiratory muscles severe enough to cause asphyxiation if the current is not removed. This could happen at 18 to 22 mA. The ventricular fibrillation can happen if the current passes through the heart. It is the rapid, disorganized cardiac rhythm. The threshold is about 75mA to 400mA. If the current increases to 1 to 6A, the entire heart muscle contracts and stops beating. This is the sustained myocardial contraction. When the current exceed 10A, the resistive heat causes burns.The thresholds of perception and let-go current are different among people and gender.The heavy the weight, the higher the threshold of ventricle fibrillation.The let-go current has different threshold if the frequency of the current changes. In high frequency, the threshold increases as the frequency.The large externally applied current is called macroshocks. The magnitude of current needed to fibrillate the heart is far greater when the current is applied directly on the body surface than it would be applied directly to heart. If a device provides a conductive path to the heart, a very small current called microshocks can induce ventricular fibrillation.If the chassis is not grounded, a fault connection can bring electric current to the surface of the chassis. If a person touches the chassis, he will get a macroshock. However, if the chassis is grounded, the current is directed to the ground and macroshock can be avoided.If there is a leak current from the power line to the chassis, the ground line can direct 99% of the current to the ground. However, in one case, if the ground is broken and the patient with a catheter touches the chassis, the leak current can go through the catheter and heart if the patient touches the chassis.The electric isolated system has two part, the power isolation and the signal isolation. In both cases, the circuits that connect to the body is electrically insulated from the rest of the circuit. The sensors and preamplifiers are isolated and the electrical signals have to be converted into other physical forms such as light to cross the isolation barrier. The diagram has shown the optical coupling of the signals. The power supply for the isolated unit can be battery or through isolated transformer. These are the codes and standard that a medical device must comply with.