Medical Electronics and Applications: Hospital Safety

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1. Electrical Safety

If we receive an electric shock there are basically two effects which occur. Firstly our nervous system may be excited, and secondly, we may suffer severe burns due to the resistive heating effect of the passage of current through our bodies. The stimulation of our nervous system may cause us injury through excitation of our muscles. However, as the heart is essentially a muscle, its stimulation represents the greatest risk through electrocution.

1.1. Levels of Electric Shock

Some individuals can sense currents as low as 100 pA externally applied at 50 Hz. However, other subjects may not be able to sense currents less than 0.5 mA. The threshold of feeling level for DC currents varies between 2 and 10 mA. These values tell us two things, that the threshold of feeling is an individual characteristic and that the body is more sensitive to AC signals than to DC signals. There is also a difference in the sensitivity of men and women to electric currents; women are generally more susceptible.

After the tingle, or threshold of feeling, the next level of electric shock is the 'let go' threshold. If you grasp a live conductor the muscles in your arm and hand are excited and contract. This causes you to grip the conductor more tightly whilst being electrocuted. At relatively low current levels you are able to overcome the current and still have voluntary control of your muscles. The approximate level above which most males cannot let go of a conducting object is 10 mA at 50 Hz. The level for women at this frequency is approximately 6 mA. At current levels above this limit there is severe pain and ligament damage may ensue.

However, this level of electrocution is not life threatening unless the sufferer is in a hazardous situation. Perhaps a man is electrocuted on a ladder. In this instance a sudden muscle contraction may cause him to fall some distance. At a higher current still, the muscle contraction may be so violent as to cause fractures.

If the level of current flowing lies between 18 and 20 mA then there is potential for chest paralysis. If someone is electrocuted between points on the right hand and the right elbow, obviously this will not occur. If, however, a current passes across the chest of the patient then the muscles which control breathing may become frozen in the contracted state and therefore unable to function. Chest paralysis is extremely painful and sufferers soon become fatigued as they are unable to maintain an adequate supply of air.

If the current which passes across a subject's chest is greater than 22 mA but less than 75 mA, the normal beating rhythm of the heart may be disrupted. At currents greater than 75 mA but less than 400 mA, ventricular fibrillation may result. This occurs when the normal coordinated beating of the heart becomes disturbed and the heart quivers or shakes and no functional beating takes place. With currents greater than this level, the heart suffers sustained contraction, i.e. both ventricles and atria may contract and remain contracted. This is strangely less dangerous than ventricular fibrillation as, following removal of the stimulation, the heart starts beating in a co-ordinated fashion as the whole heart is simultaneously returned to its normal state. In ventricular fibrillation, each section of the heart beats in an uncoordinated fashion and the heart therefore requires an external stimulus to regain its co-ordination.

If a current greater than 10 A passes through the patient then, irrespective of nervous system damage, there will be serious bums due to the heating effect of the current. Accidents of this nature usually occur where high power cables or lines are used for industrial purposes.

1.2. Physical Differences in Electrocution

As we have already stated, men and women tend to have different thresholds for the physiological effects of electrocution. However, individuals within the same sex also have different thresholds. These depend to some extent on body weight and build. The skin resistance of a patient varies significantly with sweating and, therefore, a patient or a subject who touches a live cable with moist hands will receive a greater current than a patient with perfectly dry hands. However, while you are being electrocuted, you sweat. This itself reduces the skin's resistance and tends to increase the current flowing.

The path of the shock current through the sufferer's body determines the muscle groups and nerves affected. Obviously if a patient is electrocuted between two points on one side of the body, such that the current does not flow across their chest, the likelihood of serious injury to their heart or chest is reduced; whereas a current flow across the chest is potentially the most dangerous.

The duration of the current flow through the sufferer is related to the level of damage inflicted by the equation

Imin = 116/√t

Equation 1 is empirical and expresses the length of time or the minimum current which cause ventricular fibrillation. The minimum current for ventricular fibrillation is inversely proportional to the square root of time.

FIG. 1 shows a graph of the threshold of feeling against frequency, it represents the frequency response of nervous tissue to alternating current. The effect of the current on the nervous tissue diminishes at frequencies below 10 Hz and at frequencies above 200 Hz.

Unfortunately, the effect is maximum at approximately 50 Hz; in Europe and America the frequency chosen for domestic electricity supply is between 50 and 60 Hz, which correlates with the worst frequency for electric shock.

FIG. 1 Frequency response of nervous tissue

1.3. Types of Electric Shock

An external electric shock is termed a macro shock. Potential danger to cardiac function exists with currents of greater than 10 mA. There are various situations which may result in a patient receiving a macro shock. FIG. 3 shows two possible conditions in which a patient may receive a macro shock. In FIG. 3a the live wire comes directly into contact with a patient connected lead. In FIG. 3b the combination of a break in the ground connection and a live wire coming loose and touching the casing would cause macro shock if the patient touched the case.

An electric shock applied directly to the heart is termed a micro shock. Micro shocks almost exclusively occur in the clinical environment and involve equipment which is directly connected to the heart. For instance, if a patient's blood pressure is being monitored with a catheter transducer located in the heart, then there is the potential for micro shock. The threshold current of life threatening danger to cardiac function is as low as 50 PA for micro shock.

Increasing levels of micro shock cause various levels of disruption. The first level occurs when the natural rhythm of the heart becomes disturbed. Following this, there is pump failure, when the heart no longer supplies the blood flow required for the patient, thereafter ventricular fibrillation occurs. Obviously, patient data which determines these effects is sparse.

However, a certain amount of this work has been conducted during heart operations. When a surgeon operates on a patient's heart, the heart is given a measured electric shock to elicit ventricular fibrillation, and facilitate work. Researchers have therefore been able to identify current thresholds relating to rhythm disturbance and ventricular fibrillation. The work has also been backed up by significant animal experimentation. A rhythm disturbance can be caused by a current as low as 80 pA, while 600 pA may cause ventricular fibrillation. The right atrium, where the sino-atrial node is situated, is the most susceptible part of the heart to electric shock.

FIG. 2 Macro shock possibilities

The threshold for feeling electric shock depends on the individual and circumstances, but lies between 100 and 500 pA. A danger of causing micro shock exists below the level of perception of the medical staff carrying out an investigation. Capacitive coupling from the live parts of an instrument can cause a current to flow to ground. This current is referred to as leakage current. Typical electronic instruments designed for industrial use may have leakage currents which although unnoticed by their users are above the levels which cause ventricular fibrillation if applied directly to the heart. Leakage currents are a major source of micro shock. The levels of leakage current which are permissible in medical equipment are strictly controlled. Any equipment brought in to a hospital from outside therefore represents a micro shock risk. In FIG. 3a a patient with a cardiac catheter reaches to touch a mains powered radio. The radio designed for non medical use has a potentially dangerous leakage current which flows through the patient and to ground through the cardiac catheter.

FIG. 3 Micro shock situations

The mains distribution system in hospitals uses three wires. The AC power is applied to two conductors, the live and neutral, whilst the third is connected to ground. The ground wire is commonly connected to metal screens within the instrument or to its case. If a live wire becomes loose and contacts such a screen the earth wire serves to carry fault current safely to ground and causes the circuit to fuse. The earth wire also carries leakage current due to capacitive coupling between the earthed screens in the instrument and its live parts. In the event of the earth conductor either in the power cord or the distribution system becoming broken this leakage current can no longer flow. A patient touching the faulty instrument case, as in FIG. 3a, would therefore provide a path to ground for this current. In most cases this would not be noticed; however, if the patient has a cardiac catheter then the path to ground may be through the patient's heart. In this instance micro shock results.

The distribution system in older hospitals may have evolved rather than been designed. It is possible that separate power sockets in one room are connected to different earth points.

These earth points may have different potentials. A patient connected to equipment with different earths receives a current flow owing to the potential difference. This is depicted in FIG. 3b where the patient is simultaneously undergoing ECG measurement and heart catheterization.

To minimize the risk of micro shock the majority of medical equipment used in intensive care areas incorporates isolated circuits, isolated power supplies, and earth free patient connections.

Patients during operations and in intensive care may require high concentrations of oxygen and other potentially explosive gases. These gases may build up in a fault condition and can be ignited by sparks from electrical equipment.

FIG. 4 Isolated power supply

1.4. Isolated Power Supplies

The low thresholds for micro shock make the design of electrical equipment for clinical use difficult. Power supplies produced for industrial equipment have leakage currents in excess of those permitted for medical equipment. To construct equipment with leakage current levels acceptable in the medical environment it is necessary to use isolated power supplies whose primary and secondary windings are separated by an earthed screen (see FIG. 4). The equipment must be constructed such that the capacitive coupling between the primary power supply parts and the secondary circuit and its connections is minimized. With careful design the leakage current from such a power supply can be reduced to below 25 PA. However, stray capacitance and hence leakage current can not be entirely eliminated.

1.5. Isolation Amplifiers

Isolation amplifiers allow two sections of a circuit at different potentials to be connected with a minimized leakage current flowing between them. In medical applications isolation amplifiers are used to protect the patient from both leakage currents and currents arising from fault conditions. They normally consist of a high impedance input section which must be followed by a low leakage barrier. This in turn is followed by a low impedance output, represented in FIG. 5. There are three methods of transferring information from the input to the output via a low leakage barrier. They are transformer coupling, optical coupling and capacitive coupling. In medical applications capacitive barrier isolation amplifiers are not generally used.

FIG. 5 Isolation amplifiers

1.5.1. Transformer Isolation

The differential signal (see FIG. 6) applied to the input of the amplifier is modulated and transmitted through the transformer. In the output section the signal is demodulated and amplified. Isolators fabricated in this way often incorporate feedback of the modulated signal to the input section via a second transformer winding to help correct for non-linear performance. A variety of modulation schemes is used including amplitude modulation.

FIG. 6 Transformer isolation

1.5.2. Optical Isolation

The barrier section is constructed using an optical source and detector. A Light Emitting Diode (LED) is used to transmit light to a photo diode used as a detector. The non-linear output characteristics and poor temperature stability of LEDs cause problems in the design of these devices. Similar modulation techniques to those used for transformer coupled amplifiers are employed (see FIG. 7). Both transformer and optically coupled isolation amplifiers incorporate a transformer winding to transmit power to the input section of the barrier. The device may also provide isolated power for pre-amplification stages. Isolated DC to DC converters are also used to supply isolated power to primary transducer circuits.

Isolation amplifiers must provide isolation up to approximately 5 kV before breakdown. The input stage of a isolated amplifier used in a bio-electronic recording system typically has a common mode rejection ratio of 120 dB.

FIG. 7 Optical isolation

1.6. Residual Current circuit breakers

In normal operation the current flowing down the live wire is equal to the return current flowing down the neutral wire. Discrepancies may be due to leakage currents. In fault conditions the current from the live wire may flow to ground through an alternative route. The current in the neutral conductor is then significantly less. Residual current circuit breakers sense the difference between the current flowing through the live and neutral wires of the supply and interrupt the supply if it exceeds a pre-determined limit.

FIG. 8 Residual current circuit breaker

Practical residual current detectors are built with a small symmetrical transformer placed in the live and neutral lines as shown in FIG. 8. The live and neutral are wound in opposing directions. The flux produced by the respective coils cancels when the currents balance, so a sense coil measures no induced voltage. However, if the current in the neutral wire is different from that flowing along the live wire then a net flux is induced and a voltage is produced in the sensing coil. If this induced voltage exceeds a pre-set limit a relay is switched to disable the supply. Residual current circuit breakers may be set to sense current differences of approximately 2 mA to protect against macro shock.

1.7. Electrical Safety Standards

Europe, Britain and America have standards for the electrical safety of medical equipment which specify acceptable levels of leakage current for a variety of grades of medical equipment. Equipment which is to be used in conjunction with instruments which are directly connected to the heart or equipment which makes a low impedance connection to the patient is classified as requiring higher levels of patient safety then equipment with high functional resistance at the point of application to the patient. It is important in all circumstances to design commercial and experimental research medical equipment to the safety standards in force in the intended country of use. Adherence to these standards ensures a minimum standard of safe operation is maintained. Failure to design to the safety standards may be evidence of negligence if the equipment should at any stage prove faulty or dangerous.

2. Radiation hazards

Dangers from radiation come from a number of sources. Around 20% of our normal dose of radiation comes from previously absorbed radioactive materials. As some materials localize in particular organs in the body these may be especially dangerous. Most of the remaining dose normally comes from background radiation, although in developed countries a significant proportion of this, when averaged through the population, comes from medical sources.

Additionally there is a risk of serious exposure of populations from radioactive accidents, although the authors of a UN report felt unable to make a rationally based assessment of the overall risk owing to a lack of data (UNSC, 1982). Biological damage is classified into two categories (in the same United Nations report):

1. Somatic effects, which apply directly to the irradiated individual, and cause tissue damage.

The hazard from this source depends on the affected region of the body and the age of the individual (younger people are at greater risk owing to their higher rate of cell renewal).

2. Genetic effects which cause either gene mutation or chromosomal aberrations. The former are heritable alterations of the genetic material, which may either be dominant mutations causing effects on the immediate next generation, or recessive mutations, which may not express themselves for several generations to come.

Chromosomal aberrations result in a severely disrupted chromosomal make-up which may lead to very severe abnormalities.

A more accessible description (than is contained in the United Nations report) of biological effects of radiation, precautions, and legal requirements for radiation protection is given in 'An Introduction to Radiation Protection ' by A.Martin and A.B.Harbison (Chapman and Hall, 1979). The somatic effect of radiation causes different forms of damage according to the absorbed dose. A dose of about 3 Gray causes death in 50% of individuals within 30 days of exposure.

Death at this dose level is due to depletion of white blood cells resulting in a reduced resistance to infection. When proper medical attention can be given this cause of death may be significantly reduced. At significantly higher doses the survival time reduces to typically around 3-5 days. Death is due to serious loss of cells in the lining of the intestine, which is in turn followed by severe bacterial invasion. This is called gastrointestinal death.

In both these cases, the damage caused is roughly proportional to the dose absorbed. At lower level, the damage is termed stochastic since a probability of radiation induced damage can only realistically be calculated for a population. The primary form is carcinoma inducing, where signs of damage may become apparent many years after the exposure. A dose of 1 mSv given to each of a population of 1 million people gives rise to around 13 fatal cancers. The normal incidence of cancers in a population of this size per year is around 2000.

2.1. Basic precautions

The major consideration when dealing with ionizing radiation must always be to consider whether the risks involved in its use may outweigh any possible benefits. There must be a clear strategy to minimize any exposures to radiation: this may be by using shielding, keeping a good distance and minimizing the duration of any exposure. In the case of medical exposures particularly, it may be possible to reduce the exposure of organs not under investigation by using addition shielding and restricting the width of the beam. Sensitive areas of the body should be particularly avoided, such as the gonads, as should exposure of children and pregnant women.

When a source is used with a restricted beam, it should be ensured that the radiation which escapes the working area is minimized by shielding and distance: the subsequent movement of the source may require to be restricted so that it may not be used inadvertently in an unprotected direction. Particular care should be taken of workers in radiology as they are likely to be exposed to radiation much more frequently than are their patients.

2.2. Legal requirements

Regulations for the use of radiation are not the same in all countries: there is increasing uniformity in the European Community as certain of the Euratom codes of practice are adopted. The major legislation in the UK is the Health and Safety at Work Act (1974) which defines responsibilities for safety. The statutory body which supervises the enactment of this legislation, the Health and Safety Executive, has drawn up appropriate regulations regarding the designation of types of facility in relation to their risk of causing exposure.

The UK National Radiological Protection Board is responsible for the acquisition of knowledge in the field of radiation safety and providing services to assist in that end.

Both of these topics are covered in greater detail in Martin and Harbison: anyone requiring to use ionizing radiation should however become fully conversant with their appropriate legal codes.

3. Ultrasound safety

Although ultrasound is widely regarded as being harmless, it is possible to weld materials using ultrasound and to destroy kidney stones in situ. It would be more accurate to say that the risk to patients of diagnostic ultrasound at the intensities currently used is minimal.

The passage of ultrasound through tissue causes heating as its wave energy is converted to thermal energy through relaxation processes. The heating effect of ultrasound is used for some therapeutic applications. In Doppler or echo imaging systems this process is unwanted.

In sensitive tissues, such as the brain, the heating effect of ultrasound could be dangerous.

However, the intensities used in current diagnostic equipment are such that the heating effect is negligible.

As ultrasound travels through tissue, the density compaction and rarefaction caused may reach energy levels after which cavitation occurs. It most probably would occur in continuous fields of high intensity possibly caused by standing waves. A number of mechanisms which are potentially damaging have been identified, and studies have been undertaken into the possible systemic effects of damage due to diagnostic ultrasound. They are not believed to occur at energy levels below 100 mW cm-2.

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