Biomedical Instrumentation

Transcription

1 University of Zagreb Faculty of Electrical Engineering and Computing Biomedical Instrumentation Safety of electrical medical devices prof.dr.sc. Ratko Magjarević

2 Medical technology Achievements Significant improvement of health care in all medical specialties Reducing the number of seriously ill people Reduction of mortality Consequences increased complexity of medical devices and their applications increased incidence of adverse effects of implementation of MT in the U.S. approx. 10,000 cases of patient injury due to the application of MT

3 Medical technology Causes of unintended consequences: improper handling insufficient or improper staff practice (training) in handling with MT insufficient experience example, medical staff rarely read manuals for the users (User Manual) equipment malfunction Demand for MT developers developement of safe devices (failure-safe)

4 Danger to patients In a clinical environment patient is exposed to various risks, more than a typical workplace or at home frequent invasive (blood) operations - penetration through skin or mucous membranes presence of potentially hazardous chemicals and substances - anesthetics, medicines, medical gases sources of infection - particularly "hospital infection" various sources of energy that penetrate into or through the patient: current, voltage, ionizing and non-ionizing radiation, sound and ultrasound, electric and magnetic field, UV radiation, lasers, microwave radiation, mechanical stress, etc.

5 Safe MT development Electrical MT security is considered: physiological effects of electricity possibility (risk) of failures and their consequences methods of patients and staff protection standards describing electrical safety electrical safety testing modes Understanding the possible dangers and risks Implementation in achieving security

6 Physiological effects of electricity Body (tissue) becoming a part of an electrical circuit The amount (amplitude) of electricity often depends on the ratio of voltage present and the sum of all (serially connected) impedance Usually, the highest impedance is the impedance of the skin The consequence of current flow: nerve and/or muscle tissue stimulation heating of tissues (a result of tissue resistance) burns

7 Pulsed current I t curves for sensory and motor responses

8 Physiological effects of current

9 Electricity grid Unintended consequences are usually a result of human involvement in the circuit, in some way connecting to the grid (inattention, failure) Several levels of action perception contractions (let-go) paralysis (respiratory), pain, fatigue ventricular fibrillation tetanic contraction burns and physical injuries

10 Consequences of dangerous voltage Fibrilation Convulsion Consequences depende on the voltage current relation, but also on individual susceptibility Expected values Intact skin, R K = 2-5 kω Damaged skin - injuries, surgical procedures, R O = 100 Ω 1 kω

11 Consequences of dangerous voltage Focus on the worst possible scenario, for example, a person holding two conductors at different potentials in his hands resulting muscle spasms, inability to release captured conductors In case of intact skin, R K = 2-5 kω, cramps can occur even at a voltage of few dozen ma In case of damaged skin - injuries, surgical procedures, R O = 100 Ω 1 kω, fibrillation may occur at voltages of several volts and current of few dozen ma

12 Excitability dependence of the pulse waveforms Monopolar vs. bipolar pulses, sinusoidal current Extrapolation of sinusoidal current What is the duration of the half-cycle sine current to achieve maximum excitability?

13 Safe voltages - AC Security measures in designing defining voltage values that may not appear on the conductive parts of instruments or equipment coming into contact with the patients skin or user for alternating current: safety extra low voltage, SELV of 50V, 50/60Hz medical safety extra low voltage, MSELV of 25V, 50/60Hz

14 Safe voltages - DC Security measures in designing defining voltage values that may not appear on the conductive parts of instruments or equipment coming into contact with the patients skin or user for direct current: safety extra low voltage, SELV of 120V, medical safety extra low voltage, MSELV of 60V, 50/60Hz

15 Ventricular fibrillation In cases of shock due to unwanted connection to power grid voltage, current passing through the heart is considered the most dangerous effect and can cause ventricular fibrillation (ventricles) with fatal consequences Ventricular fibrillation is completely irregular, asynchronous heartbeat; no synchronism in ventricular contractions, blood flow stops and if such heart work does not stop, it leads to death within a few minutes

16 Microshock vs. macroshock The consequences of current passage through the tissue depend on the contact point with the source of voltage Even greater power will not necessarily cause fatal consequences, eg. fibrillation, when going through the limbs

17 Microshock vs. macroshock Macroshock is caused by current passing through the body through the skin, and the current that can cause harmful effects is relatively high and significantly depends on the point of contact Microshock is caused by the passage of relatively low current, but the source of electricity was brought directly into contact with the heart, for example, during cardiac catheterization (catheter in the heart is a diagnostic procedure). Electricity sufficient to induce fibrillation is of n x 10μA. Another point of contact can be anywhere on the body (eg limbs). Current of 10 μa is considered safe limit to prevent microshock

18 Protective grounding Generally, patients are only occasionally and accidentally exposed to risk of getting in touching with devices whose conductive parts may be energized In hospitals, especially in intensive care units, patients are deliberately connected to the diagnostic and/or therapeutic electrical devices/equipment particularly careful with the isolation of any conductive parts connected to the heart or its vicinity from all other conductive parts all conductive parts in the vicinity of the patient must be connected to a single point grounding (eg, metal bed, cupboard, etc.) periodic testing of the grounding impedance must be provided Tolerable difference in potential between the grounded conductive parts in clinical areas: in the general - of 500 mv intensive care and other critical cases- of 40 mv

19 Power in clinical environments Under normal conditions, increasing patient safety is provided by: formation of isolated parts of the grid installation of automatic monitoring of impedance grounding (line isolation monitor)

20 Power failure In case of grid power failure, in critical areas in hospitals power is automatically provided within 10s after stoppage Critical components are: lighting alarms equipment in intensive care

21 Equipment safe during the first failure The principle of redundancy can be applied to the safety of medical devices and equipment (KEU) For example, it is required that electrical medical equipment is safe even in case of a breakdown on the conductive contact with patient Grounding impedance must be sufficiently low so that the patient may not get in touch with any dangerous voltage

22 Leakage currents Occur between conductors that are not in direct contact (touch) and are at different potentials For devices that are powered from the grid, leakage currents are: capacitive character (spreading capacity) operative character (current through the insulation, dust, humidity...)

23 Leakage currents pathways Pathways of leakage current through the patient a) Intact ground lead b) Broken protective ground lead, patien connected to ground through a catheter c) Broken protective ground lead, patien connected to ground by an electrode or unintentianally.

24 Leakage currents

25 Conductive paths to / from the heart In clinical settings there are current flow pathways that cause heart microshock: The external pacemaker connected to epicardial (heart area) or endocardial (inside heart) electrodes Intracardial ECG measuring electrodes Catheters for invasive heart pressure measuring taking blood samples injection of contrast or medication into the heart

26 Regulations, standards and recommendations Regulations are subordinate legislation which are obligatory and are related to the characteristics of devices and equipment that must be assured Standards stipulate features that must be assured, but their appliance is voluntary. Contain instructions on how to accomplish / verify compliance with the standard Recommendations (usually manufacturers) stipulate how to ensure the safety and prescribe manner and frequency of testing safety of devices

27 Standards Various standards institutes prescribe standards for electromedical devices and equipment These standards are mostly aligned, due to the global market The fundamental norm in Europe, IEC , Medical Electrical Equipment, Part 1:General requirements for basic safety and essential performances These codes are followed by the standard xxx, Particular requirements for...

28 Design Recommendations for Safety Reliable grounding for equipment Reduction of leakage current Double insulated equipment Operation at low voltages Driven right leg circuit Current limiters Electrical isolationisolated heart connectors

29 Safety checking Electrical Safety Analysis: general for certain types of electrical medical devices and equipment Demo practice: "General" analyzer according to Defibrillator analyzing operation

30 Determination of device categorie

31 Mind the costs, but satisfy safety requirements! Price/performance

32 Literature J. Webster: Medical Instrumentation, Chapter 14, Electrical Safety