Medical Device Safety Standards

Performance Standards

• There are approximately 10,000 device-related patient injuries in the US annually
• Most device-related injuries stem from improper device use due to inadequate training and experience
• Medical procedures pose more hazards compared to typical home or work environments
• Minimum performance standards were introduced in the 1980s

Electrical Safety Standards

• NFPA-99 is the most commonly used standard in the U.S.
• IEC 60601-1 is the most widely used standard globally

Patient Care Vicinity

• Encompasses the area for patient care, extending 1.5 meters (approximately 6 feet) beyond the bed or table
• Extends 2.5 meters (approximately 7.5 feet) vertically from the floor
• Clinical Engineering oversees patient-impact equipment within this vicinity and must adhere to NFPA-99 or IEC 60601-1 standards
• Non-patient care areas might be checked by Clinical Engineering based on hospital policy but are generally exempt from these standards

Effects of Current on the Body

• Electric stimulation is a phenomenon when electric current flows through biological tissue
• Resistive heating of tissue also known as the warming of tissue
• Electrochemical burns and tissue damage can occur from higher voltages

Physiological Effects of Electricity

• Exposure to 60 Hz current via copper wires held in hand for 1-3 seconds can cause the following:
  • Tingling sensation at a threshold of perception
  • Nerves and muscles vigorously stimulated result in pain and fatigue.
  • Contractions of respiratory muscles severe enough to bring about asphyxiation if the current is not interrupted.
  • Heart rate increase to 300 beats per minute if the cardiac activity is disrupted
  • Pumping action of the heart ceases and death occurs within minutes.

Threshold of Perception

• A tingling sensation occurs when the local current density is high enough to excite the nerve endings in the skin
• The minimal current that an individual can detect represents the current perceived
• Threshold varies considerably among individuals
• The lowest thresholds are about 0.5 mA at 60 Hz when moistened hands grasps small copper wires
• Thresholds for DC current range from 2 to 10 mA

Let-Go Current

• Higher levels of current stimulate nerves and muscles resulting in pain and fatigue
• Involuntary muscle contractions can prevent voluntary withdrawal
• The let-go current is the maximal current where voluntary withdrawal is still possible
• The minimal threshold for let-go current is 6 mA
• Minimal let-go current is found at commercial power-line frequencies of 50-60 Hz

Respiratory Paralysis, Pain, and Fatigue

• Involuntary contraction of respiratory muscles at higher currents can cause asphyxiation if the current is not interrupted
• Respiratory arrest can occur during let-go experiments at 18 to 22 mA
• Strong involuntary muscle contractions and nerve stimulation can lead to long-term pain and fatigue

Ventricular Fibrillation

• A portion of the current flows through the heart
• If the current is sufficient, disruption of normal electrical activity in the heart muscle happens
• Heart rate can reach 300 bpm as re-entrant wavefronts of depolarization rapidly sweep over the ventricles
• Contractions in the heart cease and result in death within minutes
• Ventricular fibrillation threshold ranges from about 75 to 400 mA for an average-sized human

Sustained Myocardial Contraction

• The entire heart muscle contracts when the current is high enough
• The heart stops beating while the current is applied; interrupted current leads to a normal rhythm, similar to defibrillation
• Minimal currents for complete myocardial contraction range from 1 to 6 A, according to AC defibrillation experiments on animals
• Brief current applications do not cause irreversible damage to the heart tissue

Burns and Physical Injury

• Little is known about currents over 10 A, especially with short durations
• Resistive heating usually causes burns where skin resistance is high
• Voltages exceeding 240 V can puncture the skin
• High currents cause the brain and nervous tissue to lose functional excitability
• Excessive currents can stimulate muscle contractions resulting in detachment of the muscle from the bone.

Current Path to Ground

• Electrical safety revolves around current path to ground
• Birds are not electrocuted because they're not grounded, meaning there is no path to ground

Point of Entry

• Only a fraction of the total current flows through the heart when current is applied at two points on the surface of the body
• Macroshocks are large, externally applied currents
  • I = 120V / 100 kΩ = 1.2 mA = 1200 μA through the skin
• Invasive devices placed in direct contact with cardiac muscle make patients vulnerable to electric shock
• Microshocks are small currents that can induce ventricular fibrillation when a device provides a conductive path to the heart
  • I = 120V / 25 kΩ = 4.8 mA = 4800 μA under the skin

Classes of Medical Electrical Equipment

• Equipment is categorized based on electric shock protection methods

Class I Equipment

• Class I equipment has a protective earth
• Live parts and exposed conductive parts are insulated
• Protective earth comes into effect if a fault causes an exposed conductive part to become live

Leakage Current

• Leakage current results from both capacitive and resistive factors
• Capacitive results from intrinsic capacitance between conductor
• Resistive is caused by imperfect insulation

Class II Equipment

• Double insulation protects against electric shock
• Basic protection is first insulation layer with supplementary protection as the second
• Physical separation of live conductors from equipment enclosure may serve as basic insulation where the insulating material is air
• The enclosure material forms the supplementary insulation when made with a non-conducting plastic

Class II Equipment Symbol

• The symbol consists of two concentric squares that illustrate double insulation

Class III Equipment

• Protection from electric shock relies on voltages no higher than safety extra low voltage (SELV) level
• No more than 25V AC current or 60V DC current
• Equipment is either battery operated or supplied by a SELV transformer
• Current IEC standards do not recognize Class III equipment; voltage limitation alone does not ensure patient safety
• All mains-connectable medical equipment must be Class I or Class II

Types of Medical Electrical Equipment

• Degree of protection is defined by the type designation
• Type designations account for different application areas and varying electrical safety requirements

Type B

• Applied parts are non-conductive and readily released from the patient
• Non-invasive BP monitors are an example

Type BF

• Devices or parts have direct or long-term contact with the patient
• ECG monitors are representative

Type CF

• Applied parts provide direct contact with the heart
• Invasive pressure monitors and defibrillation paddles

Isolated Power System

• Ground faults can occur even with a separate grounding system for each patient
• Ground faults are short circuits which inject large currents into the grounding system (e.g., damaged wiring) between the live conductor and ground
• Shortcuts can occur through conductive items that may lead to electric shock (e.g., metal appliance casings)

Isolated Power System – Isolation Transformer

• Isolation of both conductors from the ground is achieved with an isolation transformer

Isolated Power System – Line Isolation Monitoring

• The line isolation monitor (LIM) is a device that monitors the isolation power system continuously
• The LIM measures the impedance to ground of each side of the IPS

LIM Operation

• Impedance would be infinitely high and there would be no current flow with perfect isolation
• The amount of leakage will be indicated (in milliamperes) as a result of capacitance or electrical wiring being measured by the meter
• Activation happens at 2 or 5 mA, based on the system's age and brand.
• Exceeding the preset limits visual and audible alarms are triggered, signaling isolation from ground has been compromised

Isolated Power System – Optical Isolator

• Ground loops, caused by noise or high-return currents in ground wires are broken effectively by opto-isolators and transformers
• The opto-isolator contains a source (emitter) of light, almost always a LED, that converts electrical input signal into light, a closed optical channel, and a photosensor, which detects incoming light and either generates electric energy directly
• Transformers and opto-isolators offer reinforced protection and one physical isolation barrier, but protection is equal to double isolation