Specialist electronics design company TT electronics Welwyn Components has pioneered a new high-voltage resistor to be used in the manufacture of life-saving technology for cardiac arrest patients. The new resistor is a core component in automated external defibrillators (AEDs) - portable systems which give patients immediate access to essential treatment.

Sudden cardiac arrest (SCA), exposure to high voltage power lines, and other trauma to the body can result in ventricular fibrillation. Ventricular fibrillation is the rapid and uncoordinated contraction of the myocardium of the heart. Ventricular fibrillation is a life-threatening medical condition in which the electrical activity of a person’s heart becomes unsynchronized, resulting in a loss of ability of the heart to pump blood into the person’s circulation system. But unlike other health problems of this magnitude, SCA is treatable. The treatment for most cases of SCA is immediate treatment with a defibrillator, a device that shocks the heart out of a fatal rhythm, allowing a normal, healthy rhythm to resume. A defibrillator provides an electrical shock to the heart, in order to convert the irregular heat beat to a normal sinus rhythm.

Science and industry have developed an automated or semi-automated external defibrillator (AED) that provides a safe and effective treatment, and is automated enough to be used by non-medical personnel. AEDs can be placed at dispersed, non-hospital locations throughout a community to provide victims of SCA quick access to this treatment. The use of external defibrillators to restore the heart beat to its normal pace through the application of an electrical shock is a well recognized and important tool in resuscitating patients. External defibrillation is typically used in emergency settings in which the patient is unconscious. External defibrillators may be manually operated, as are typically used in hospitals by medical personnel or may be semi-automatic, semi-automated, fully automatic, or fully automated devices, where they can be used in any location where an unanticipated need may occur. An AED is a portable battery-operated device that analyzes a patient’s heart rhythm, and, if appropriate, administers an electrical defibrillation shock (automated) or instructs an operator to administer the shock (semi-automated) to the patient via electrode pads. For example, such a defibrillator shock can often revive a patient who is experiencing ventricular fibrillation (VF).

Previously, patients who had suffered cardiac arrests had to wait until they had been transported to hospital before they could receive defibrillation treatment. But in an attempt to reduce the delay in receiving this treatment, and thus improve cardiac arrest survival, many healthcare providers are now using the portable AEDs. Welwyn was approached by a market leader in the development and manufacture of AEDs in a bid to improve the reliability of the portable systems. The challenge was to adapt hospital-based defibrillation equipment by miniaturising the defibrillator to a small portable device whilst ensuring it could withstand harsher temperatures and humidity environments without compromising reliability. The stable and repeatable measurement of the charging voltage is a critical function of this circuit, as this determines the amount of electrical energy delivered to the patient. In this instance R1 was a high voltage resistor with a value of 20M and 1% tolerance, which together with a standard chip resistor R2, forms a potential divider for voltage feedback.

In addition to the requirement for a physically smaller component with better environmental performance, the high-voltage resistor needed to have excellent linearity - expressed by voltage coefficient (VCR) and temperature coefficient (TCR) - and long-term stability under voltage stress. Welwyn’s design team developed a new high-voltage planar resistor incorporating a thick-film resistor ink based on a ruthenium oxide/glass system which is subsequently encapsulated with a specially formulated high-density epoxy protection to give improved environmental performance over the traditional porous conformal coat protection.

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