How Are Pressure Transducers Used in Ventilators?

Introduction

The global COVID-19 epidemic now threatens the health and mortality of millions of individuals. Similar to other related viruses like MERS and SARS, the new coronavirus attacks and damages the patient's lungs, and it continues to tax international health care resources in many ways. Medical ventilators play a key role in COVID-19 treatment.  Different types of sensors are present inside medical ventilators to monitor oxygen, flow, pressure, humidity, and others. Pressure transducers are fitted inside these life-saving machines for oxygen and air pressure control, and are used to ensure that the ventilator operates smoothly and safely. In this Tech Spotlight article, we will discuss the role of pressure transducers in ventilators.

Overview

What is a medical ventilator? Ventilators are sophisticated medical devices that transport breathable air, occasionally with higher oxygen levels, in and out of a person's lungs when the patient cannot breathe independently. Ventilators are primarily used in emergency care, intensive care, home care, and as a part of anesthesia machines.

How Does a Ventilator Work?

A ventilator utilizes pressure to transport air, or a mixture of gases (e.g., oxygen and air), into the lungs. Such pressure is termed positive pressure. The ventilator performs the human function of exhaling air and can be programmed to "breathe" a particular number of times over a predetermined period, usually minutes. It can also be programmed to switch on the ventilator to force air into a patient's lungs. However, even if the patient does not trigger it within a specified period, the machine spontaneously supplies air to keep the patient breathing.

The Use of Pressure Transducers in Ventilators

Differential pressure sensors or transducers measure flows in several devices. The airflow is proportional to the pressure drop across a curtailment in the flow path. The transducer measures pressure difference, and a signal is sent to the ventilator display. The ventilator controller's feedback loop permits the sensor to maintain and monitor a desired set of conditions. Ventilator design can vary, but all of them have common areas where pressure sensors are used.  These are explained in the following diagram:

Figure 1: Typical components and potential locations for pressure sensor/transducers in a ventilator

1. Pressure measurement between filter and regulator from initial air and oxygen inputs.

2. An oxygen sensor first measures the air/oxygen mix, and then a pressure sensor measures the inhalation (or pressure applied to the patient). A few ventilators may measure the gas pressure leading to the attached external humidifier.

3. Pressure measurement is taken from the patient when that individual exhales (breathes back) into the ventilator.

4. The ventilator's physical location determines the barometric pressure measurement obtained to offset any elevation changes.

The accuracy of such a measuring technique hinges on a combination of the pressure sensor and the component employed to extract the pressure drop, usually a linear flow element or an orifice. These pressure sensors must have high measurement accuracy and short response time. The sensors must also be rugged and have durable stability to minimize maintenance and re-calibration.

Focus: Pressure Transducers

A pressure transducer converts pressure to a digital or analog signal. Multiple technologies can be used to reach this objective, but the strain gauge is the preferred technology for critical applications. The popular strain gauges are made of a semiconductor or a metal foil. Metal foil devices are generally crafted from copper-nickel alloy or nickel-chrome foils in a grid pattern arrangement, and use the resistance change that results from the deformation of the foil elements. Semiconductor devices use silicon or germanium strain gauge and use the piezoresistive properties of these materials.

The strain gauge elements should be made of metal or semiconducting material. The resistance change in metal strain gauges is mainly due to changes in the cross-sectional area and length (geometry) of the material. The disparity in sensor resistance is generally measured through a Wheatstone bridge circuit, as shown in Figure 2. This permits small changes in sensor resistance to be converted into an output voltage.

Figure 2: Piezoresistive strain gauge measurements made using Wheatstone bridge circuit

An excitation voltage must be given to the bridge. A zero volts output is obtained if the strain is absent and if all bridge resistors are balanced.  Any pressure variation will lead to resistance change in the bridge resulting in corresponding current or output voltage. The calculation is done as shown in the following formula:

Enhanced performance can be obtained by using two or four sensing elements in the bridge, with elements in each pair being put through to an opposite or equal strain. This escalates the output signal and minimizes temperature effects on sensor elements. The system can be calibrated to offer a sensitivity factor expressed in mV/V/psi or mV/V/pound-force. This pressure sensor function within ventilators controls air pressure or oxygen control to ensure its safe and smooth service.

The pandemic of COVID-19 demands a growing number of ventilators. The machines must be rapidly manufactured, with particular care of their critical subsystems, new test stands, and integrated sensors.  It is thus vital to choose the right pressure transducers.

The strain sensitive pressure sensors are based on the fundamental piezoresistive Wheatstone Bridge function.  These piezoresistive strain gages attached to a pressure-gathering diaphragm using many materials ranging from glass frit to super-strength epoxies.  The pressure mustering diaphragm is usually circular and made from various materials like ceramic or steel.

Omega's Pressure Transducers

OMEGA's new piezoresistive pressure transducers in their PX409 and PX309 Series use a highly stable silicon wafer micro-machined to precision tolerances and has molecularly-embedded strain gauges. Such construction results in an extremely hardy transducer of exceptional accuracy, stability, and thermal effects. The design toughens the transducers by offering secondary fluid containment in the case of a diaphragm rupture. The micro-machined silicon design is ideal for ventilators that need a high accuracy transducer.

Examples of Pressure Transducers