Breathing new life into ventilator technology

[First published on 11 December 2020 ; Nature India Special Issue: COVID-19 Engineering Solutions]

While medical teams in hospitals across India are striving to keep people alive during the pandemic, engineers are bringing their expertise to health care, working to find solutions that make life-saving work easier and safer. Ventilators are the last chance for acute patients in the advanced stages of COVID-19. The level of engineering prowess which goes into designing this lifeline equipment is often overlooked. General precautionary measures, hand sanitizers, face masks, and gloves are readily available, but the desperate need for ventilators remains.

For decades, ventilators produced by internationally acclaimed companies have been the go-to apparatus in newly built intensive care units. A major complaint in the Indian medical community has been the cost of importing and installing ventilators from abroad. Since the pandemic started, the demand for ventilators has skyrocketed worldwide, and the supply chain has lagged significantly. As the pandemic rages, shipping costs, import costs, and installation costs are at an all-time high, a huge burden for Indian hospitals and COVID-19 clinics.

This need has prompted the manufacture of several low cost ventilators in India. Some use the non-invasive route as an alternative to intubation in an intensive care unit to address challenges such as avoiding lung injury while facilitating gas exchange. The domestically designed low-cost, easy-to-build ventilators combine both invasive and non-invasive options. Just like high-end imported versions, they can be used on adults or children, feature a trend graph, and options to alter Inspiratory versus Expiratory ratio (I:E) according to patient need. Some of the domestic ventilators have open-source hardware descriptions, which allows free replication.

Critical care ventilators bring a host of unique features to provide lung protective ventilation therapy for both adults and children. These ventilators play a pivotal role in lung assessment and pulmonary pressure measurement. Exemplified by their ability to provide ventilation and real-time monitoring of pressure, volume and flow waveform more than several hundred times per second, the new range of ventilators detect patient response as swiftly as possible, and support inspiration and expiration.

Ventilators are broadly used in two modes: pressure control, and volume control, both of which can be programmed into target specific modes on demand, depending on patient need. The key factors that differentiate each mode are: trigger (which initiates the inspiratory phase), cycle length (which terminates the inspiratory phase), and primary output (a preset tidal volume or a preset pressure).

Crucial ventilator modes

Continuous mandatory ventilation (CMV) is a mode of mechanical ventilation in which the ventilator has complete control over the patient’s respiration. All breathing work is done by the machine for patients who are under anesthesia, in a coma, or heavily sedated. In this mode, all the parameters are set by the clinician, and the patient cannot trigger the vent. This mode is aimed at severely distressed/ agitated patients with rapid inspiratory efforts and chest injuries. positive inspiratory pressure (PIP), respiratory rate (RR), oxygenation (FiO ₂ ), positive end expiratory pressure (PEEP) are the crucial parameters.

Alternately patients can also be sustained with volume-limited continuous mandatory ventilation, in which the operator can set the required tidal volume (mL) to be delivered during inspiratory and PEEP (cmH ₂ O) level to be maintained during expiration. Tidal volume, respiratory rate, and oxygenation are the crucial parameters. All the volume based ventilation modes are dictated by tidal volume more than by positive inspiratory pressure.

The main emphasis in the continuous mode is to sustain the patient with a remnant pressure, that is, pressure remaining within alveoli at the end of the expiration, PEEP. In essence, the ventilator is in total control. In the assist-control mandatory ventilation, the ventilator performs the work of breathing at a set volume and rate. If the patient triggers the ventilator, assist breath is initiated. The patient may trigger the inspiratory phase on the ventilator (pressure/ flow trigger), which in turn assists the patient by delivering a set volume of air to complete inspiration. This is a volume control mode where the volume and minimum respiratory rate is constant and pressure is monitored to determine the compliance of the lung. (If pressure increases excessively, there is reduced compliance, indicative of lung pathology.)

Unlike controlled breaths, which initiate inspiration at set intervals, assist breaths will be delivered to patient if and when they attempt to breathe. For an assisted breath, the patient must trigger the ventilator, made possible by setting low trigger sensitivity. Synchronized intermittent mandatory ventilation (SIMV) is a type of intermittent mandatory ventilation in which the ventilator delivers either assisted breaths to the patient at the beginning of a spontaneous breath or time-triggered mandatory breaths. The mandatory breaths are synchronized with the patient’s spontaneous breathing efforts so as to avoid stacking. This mode is often used as a weaning mode by decreasing the SIMV phase and allowing the patient to breathe spontaneously. If a breath is not triggered by patient effort within a set time, the ventilator delivers a set tidal volume with a constant flow or operator-selected flow pattern.

Combined PV modes to wean patients: Pressure regulated volume control (PRVC) is an assist control mode that combines the best attributes of volume control and pressure control. The clinician selects a desired tidal volume, and the ventilator gives that tidal volume with each breath, at the lowest possible pressure. Expiration occurs at the end of inspiration and a predefined maximum pressure ensures that over inflation and excessive airway pressure do not occur, causing the ventilator to cycle into the exhalation phase, thus preventing lung injury.

Continuous positive airway pressure (CPAP) increases lung volumes, improving oxygenation. It is a type of positive airway pressure mode, where a preset pressure is maintained in the airway. Patient must breathe spontaneously, with no mechanical breaths delivered. CPAP is a way of delivering PEEP that maintains set pressure throughout the respiratory cycle, during both inspiration and expiration. Apnoea backup provides ventilation after the apnoea time passes with no breath attempts detected. When this occurs, the ventilator immediately switches into apnoea backup ventilation.

Solving complex lung dynamics

The entire function of the ventilation is to ensure required alveolar ventilation by constantly measuring the gas exchange by monitoring the volume of gas entering the lungs that actually participates in the gas exchange. Alveolar ventilation is crucial since the rate at which tissue metabolism produces carbon dioxide determines the oxygenated arterial blood flow. It is also important to observe that the bedside mechanical ventilators measure only pressure and flow with volume being derived from an integration of flow by means of trapezoidal rule. The trapezoidal rule allows flow and time to obtain volume of the integrated area under the flow curve. The other parameters vital for the lung dynamics include: respiratory rate (breaths per minute) of 6 – 40; tidal volume of 200–800mL; I/E ratio (inspiratory/expiration time ratio) of 1:2; and PEEP of 5–15 cm H ₂ O.

Global impact

The critical care ventilators are distinctly different from other low cost ventilators that have emerged during the pandemic, which have limited application. These low-cost clinically viable ventilator designs, like AmbuBox, have a controllable pneumatic enclosure and standard manual resuscitators good enough to provide emergency support until the patient is extended critical care. Patient-specific parameters such as air pressure, air volume, flow speed, and general parameters such as air leakage, mechanical failure, power failure, battery backups, oxygen tanks, and remote control are all equipped with sensors and monitors for the critical care ventilators.

Growth in the ventilators market is driven by the pandemic, a high prevalence of respiratory diseases, an increase in the geriatric population, and an increasing number of preterm births. Technological advances in microprocessor-controlled ventilation integrated with the complexity of new modes have provided opportunities to improve care.

*Director General, Pandit Deendayal Petroleum University, Gandhinagar, Gujarat. **Managing, Director, Hild Defense and Aerospace Private Limited, Chennai.