The COVID-19 pandemic has upended the daily lives for much of the world and several reports forecast that the disease is here to stay. In critically ill patients, the disease can result in acute respiratory distress syndrome (ARDS), where the patients may need to be put on ventilators. In March, due to the shortage of ventilators, the Italian doctors faced an extraordinary challenging decision, i.e., choosing which patients to save. Along the same lines, several states in the US issued guidelines on rationing the use of ventilators.
Most studies estimate that there is a shortage of ventilators around the world. However, the quantitative estimates of the shortage vary widely, mainly because a lot is still unknown about the disease and its effects. To address this shortage, research and engineering have progressed at breakneck speed to come up with innovative solutions. Numerous universities and corporations are designing and manufacturing ventilators. In light of these developments, the main purpose of this article is to focus on the following questions:
1. What is the purpose of a ventilator?
2. What is the difference between invasive and non-invasive ventilation?
3. What are the main types of mechanical ventilators?
4. Why are ventilators believed to be important for COVID-19?
5. What are some of the innovative solutions to ventilator shortage that are being deployed to fight the COVID-19 pandemic?
What is the purpose of a ventilator?
In a healthy individual, lungs perform the critical function of supplying oxygen and removing carbon dioxide from the bloodstream. This exchange of gases, i.e., intake of oxygen and outtake of carbon dioxide from the bloodstream, occurs inside millions of alveoli, which are cup shaped cavities inside the lungs.
In an individual suffering with acute respiratory distress syndrome (ARDS), the alveoli may inflame and fill up with fluid, thereby reducing the capacity of the lungs to breathe normally. In such patients, ventilators provide assistance in breathing. Note that mechanical ventilation (explained below) is the primary form of treatment for ARDS. The essential goal of ventilation is to “recruit” the alveoli that are unable to participate due to inflammation. Note that the recruitment of alveoli refers to the reopening of previously gasless alveoli.
Healthy vs inflamed lung. Image source: here.
It should be noted that ventilation is different from oxygen therapy. In oxygen therapy, a patient is supplied with air containing a higher concentration of oxygen so as to increase the level of oxygen in the blood. However, if the patient has inflamed alveoli, oxygen therapy alone will be unable to recruit the underventilated alveoli.
What is the difference between invasive and non-invasive ventilation?
A ventilator can be invasive or non-invasive. In the invasive mode, the patient is sedated, and a tube is inserted into the patient’s throat or trachea, or the process known as intubation. Here, the patient’s breathing is assisted through control of tidal volume, breathing rate etc. Invasive ventilation can have a broad range of modes and the interested readers are referred here for more details.
In contrast, in the non-invasive mode, the ventilation is provided through a sealed mask or a helmet. The primary mode here is referred to as CPAP, i.e., continuous positive airway pressure. Here, the patient inhales and exhales spontaneously but at a higher pressure. The higher pressure can help recruit some of the underventilated alveoli while the patient recovers. CPAP is utilized commonly in sleep apnea patients and is a milder form of treatment for respiratory patients when compared to invasive mode.
What are the main types of mechanical ventilators?
We now focus on the different types of mechanical ventilators. Mechanical ventilators are mostly used during the invasive mode of ventilation, though they can also be employed during the non-invasive mode. Several types of such ventilators exist, as shown in the plot below.
Summary of different types of ventilators. The plot has been adapted from Husseini et al. 2010. The image of bag valve masks has been taken from here. The image of a typical hospital ventilator has been taken from here.
The commercial ventilators used in hospitals are sophisticated equipment and possess a broad range of modes to control and monitor a patient’s health. For instance, these ventilators can be used in continuous modes, intermittent modes, pressure control mode, volume control mode, among several others. Due to the precise control and flexibility these instruments provide, their cost ranges from $10,000 to $30,000, and possibly even more. However, their enormous cost and lack of portability make them less ideal to be the only choice in a pandemic.
In contrast, the cheapest form of mechanical ventilation is the bag valve mask (BVM), also commonly known as the Ambu bag. In a BVM, the patient is provided air through manual squeezing of the bag. The bag consists of several valves to assist in inhalation and exhalation. The cost of BVM could be as low as $10. However, due to its basic design, the bag valve mask cannot provide control over the critical parameters like air volume and breathing rate. In addition, up to two personnel may be required for the usage of each BVM, making it unsuitable for large scale use. Other types of ventilators such as electric and pneumatic also exist, and the interested readers can read more about them here.
Why are ventilators believed to be important for COVID-19?
The enormous challenge with COVID-19 is that it spreads rapidly with estimates of its reproduction number R0 ranging from R0 = 2-6. The value of R0 estimates how many infections might be created through an infected person. If the value of R0 < 1, the infection will likely die after some time. However, if R0 > 1, the infection will spread. Note that R0 is not a biological constant for a pathogen, and can be decreased through policies like social distancing.
Given the relatively large reproduction number of COVID-19, it is expected that COVID-19 is here to stay until a vaccine or a drug is developed. Since COVID-19 can cause acute respiratory distress (ARDS), several reports estimate that there will be a significant shortage of ventilators (note that ventilators are the primary form of treatment for ARDS), with the estimates of shortage varying widely.
It should be noted that more recent developments paint a more encouraging picture, i.e., the shortage of ventilators may not be as dire as initially predicted. Moreover, there are also concerns that invasive ventilation may not be as crucial for critically ill COVID-19 patients as it sometimes predicted to be. Nonetheless, it is still generally accepted that ventilator supply are critical, especially to prepare in case of a future outbreak.
What are some of the innovative solutions to ventilator shortage that are being deployed to fight the COVID-19 pandemic?
In the last 2-3 months, numerous designs and approaches have been developed to address the shortage of ventilators. Here, in the interest of time, we will focus on two interesting approaches: the e-vent design and the helmet design.
The e-vent design
The design was first conceived at MIT in 2010 (link to the original paper, link to project website). The basic idea is to exploit the widespread availability of BVMs and engineer the BVMs to provide automated control of parameters like air volume, breathing rate, I:E ratio etc. A sketch of the design is provided below.
The basic e-vent design. The image has been taken from Husseini et al. 2010. For COVID-19, the variants of this design are being developed for basic mechanical ventilation.
The control of air volume and breathing rate is obtained through a cam concept, i.e., a crescent-shaped cam that enables smooth and reproducible deformation; see figure above and the video below. This enables the air volume to be controlled from 200 mL – 750 mL and the breathing rate to be controlled from 5 – 30 bpm.
The e-vent design can be customized for both invasive and non-invasive modes. The video shared above demonstrates a successful implementation of a variant of the e-vent design in Chennai, India.
Naturally, the e-vent design does not have all the functionalities and modes of the expensive hospital ventilators. However, it does provide the functionality of a mechanical ventilator and can be customized for basic modes such as PEEP. Therefore, the e-vent and its variants can at least support those patients who are in less critical condition.
Given the “relatively” simple design and few mechanical parts, the e-vent design can cost about $200-500 and could weigh as little as 4-8 kgs. Therefore, the design is suitable for rapid deployment.
The helmet design
As discussed previously, the helmet design is particularly suited for non-invasive ventilation. Just to reiterate, in non-invasive mode, the patient breathes spontaneously but at a higher ambient pressure (the CPAP mode). A sketch of the design is provided below. The helmet design can assist in treating less severe patients and/or reduce the time spent by a patient in the ICU. Doctors have also reported that the patients are more comfortable in wearing helmets as opposed to intubation. Furthermore, the medical staff is less exposed to viral droplets that may come from an infected patient.
The helmet design. The ventilation through helmet design (left). The images have been taken from here. A flowmeter (right) can be added onto the helmets to monitor respiratory parameters, as developed by the Princeton University's open ventilation monitor project.
The helmet design does present its challenges – it may be only suited for less severe patients. Furthermore, it was suggested that a low supply of fresh gas could result in increased level of CO2, which could even result in asphyxiation. To resolve this challenge, the open ventilation monitor project at Princeton University has developed an inexpensive flowmeter that enables monitoring of respiratory parameters – tidal volume, breathing rate, and the I:E ratio, among others.
Let us review
The ventilators provide assistance in breathing through invasive and non-invasive modes. The sophisticated ventilators used in hospitals provide fine control of parameters but may not be suitable for widespread deployment, as required during the COVID-19 pandemic, To tackle this, researchers are designing innovative solutions to control or monitor respiratory parameters in existing setups such as bag valve masks or helmets.
Let me use this opportunity to thank all the frontline health workers and essential personnel who have been working (for all of us) through the pandemic. Also, let us all hope that we never have to use these ventilators!
Acknowledgments
Thanks to Viplove Arora, Ravi Suman, Himani Gupta, and Charu Mehta for reviewing the blog. I also want to thank Dr. Dinesh Kumar Gupta (my father), who introduced me to basic medial terminology related to mechanical ventilators.
Comments