The Difference Between High-flow Oxygen Therapy and a Ventilator

"My neighbour has been tested positive for Covid and taken to a nearby hospital", reported one WhatsApp group member a few days ago. Another member asked if she was on a ventilator. The first member replied that she was actually receiving "oxygen therapy". A third member interjected: "Oh! That's not bad. My mother has been on an oxygen machine for almost 2 years now. Another knowledgeable member commented, "It's not the same. The oxygen machine is a low-flow oxygen therapy and the hospital uses it for acute patients as a high-flow oxygen therapy.
Everyone else wants to know what the difference is between a ventilator and oxygen therapy - high flow or low flow?!
Everyone knows that using a ventilator is serious. How serious is it to receive oxygen therapy?
Oxygen Therapy and Ventilation in COVID19
In recent months, oxygen therapy has become the buzzword in the treatment of patients with COVID19. March to May 2020 saw a mad scramble for ventilators in India and globally. Governments and people around the world have become very aware of how COVID19 can lead to reduced oxygen saturation in the body. It has been noted that some patients with respiratory distress arrive at hospital emergency rooms with oxygen saturation or SpO2 levels as low as 50-60% without much else to feel.
The normal oxygen saturation range is 94-100%. An oxygen saturation of <94% is described as 'hypoxic'. Hypoxia or hypoxaemia can lead to breathing difficulties and cause acute respiratory distress. Everyone largely believes that a ventilator is the answer for patients with acute Covid19. However, recent statistics show that only about 14% of COVID-19 patients will develop moderate to severe disease requiring hospitalisation and oxygen support, and only another 5% will actually require admission to an intensive care unit and supportive care, including intubation and ventilation.
In other words, 86% of people who test positive for COVID19 are either asymptomatic or exhibit mild to moderate symptoms.
These people do not require either oxygen therapy or ventilation, but the 14% mentioned above do. WHO recommends immediate supplemental oxygen therapy for people in respiratory distress, hypoxia/hypoxaemia or shock. The aim of oxygen therapy is to restore their blood oxygen saturation to >94%.

Just in case you or a loved one happens to fall into the 14% category above - you may want to find out more about oxygen therapy.
You may want to know how oxygen therapy differs from a ventilator.
What are the oxygen units and delivery systems?
How do they work? What are the components?
How do these devices function differently?
How do they differ in terms of benefits and risks?
What are the indications - who needs oxygen therapy and who needs a ventilator?
How is an oxygen therapy device different from a ventilator?
To understand the differences between oxygen therapy equipment and ventilators, we must first understand the difference between ventilation and oxygenation.

Ventilation and Oxygenation
Ventilation - Ventilation is the activity of normal voluntary breathing and involves the processes of inspiration and expiration. If a patient is unable to perform these processes on their own, they may be placed on a ventilator, which performs them for them.

Oxygenation - Ventilation is essential for the process of gas exchange, which is the delivery of oxygen to the lungs and the removal of carbon dioxide from the lungs. Oxygenation is only the first part of the gas exchange process, i.e. the delivery of oxygen to the tissues.

The difference between high flow oxygen therapy and a ventilator is essentially as follows. Oxygen therapy only involves giving you extra oxygen - your lungs still carry out the activity of taking in oxygen-rich air and breathing in carbon dioxide-rich air. A ventilator not only gives you extra oxygen, it also does the work of the lungs - inhaling and exhaling.

Who (what type of patient) needs oxygen therapy and who needs ventilation?
In order to apply the appropriate treatment, it needs to be determined whether the patient's problem is poor oxygenation or poor ventilation.
Respiratory failure can occur due to the following reasons
Oxygenation problems resulting in low oxygen but normal - low levels of carbon dioxide. Also known as hypoxaemic respiratory failure - this usually occurs when the lungs are unable to absorb sufficient oxygen, usually due to acute lung disease causing fluid or sputum to occupy the alveoli (the smallest sac-like structures of the lungs, used to exchange gases). Carbon dioxide levels may be normal or low because the patient is able to exhale normally. Patients with this condition - hypoxaemia - are usually treated with oxygen therapy.

A ventilation problem that results in low oxygen and high carbon dioxide levels. Also known as hypercapnic respiratory failure - this condition is caused by the patient's inability to ventilate or exhale, resulting in a build-up of CO2. The build-up of CO2 then prevents them from taking in enough oxygen. This condition usually requires the support of a ventilator to treat the patient.
Why is low-flow oxygen therapy equipment not suitable for acute cases?
In acute cases, why do we need high-flow oxygen therapy rather than a simple oxygen concentrator?
The tissues in our bodies need oxygen to survive. Prolonged (more than 4 minutes) lack of oxygen or hypoxia in the tissues can lead to serious injury and ultimately death. While it may take some time for a doctor to assess the underlying cause, in the meantime increased oxygen delivery can prevent death or disability.
A normal adult breathes 20-30 litres of air per minute at moderate activity levels. 21% of the air we inhale is oxygen, or about 4-6 litres per minute. In this case, the FiO2 or inhaled oxygen is 21%.
However, in acute cases, the solubility of oxygen in the blood may be very low. Even with an inhaled oxygen concentration of 100%, dissolved oxygen may provide only one third of the resting tissue oxygen requirement. Therefore, one way to address tissue hypoxia is to increase the proportion of inhaled oxygen (FiO 2 ) from the normal 21%. In many acute situations, short periods (even up to 48 hours) of 60-100% inhaled oxygen concentration can be life-saving until more specific treatment can be decided and given.

Suitability of low-flow oxygen equipment in acute care
Low-flow systems have lower flow rates than inspiratory flow rates (as mentioned above, normal inspiratory flow rates are between 20-30 l/min). Low-flow systems such as oxygen concentrators produce flow rates of 5-10 litres/min. Even though they deliver oxygen concentrations of even up to 90%, as the patient needs to inhale room air to compensate for the need to balance the inspiratory flow - the overall FiO2 may be better than 21%, but still insufficient. In addition, at low oxygen flow rates (<5 l/min) there may be significant rebreathing of stale exhaled air as the exhaled air is not sufficiently flushed from the mask. This leads to a higher retention of carbon dioxide and also reduces the further intake of fresh air/oxygen.

In addition, the oropharynx or nasopharynx (airway) provides sufficient humidification when oxygen is delivered at a flow rate of 1-4 l/min through the mask or nasal plug. At higher flow rates or when oxygen is delivered directly to the trachea, additional external humidification is required. Low-flow systems are not equipped to do this. In addition, FiO2 cannot be set accurately in low-flow systems.

Overall, low-flow oxygen systems may not be suitable for acute hypoxia cases.
Suitability of high-flow oxygen equipment in acute care
A high flow system is one that can match or exceed the inspiratory flow rate, i.e. 20-30 litres/minute. Currently available high flow systems can produce a flow rate of 2-120 l/min, just like a ventilator.FiO2 can be set and monitored precisely.FiO2 can approach 90-100% because the patient does not need to breathe any atmosphere and gas loss is negligible. Rebreathing of exhaled gas is not a problem as the mask is flushed by the high flow rate. They also improve patient comfort by retaining moisture in the gas and enough heat to lubricate the nasal passages.
Overall, high-flow systems not only improve the oxygenation required in acute cases, but also reduce the work of breathing, thereby significantly reducing the pressure on the patient's lungs. They are therefore ideally suited to this purpose in cases of acute respiratory distress.
What are the components of a high-flow nasal cannula and ventilator?
We have seen that at least one high-flow oxygen therapy (HFOT) system is needed to treat cases of acute respiratory failure. Let's look at how a high flow (HF) system differs from a ventilator. What are the different components of the two machines and how do they function differently?
Both machines need to be connected to a hospital oxygen source, such as a tube or cylinder. The high flow oxygen therapy system is simple:
Flow generator, air-oxygen mixer, humidifier, heating tube and delivery device such as a nasal cannula.
How ventilators work
The ventilator, on the other hand, has a much wider range. Not only does it consist of all the components of the HFNC, but it also has a breathing, control and monitoring system and alarms to perform safe, controlled, programmable ventilation for the patient.

The most important programming parameters in mechanical ventilation are
Ventilation mode (volume, pressure or dual ventilation) mode (controlled, assisted, support ventilation), and respiratory parameters. The main parameters are tidal volume and minute volume, peak pressure (pressure mode), respiratory rate, positive end-expiratory pressure, inspiratory time, inspiratory flow, inspiratory-to-expiratory ratio, pause time, trigger sensitivity, support pressure and expiratory trigger sensitivity.

Alarms - To detect problems with the ventilator and changes in the patient, tidal and minute volumes, peak pressure, respiratory rate, FiO2 and apnoea alarms can be used.

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