As the number of COVID-19 cases rises across Canada, doctors are discussing the delicate decisions some patients and families will need to make if physical distancing, handwashing and travel restrictions fail to slow down the spread and intensive care units become overwhelmed.

In critical cases, COVID-19 attacks the lungs and restricts breathing to the point where a patient needs a machine called a ventilator to stay alive.

However, in Canada, as in most other countries, there is a limited supply of these devices.

Faced with spikes in critical cases, physicians in Italy are already facing the reality of having to make life-and-death decisions about who gets a ventilator and who doesn't.

Here are some answers to common questions in this country about the supply and use of ventilators during the pandemic.

Who needs a ventilator?

The World Health Organization (WHO) estimates 80 per cent of people with COVID-19 get better on their own, without needing to be treated in hospital.

Those who are hospitalized are often given oxygen to help them breathe, either through a nasal mask, sometimes called a respirator, or a mouthpiece.

But when someone suffers from severe pneumonia, their lungs aren't able to do their job of getting oxygen in and carbon dioxide waste out. The lungs start filling with fluid. It can feel like you are drowning.

In such serious cases, a mechanical ventilator may help.

What does a ventilator do?

Dr. Anand Kumar, a critical care physician in Winnipeg, said patients critically ill with COVID-19 need specialized ventilator care.

"If a person with a critical illness is going on a ventilator, that's the only option," Kumar said. "They will otherwise die."

The goal of mechanical ventilation is to allow air to go in and out of the chest and lungs until the patient recovers.

The device has regulators to set the right mix of air and oxygen and a fan or turbine that manages the flow to the patient. The patient must be intubated, meaning a tube will be inserted beyond the vocal cords into the airway, or trachea, and sealed to ensure the air goes where it's supposed to.

Air will pass through a humidifier and into the patient's lungs, before being released through a separate tube.

A ventilator provides gas enriched with up to 100 per cent oxygen.

How many ventilators does Canada have?

Dr. Theresa Tam, Canada's chief public health officer, said Monday that of the 220,000 people who've been tested for COVID-19 in this country, three per cent have been confirmed positive. Of the more than 6,000 cases diagnosed so far, seven per cent have required hospitalization, three per cent are critical, and one per cent have died. However, the testing numbers have limitations.

In Ontario alone, confirmed or suspected COVID-19 cases now account for about one out of four patients currently in intensive care units in the province.

Other data compiled by CBC News/Radio-Canada suggests that Canada has 7,752 total ventilators across all provinces, with another 371 on order. Currently, about 80 per cent of the capacity is devoted to non-COVID-19 cases.

Is that enough ventilators?

Dr. Srinivas Murthy, an associate professor of medicine at the University of British Columbia and a critical care physician, says there is concern about the number of critically ill patients who could arrive in hospital over the next few weeks across Canada, despite preparations and collective efforts to flatten the outbreak's curve.

"I think the burden on our system will increase," Murthy said. "It means busy providers running around from patient to patient. It means not enough providers, nurses, doctors, respiratory therapists. It means full ICUs, meaning not enough beds to put them in. And it means ambulances and emergency rooms being full with patients as well."

WATCH | Why Canada could face a shortage of ventilators within weeks:

Canada could face a shortage of ventilators within a few weeks if cases of COVID-19 continue to surge. 2:00

Both Kumar and the WHO have said no country in the world has sufficient ICU beds or ventilator capacity to handle the kind of dramatic surge in critical cases that Italy experienced.

The only approach to try to prevent such a crisis here is to continue to practice physical distancing and isolation, Kumar said.

What guidance do physicians receive on allocating ventilators?

Guidance from the WHO and the Public Health Agency of Canada doesn't go into details on who should receive a ventilator and in what situations.

Instead, teams of doctors, nurses and respiratory therapists and clinicians not directly involved in a patient's care work with the patient and their relatives to decide whether to go on a ventilator.

While such critical care can help people with serious illnesses, it can also cause discomfort.

Palliative care physicians say that since many people who are close to death from a serious illness never return to their original level of health and independence, some choose not to receive CPR or a ventilator and opt for comfort care instead.

Dr. Robert Fowler, a critical care physician at Sunnybrook Health Sciences Centre in Toronto, said it's important for people to consider and decide in advance what measures might be acceptable to them should they need to go to hospital — and to ensure their wishes are expressed to their care team.

"We always want to make sure that the aggressiveness of the care that we bring to the patient's bed is in line with exactly what they might want," Fowler said.

Some recommendations on caring for critically ill adults with COVID-19 were published this week, compiled by 36 experts in 12 countries.

How about in a worst-case scenario?

Dr. Ross Upshur, a professor at the University of Toronto's Dalla Lana School of Public Health, co-authored an article in the New England Journal of Medicine titled, "Fair allocation of scarce medical resources in the time of COVID-19."

As a primary care physician, he has worked on pandemic preparedness scenarios since the 1990s, including during SARS, H1N1 and the Ebola outbreak in West Africa.

This ventilator at Humber River Hospital is one of about 1,300 across Ontario. Companies are being asked to switch gears and help manufacture the equipment. (Craig Chivers/CBC)

He says when the available resources, including staff and equipment, are less than the number of sick people who need care from a virus that could overwhelm health systems — as is the case in Italy and Spain, and was initially in Wuhan, China — then the decision-making tends to become utilitarian.

For instance, if an 18-year-old and a 40-year-old both need a respirator, the person with more chronic conditions, such as hypertension, diabetes or osteoarthritis, has more risk factors for severe disease and a negative outcome, making them less likely to get the potentially live-saving treatment. The decision depends on who has more chronic conditions rather than age, he said.

The purpose of triaging isn't to deny people care, but to make sure care is being given to the people who can most benefit from it.

Upshur outlined guiding principles for allocating health-care resources in a situation of absolute scarcity, which he said hasn't been reached in Canada. They include:

Maximizing benefit, either saving the most individual lives or by giving priority to patients most likely to survive longest after treatment.

Prioritizing health-care and other essential workers, given their willingness to take great risks and the chance they'll recover and contribute once again.

Treating people equally, such as through a random lottery.

What are the risks of ventilator use?

Spending several days or longer on 100 per cent oxygen can damage the lungs, Kumar said.

With high pressure from the ventilator, unhealthy lungs tend to become stiffer, so it takes more pressure to expand them, which can also cause lung injury.

"If you use high pressure on patients with lung injury, they do worse," he said.

Dr. Rob Fowler, right, and Dr. Tom Fletcher, left, seen providing clinical care to patients with Ebola in Guinea back in March 2014. Fowler says there's a range of survival rates when it comes to COVID-19 patients who have been put on a ventilator. (royalcollege.ca)

Also, when health-care workers intubate a patient, they're very close to the mouth of a person excreting large amounts of virus, which is dangerous for them, he said.

"It's very easy to acquire infection unless you take appropriate and very stringent precautions."

What's the survival rate of those put on a ventilator?

"Four in 10 might not make it," said Fowler, based on early North American data of COVID-19 patients who have required ventilators. Survivors are often younger, without underlying health conditions.

Murthy said COVID-19 survival rates after ventilation depend on many variables, such as the severity of lung damage and whether the patient has other illnesses such as heart or kidney failure.

The data is still fairly sparse in Canada and worldwide.

Dr. James Downar is head of palliative care at the University of Ottawa and a critical care physician at the Ottawa Hospital, where he's helping to develop Ontario's triage criteria for the pandemic.

Downar said the criteria haven't been implemented — and hopefully will never have to be.

"We want to cure people when we can," he said. "But when you can't cure somebody and you can't help them get better, you want to keep them comfortable and support them in what might be their final time with us."