Ventilatory support (NIV and HFNC oxygen)

Internal Medicine Rapid Refreshers is a series of concise information-packed videos refreshing your knowledge on key medical issues that general practitioners may encounter in their daily practice. This video provides an overview of fundamentals of noninvasive ventilation and high-flow nasal cannula oxygen.

Useful links

Chapter on respiratory support from the McMaster Textbook of Internal Medicine

Transcript

Objectives

Hi, I’m Mark Soth, respirologist and chief of critical care at St Joseph’s Healthcare in Hamilton, Canada, as well as associate professor in the Department of Medicine at McMaster University. In this video we’ll go over the basics of noninvasive ventilation (NIV) and high-flow nasal cannula (HFNC) oxygen. I’ll cover some basic applications and also put that into the context of the current coronavirus disease 2019 (COVID-19) epidemic. This video is intended to be a refresher for those returning to practice in this setting.

General principles

When you’re thinking about NIV, there are some general principles to keep in mind. Basically it’s like a ventilator, with the main difference lying in how it’s interfaced to the patient. Instead of an endotracheal tube, you have to interface it through a type of mask. That tells you a little bit about what the most important features are to consider when deciding who would be a good candidate.

A good candidate needs to have a patent airway, because there is no endotracheal tube there. It’s also best if they don’t have any copious secretions, because you can’t suction directionally below the vocal cords; the patient has to be able to cough any secretions up and have those orally suctioned. It’s also best if it’s a rapidly reversible problem, because people tend not to tolerate masks for a longer period of time and develop other complications, like pressure ulcers.

Because you are dependent on the delivery of pressure through the mask, the seal can also become an issue. If you want to ventilate using higher pressures, like the ones you’d use with a conventional ventilator, you run into problems because of air leak due to the mask fit.

Interface selection

Part of the process is selecting the right type of interface. There are different kinds to choose from.

The simplest interface would be a nasal mask, which is what you’d see used by people who have a continuous positive airway pressure (CPAP) machine for obstructive sleep apnea. It covers the nose and delivers pressure through there. However, that doesn’t work as well with people with acute respiratory failure. They tend to mouth breathe sometimes and unless you can keep their mouth closed, they won’t deliver the same amount of pressure. In that setting we’d use a full face mask.

A full face mask covers both the nose and the mouth. The downside of a full face mask is that it does put more pressure onto the bridge of the nose and can cause some stress ulcers there when used for longer periods of time.

The other difficulty you could have with a full face mask is that sometimes it may not have a good seal in the area around the mouth if it the patient does not have the right facial structure.

If that’s an issue, the other option you have got is a total face mask. This is a mask that goes around the entire face, like a hockey goalie mask. The advantage of this type of mask is that because it’s spreading out pressure over a wider surface area, it does not cause the same issue with pressure ulcers that a full face mask may have. People also tend to have less variability around the outside of their face than they do around the mouth area, so it’s much easier to get a good seal on a wide variety of patients.

In Europe they also have a helmet mask, or a helmet interface, but that’s not available here in North America.

Modes of NIPPV

There are 2 main types of noninvasive positive pressure ventilation (NIPPV).

One type is CPAP. In this mode the machine delivers a constant pressure and the patient can breathe spontaneously around that. The machine will just maintain that constant pressure. The CPAP is analogous to positive end-respiratory pressure (PEEP), or what you’d have delivered with a conventional ventilator.

One purpose of CPAP can be to help splint open the upper airway because we don’t have an endotracheal tube. The other purpose would be to help recruit more of the alveoli, which gives us more surface area for oxygen transfer and better oxygenation. So, CPAP or PEEP helps with oxygenation.

The other mode of ventilation is bilevel positive airway pressure (BiPAP). In this mode of ventilation there’s the expiratory positive airway pressure (EPAP) and the inspiratory positive airway pressure (IPAP). The EPAP is the same as CPAP or PEEP: a constant pressure at the end of expiration that helps with oxygenation.

The IPAP is the maximum pressure that the machine delivers, and it delivers that extra pressure when a breath is triggered. It’s like giving an extra boost to inspiration. This helps to offload the respiratory muscles and improve tidal volumes, which improves ventilation. It can help you with work of breathing or CO₂ retention.

One of the key differences between using BiPAP, or a bilevel noninvasive ventilator, compared to using a usual conventional ventilator is that the IPAP is the maximum pressure that the ventilator delivers. The amount that the patient gets for the extra boost is the difference between the IPAP and the EPAP. If you have someone with an IPAP of 15 and an EPAP of 10, that’s going to give a boost of the difference, which is 5 cm H₂O, with every breath that they take.

Main indications for NIPPV

The main indications that have been well studied are both in acute exacerbation of chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF). These are conditions where the person has a patent airway. They also tend to meet the other criteria where NIV is a good option.

There have been multiple randomized trials looking at the usage of NIV compared to usual care where you use oxygen; if these fail, you’d just intubate the person. The studies have shown both prevention of intubation and mortality benefit. For every 4 patients that you treat with NIV you may prevent 1 intubation, and for every 10 patients that you treat you may prevent 1 death. When intubating someone and putting them on a ventilator you may feel like you’re doing definitive management, but this also means you’re exposing them to all the risks of invasive ventilation. If we can avoid that, we can decrease mortality.

In CHF or pulmonary edema from volume overload it’s a similar story: 10 patients to prevent an intubation and 20 patients to prevent a death. Although it is not quite as strong of a signal as in COPD, NIV can still be considered a very useful intervention for those patients.

Clinical application pearls

How do we actually perform NIV in practice? We have to select the pressures that we want to deliver. I talked a little bit about CPAP or EPAP, and we generally put that in the range of 8 to 12 cm H₂O.

I’d say that you could do that for just about any condition. Let’s say the patient has obstructive sleep apnea and you put them on NIV. They’re finally relaxed, doze off, and fall asleep. If they have obstructive sleep apnea, you need to be providing enough CPAP or EPAP to be splinting open their upper airway and allow it to be patent for this mode of ventilation to work. The other example would be a patient with COPD. In this case they wouldn’t have to do the pursed-lip breathing to generate their own PEEP if you’re giving them some PEEP through the machine. In CHF having some PEEP to cause preload reduction helps with the underlying pathophysiology. So having a PEEP in the range of 8 to 12 cm H₂O would be useful.

The IPAP is the amount that you give for that extra boost to decrease the work of breathing. It would be something in the range of 15 to 20 cm H₂O if you were to try to emulate what they did in the studies that showed mortality benefit.

Some people are hesitant to try to use those higher pressures, thinking that patients won’t tolerate it. If you were to put on a BiPAP machine with your normal lungs, you’d be feeling incredibly uncomfortable with those types of pressures. But someone who has stiff noncompliant lungs or is in respiratory failure with respiratory muscle weakness may need those higher pressures and may actually feel worse if you put a mask on their face and provide inadequate pressures.

So we’d want to have an EPAP in the 8 to 12 cm H₂O range and an IPAP in the 15 to 20 cm H₂O range.

When starting, we acclimatize the patient to NIV and we want to see what the response is. Then we would titrate the pressures to their clinical response, which I’ll talk a little bit more about later.

Times to avoid NIPPV

There are some times when we consider avoiding NIV. One of these would be in pneumonia, particularly if the patient has a lot of secretions. As I mentioned, we don’t have direct access to suctioning below the vocal cords. Because of that and because of the natural history of pneumonia, where it often gets worse before it gets better over many days, there’s a high failure rate of NIV.

Patients who have active cardiac ischemia may also be better off with sedation and intubation than with NIV.

Patients with acute respiratory distress syndrome (ARDS) also have a high failure rate. One of the reasons for that is that ARDS is a condition that takes a long time to get better. The other reason for avoiding NIV in ARDS is that we want to use protective lung ventilation. When we are doing NIV we know what pressure the machine is delivering but we don’t know what pressure the patient is generating on their own. When you take the difference between the positive pressure delivered by the machine and add in the negative pressure that the patient is generating on their own, you can have very high transpulmonary pressures, which causes a lot of lung stress. In those patients we may move more towards intubation and invasive mechanical ventilation.

Assessing clinical response to NIPPV

When starting NIV, you want to titrate it to clinical response and adjust as needed. But how do you know it’s working?

One way is to look at the respiratory rate (RR), which is the key piece of data. This is something subjective and sensitive. If someone is improving on NIV, you’ll often see their respiratory rate come down nicely. There are other more subjective things, such as their work of breathing (WOB), how distressed they look, their accessory muscle use, or if they’re tachycardic because of all that stress. You may see improvements of all those factors.

People often get caught up on getting arterial blood gas (ABG), which may be helpful. However, you’re looking for fast turnovers of results and a quick response. The key is to actually look at the patient. You should see a good response within the first hour of NIV.

If you don’t see a good response and are worried that NIV isn’t working, you have to stop and think about what’s going on. Do you need to adjust the pressures? Perhaps you don’t have them quite right? Maybe you need to go a little bit higher on the IPAP if the patient’s respiratory rate is still high. If they’re still a little bit hypoxic, maybe you need to go higher on the fraction of inspired oxygen (FiO₂) or the oxygen being delivered? A little bit higher on the EPAP? Maybe there’s a leak in the mask and you need to try to adjust the mask or switch to a different one?

Or is it just that you need to be patient? Let’s say the patient has pulmonary edema and they’re just starting to diurese. Maybe you just need to be patient and wait for them to diurese a little bit further. It’s not something that you can fix.

The question is if there’s harm in persisting. The answer is that there probably is. In people that are failing NIV, there is probably harm associated with persisting with NIV till they fail completely.

It’s one of those interventions where you start and then adjust as needed. If you aren’t having a good response, you need to change course and consider intubation and invasive mechanical ventilation.

High-flow nasal cannula (HFNC)

HFNCs are another way of delivering oxygen and a little bit of CPAP to patients.

What makes it different from regular oxygen delivery is that you have a higher flow rate of the oxygen—40 to 60 L/min—through a humidifier and heater and delivery through nasal cannulas. This is different from regular oxygen in that you’re able to humidify and heat the oxygen at a much better rate, so you’re able to use higher flow rates for the patient.

HFNC has had a number of demonstrated benefits. It is much more comfortable for most patients than NIV. It generates some CPAP, in the range of 3 to 5 cm H₂O, so that should help with oxygenation. With the turbulence that’s created within the large airways, it also helps with some CO₂ washout. The humidification helps to keep secretions nice and viscous so that they’re easier to cough out, and because you are not covering the patient’s mouth, it’s easier for them to cough them up into their mouths and suction them.

HFNC and COVID-19

How do we use these interventions in the setting of COVID-19? HFNC is probably one of our first-line therapies for patients that can’t tolerate using nasal prongs or who need more oxygen with nasal prongs. One of the reasons for that is because we can put an HFNC on the patient and use a surgical mask to help decrease aerosolization. We see variability in practice on that: some patients tolerate having a surgical mask and others don’t.

Either way, if possible we would try to have this done in a negative-pressure room with full personal protection equipment (PPE), because this is considered an aerosol-generating medical procedure. It may be less aerosolizing than a Venturi mask, but you still need to take full precautions.

What if the patient is failing HFNC? I wouldn’t wait until the last minute to intubate them. If someone’s getting on higher amounts of HFNC oxygen, then you need to consider what the next step is. In some patients you may continue to watch on it while in others you may move on to intubation. This is definitely a time when you want to be consulting an intensivist to help you with the decision.

The reason why you don’t want to wait until the last minute is because intubating someone in the setting of COVID-19 can be a complicated procedure to do safely. Even without COVID-19, it’s much more difficult to intubate someone who’s already on high amounts of oxygen and failing completely than someone in a more controlled situation.

Take-home messages

The take-home messages would be to consider NIV or HFNC when thinking about ventilating someone.

The main difference between noninvasive and invasive ventilation is the interface. You want to choose the right type of mask and the right types of pressures. You also want to consider this intervention for diseases in which it has mortality benefits. COPD and CHF are still very prominent diseases in the setting of COVID-19, or at any other time, and this would be considered the first line to treat them.

How are we approaching this in COVID-19? HFNC would be our preferred method of oxygen delivery for patients with COVID-19 who tend to be hypoxic yet still have very compliant lungs. Using added pressures for inspiration, like in BiPAP or with invasive ventilation, may not be necessary and could even be harmful. If you are able to use HFNC, that would be our first choice. If not, you could consider NIV in some settings or you would move on to intubate the patient.

If the patient has COPD or CHF, you would want to use NIV as one of our first-line therapies. If they’re COVID-19–negative, you would continue that. If they’re COVID-19–positive, how much you’d persist with that is a little more controversial. That would be another time where you’d want to consult with an intensivist or respirologist to help you with the decision-making.

Because this is an aerosol-generating medical procedure, it needs to be done in a negative-pressure room with full PPE.

If the patient is failing NIV, you should move on to an invasive ventilation approach.