Can respiratory support devices create air hunger through flow dyssynchrony?

Content provided by Mercury Medical

By Steven C. LeCroy, MA, CRT, EMTP

Have you ever had this happen? You are treating a patient with a chief complaint of difficulty breathing. The patient is awake and alert, respiratory rate of 26, pulse-oximetry is 88% with an EtCO₂ of 46 mmHg with a history of CHF. After placing the patient on a nasal cannula at 6 LPM there is no change in the work of breath or oxygen saturation. You decide to place the patient on a non-rebreather mask at 10 LPM (bag stays inflated during inhalation). After several minutes the patient’s respiratory rate is 32, work of breathing has increased, oxygen saturation is 84%, patient is anxious and trying to pull the mask off. How did the patient’s work of breathing go up and oxygen saturation go down when you went to a mask with a higher oxygen concentration and flow rate? Did the patient deteriorate or was it something you did?

Dyssynchrony is the effect of the patient’s respiratory demands not being appropriately met by the device. The patient has their own idea about how to breathe and the device supporting them. Instead of making breathing easier, the device interferes with ventilation or oxygenation while increasing work of breathing. For patients on ventilators this phenomenon is called Patient-Ventilator Dyssynchrony or PVD. However, the problem can occur with any ventilation or oxygenation support device.

Patients such as the one in the scenario need to have oxygen/air supplied at a velocity that meets their increased demand. This is commonly referred to as peak inspiratory flow rate or PIFR. Considering patients in respiratory distress who often need a PIFR of greater than 40 L/min. In these cases when a mask is applied at 10 L/min (100% oxygen), this would require patients to draw in at least 30 L/min of room air (21%) around the mask (increasing the work) while diluting the oxygen percentage to 40.75%.

Here’s the math using the FiO₂ formula:

FiO₂ Formula = (Air flow x 0.21) + (Oxygen flow x 1.0) /Total Flow

FiO₂ = (Air Flow x 0.21) + (Oxygen Flow x 1.0)

Total Flow

(30 L/min x 0.21) + (10 L/min x 1.0)

40 L/min

(6.3 + 10)/40 = .4075 X 100 = 40.75

FiO₂ = 40.75%

It should not be a surprise that the work of breathing can go up and oxygen saturation can go down when the patient and the equipment are not in sync.

This example shows that even though the oxygen flowrate increased from 6 to 10 L/min the percentage of oxygen the patient received went down. The biggest change was the increase in the patient’s inspiratory flow demand and how much that “diluted” the pure oxygen being delivered. If the flow rate from the device exceeds the patient’s peak inspiratory flow rate, there is little need to draw in room air (diluting the oxygen).

Flow dyssynchrony can occur when either the device is not set correctly or not capable of meeting the patient’s flow demand.

Why is it bad?

• Work of breathing increases: Remember the point is to make breathing EASIER
• With increased work, oxygen demand increases: Increased workload can be catastrophic for some patients
• Dyssynchrony can give a feeling of asphyxiation causing the patient’s stress level to go up leading them to take the device off

If dyssynchrony is not corrected, the unsuspecting clinician may believe the patient is just anxious or claustrophobic over wearing a mask and attempt to sedate the patient. Giving sedation in this scenario is a pending disaster for both the clinician and patient.

It should be clear how low flow oxygen administration devices can lead to an increase in work of breathing and/or hypoxia. With the increase in the pre-hospital use of Continuous Positive Airway Pressure (CPAP) disposable devices, could flow dyssynchrony be a problem? Especially if the device uses lower gas flows (5 – 15 LPM). Not all low-flow CPAP devices provide positive pressure during inhalation and are essentially nothing more than PEEP (Positive End Expiratory Pressure) devices with limited supplemental oxygen capability, some as low as 30% FiO₂. PEEP alone does not constitute CPAP and does not provide the same respiratory benefits. Flow dyssynchrony and low FiO₂ is why some patient deteriorate when placed on CPAP.

Disposable CPAP devices vary on their ability to meet or exceed a patient’s peak inspiratory flow needs. Some manufacturers claim their CPAP device can achieve 5 cmH₂O on as little as 5 L/min of gas flow. In most cases this claim is more likely than not to be true. What they don’t claim is consistent pressure during both inhalation and exhalation nor do they mention peak flow rates. If a device does not maintain pressure, it’s not CPAP and if the peak flow does not match the patient’s demand, we know work of breathing goes up and oxygen saturations go down.

As stated earlier, patients in respiratory distress may need flow rates in excess of 40 L/min. This does not mean low flow disposable CPAP devices can’t meet the demand. Some manufacturers have integrated ways to increase peak flow some as high as 65 L/min while only using 8 L/min. It’s important to remember that in this case 65 L/min refers to velocity and not volume. Patients in respiratory distress may not need more air but they will always need it faster.

There are two ways to determine if a disposable CPAP device is meeting the patient’s flow demand:

1. Place a manometer inline or use a device with an integrated manometer. Observe the manometer, if the reading goes to zero when the patient inhales the flow is not adequate. An inadequate flow will force the patient to draw in room air increasing work of breathing, possible hypoxia and a feeling of suffocation.
2. Evaluate the patient, look for nasal flaring, use of accessory muscles, increase in inspiratory effort, increase in heart rate and respiratory rate, hypoxia, increased agitation. These signs and symptoms can be caused by several issues, however device flow dyssynchrony should be at the top of the list.

So next time you are assessing and treating a patient in respiratory distress who is trying to inhale like their lives depend on it (it probably is), make their breathing easier and give them some additional flow! The increased flow may also go a long way towards solving the patient’s low oxygen saturation.

Rules to live by:

1. Patients with oxygenation issues need an increase in FiO₂.
2. Patients fighting to move the air need more flow.
3. Patients that have oxygenation issues and trouble moving air need both.

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