Hi I'm Dr. Mahandevan and I'm Dr. Strehlow. This lecture will cover the second part of treating
the moderately dyspneic patient. Specifically we will focus on two major modalities, high
flow nasal cannula and non-invasive respiratory support. What is high flow nasal cannula therapy?
It's a specialized delivery device that is simple and safe it delivers high oxygen flow rates
greater than 30 liters per minute that provide more efficient oxygenation than conventional
oxygen therapy. It allows for heating and humidification preventing desiccation of the nasal
mucosa and that intolerable burning. The silicone nasal cannula fits without occluding the nose and
this provides better comfort when compared with the masks used in non-invasive respiratory support.
High flow nasal cannula consists of an open circuit system made up of a flow generator
which blends air and oxygen an active heated humidifier and a single heated non-condensing
circuit that connects to a silicon nasal cannula which is sized to fit the patient's nostrils.
There are two major settings that can be adjusted with high flow nasal cannula:
therapy the flow rate or air delivered in liters per minute and the oxygen concentration.
Since the flow seems to help these patients start with a fixed flow of 30 liters per minute.
Although some advocating for starting higher at 60 liters per minute and start with an fio2 of a100%
Start with a temperature of 37 degrees Celsius and adjust it down to 34 or 31
degrees as tolerated by the patient. When weaning a patient start by decreasing the fio2 first
then decreasing the flow rate in increments of 5 liters at a time. Why does high flow nasal cannula
therapy work with COVID patients? First, it provides for efficient oxygen delivery
because you're able to maintain a high fio2 by delivering flows higher than spontaneous
inspiratory demand, and it also reduces the anatomic dead space. During normal ventilation
approximately 30% of the tidal volume is wasted in anatomical dead space that does
not participate in gas exchange. High flow nasal cannula washes out the co2 of the upper airways
leading to improved ventilatory efficiency and reduced work of breathing and finally high flow
nasal cannula also delivers a small amount of PEEP. One centimeter of water for every 10 liters per
minute of oxygen high flow- which increases lung volumes and improves gas exchange. So
what is the aerosolization risk associated with high flow nasal cannula therapy? Well we know from
prior studies that a standard surgical mask could significantly reduce aerosolization with high flow
nasal cannula therapy. While a face mask should be placed on patients who are receiving low or high
flow nasal cannula therapy such a strategy does not entirely prevent particulate in airflow escape
this highlights the importance of personal protective equipment, hepa filtration, and
negative pressure rooms as important adjunctive measures. What is non-invasive respiratory support?
It's a pressure targeted respiratory modality that prior to COVID was primarily used for COPD and
cardiogenic pulmonary edema. The device comprises a mechanical ventilator attached to a tight-fitting
mask and it keeps small airways and alveoli open by providing peak positive end expiratory pressure
and it also reduces the work of breathing by assisting the respiratory muscles when you add in
biphasic positive airway pressure. Without PEEP the small airways and alveoli of covet-infected
lungs collapse at the end of expiration. With the application of PEEP however
you can see that these small airways and alveoli remain open increasing end expiratory lung volume.
There are two modalities of non-invasive ventilation CPAP or continuous positive
pressure ventilation and BiPAP or bi-level positive pressure ventilation
both modalities can provide oxygen delivery up to 100 and positive end expiratory pressure. BiPAP
also provides some degree of inspiratory pressure. So what should I start with CPAP or BiPAP?
Well, many patients with covet related hypoxemia have a tolerable work of breathing
and so they don't need mechanical inspiratory support which is what BiPAP would deliver but they
would benefit from CPAP which provides enough PEEP to recruit additional alveoli for gas exchange
without harmful increases in tidal volumes. So when should I use BiPAP? Consider using
BiPAP in patients with hypercapnic respiratory failure and muscular fatigue, such as those with
COPD. It is important to note that CPAP does not actively reduce a patient's respiratory effort
but through alveolar recruitment improved lung compliance and reduced ventilation perfusion
mismatch it reduces the degree of hypoxemia even in cases that may have failed conventional
oxygen therapy. In non-COVID patients you might start with CPAP of 5 to 10 centimeters of water
but in COVID patients generally start higher at approximately 10 centimeters of water and
slowly increase as high as 15 to 18 centimeters of water as tolerated by the patient. Titrate the fio2
and peep to keep the respiratory rate less than 25 breaths per minute and to reach your target oxygen
saturation. Consider using BiPAP with a low driving pressure in patients with comorbidities like COPD.
Normally you might provide five centimeters of EPAP and pressure support of 10 centimeters of
water. However for patients with COVID start with a higher expiratory positive airway pressure or EPAP
like 12 centimeters of water and an inspiratory positive airway pressure IPAP of 16 centimeters
of water titrate the IPAP and EPAP to reduce the work of breathing and keep the respiratory
rate below 25 breaths per minute titrate the fio2 to achieve your target oxygen saturation.
As with conventional oxygen therapy and high flow nasal candle therapy there are theoretical
concerns about the risk of COVID transmission with the use of non-invasive respiratory support. If
you are going to employ non-invasive respiratory support these risks can be mitigated by the use
of viral filters. Viral filters are important to creating a closed system and limiting transmission.
Attach the filter directly to the mask as long as the filter and the mask are compatible.
Or, when using specialized non-invasive machines which have one limb systems attach one filter to
the inflow rim and another filter to the exhaust port. Ensuring a proper mask seal is also important.
If you're going to employ non-invasive respiratory support
there are some requirements to keep in mind. The patient has to be conscious,
able to initiate their own breaths, able to maintain their own airway, and they can't have
facial abnormalities that would prohibit a proper mask seal. So how do I choose between high flow
nasal cannula therapy and non-invasive respiratory support. If your patient is in respiratory distress
but does not appear fatigued consider high flow nasal cannula therapy. Assume that your patient
is fatiguing if they have rapid shallow breathing, are tiring after prolonged respiratory distress,
or have asynchronous movements of the chest and abdomen.
If your patient appears to be fatiguing and fails a trial of high flow nasal cannula therapy
then consider non-invasive respiratory support. This lecture covered the second
part of treating the moderately dyspneic patient with a focus on high flow nasal cannula therapy
and non-invasive respiratory support. We hope you have enjoyed it.