Nasal cannula
Nasal cannula is a common form of oxygen delivery on the ward. It can be used for short or long term therapy and is best for patients who require low amounts of oxygen. It can deliver oxygen at a concentration of 24 to 40 % and at a rate between 1 to 6 liters of oxygen/min. It may cause nares to become dry if the rate is consistently set to above 4 L / min (consider adding humidifier above 4 L/min). Useful and commonly used as patients can continue to talk and eat while receiving oxygen.
Simple oxygen face mask
Oxygen mask is the next level of oxygen therapy available. It can provide oxygen at concentrations between 40 to 60 % and can deliver oxygen at a rate of 6 to 10 L/min. Disadvantage includes that it can be claustrophobic and that patients cannot eat with the mask on.
Venturi mask
High flow oxygen delivery system that can provide humidified oxygen at concentrations between 24 to 60 % O2 at 6 to 10 L/min. It allows for the precise delivery of oxygen by changing the adaptor used. This is important for patients with COPD for whom you do not want to provide too high concentrations of oxygen and suppress the ventilatory drive.
Heated humidified high flow therapy (also known as high flow nasal cannula)
High flow nasal cannula is commonly seen in the ICU (usually a step down from non-invasive ventilation i.e. BIPAP). Because the delivery of cold and dry gas irritates the respiratory mucosa and can cause discomfort and epistaxis, when we provide O2 at higher rates, the gas must be humidified and heated.
High flow nasal cannula can deliver flow rates up to 60 L/min. It can also provide PEEP if mouth is closed (~1 cm/H2O of PEEP for every 10 L provided). Used for acute hypoxemic respiratory failure or as a bridge to/from mechanical ventilation but NOT in hypercapnic respiratory failure.
Why does oxygen get diluted with a regular nasal cannula?
Even with quiet breathing, the inspiratory flow rate at the nares of an adult usually exceeds 12 liters per minute, and can exceed 30 liters a minute for someone with mild respiratory distress. Traditional oxygen therapy through nasal cannula is limited to six liters per minute and does not begin to approach the inspiratory demand of an adult, and therefore the oxygen is diluted with room air during inspiration. With high flow nasal cannula, the goal is provide oxygen flow at a rate that is sufficient to meet or even exceed the patient’s inspiratory flow rate.
Non re-breather mask
Consists of a simple mask and a small reservoir bag attached to the oxygen tubing connecting to the flow meter. It’s called a non re-breather because it ensures that there is no rebreathing of exhaled air and is therefore one of the purest ways to deliver oxygen. It has 3 one way valves. The first one is between the bag and the mask and allows pure oxygen to pass from the bag to the mask but not vice versa. The second and third valves are exhalation valves found on the sides of the mask; these allow the exhaled air to pass from the mask to outside the mask during exhalation, but does not allow air from outside the mask get into the mask during inhalation (although in reality one of these valves is usually removed so that the patient is able to get some inhaled air from outside and not through the bag, just in case there is an issue with the oxygen delivery into the bag so the patient can get some air during inspiration). It delivers between 60 and 80 % FiO2 at a rate of 10 to 15 L/min.
In a partial re-breather, the bag has no one-way valve, so the expired air mixes with the inhaled air. Partial rebreather masks are designed to capture the first 150ml of the exhaled breath into the reservoir bag for inhalation during the subsequent breath. This portion of the breath was initially delivered at the end of inhalation and was therefore delivered to the “deadspace” anatomy where gas exchange did not occur. Therefore, there would be no depletion of oxygen nor gain of carbon dioxide during the rebreathing component.
Non-rebreather masks are to only be used in monitored settings for short term use and for emergencies. They are not to be used on the wards regularly.
CPAP
CPAP stands for continuous positive airway pressure. CPAP machines deliver a continuous flow of air at a steady, defined pressure. Since CPAP machines use the same pressure setting for both inhalation and exhalation, some people find it tougher to exhale comfortably and may need to transition to BiPAP.
BIPAP (non-invasive ventilation or NIV)
A noninvasive form of ventilation that maintains two different levels of positive pressure in the airway, such as a higher level (e.g., 10 mm Hg) during inspiration and a lower level (e.g., 4 mm Hg) during expiration. Commonly used to treat acute exacerbations of COPD and asthma. Maintaining a slightly positive pressure prevents airway collapse, decreases the work of breathing, improves alveolar ventilation, and increases the diffusion of oxygen from the alveoli into the blood.