One of my readers recently asked a very important question about ventilating a patient with a bag-valve-mask device: “Is there an outlet for the expired air of the patient?” The answer is yes. When ventilating a patient we are concentrating, and rightfully so, on watching the lungs expand and verifying that we hear breath sounds. It is just as important to verify that your patient can exhale. All ventilation devices have a built in pressure relief valve, also called a pop-off valve, which allows you to balance the force needed to expand the lungs with the ability to the patient to passively exhale. Failure to allow exhalation can lead to patient injury from barotrauma.
During spontaneous ventilation, the muscles between the ribs (intercostal muscles) and the diaphragm contract during inhalation. Contraction of the intercostal muscles lifts the ribs upward and outward, increasing the volume of the chest cavity. As the diaphragm contracts, it moves downward, further expanding the chest cavity. When the volume of a container increases, the pressure inside goes down. A good analogy is using a syringe. When you pull the syringe plunger, the chamber inside becomes larger, the pressure inside goes down, and the fluid is drawn into the chamber. Chest expansion lowers the pressure inside the chest cavity, the intrathoracic pressure, below atmospheric pressure. If the airway is open, air flows into the lungs until the two pressures are again equal.
When we exhale, normal elastic recoil of our chest wall compresses the rib cage. The diaphragm relaxes. The chest cavity becomes smaller. When volume decreases, pressure increases. Think of pushing the plunger on our syringe inward. As intrathoracic pressure rises higher than atmospheric pressure it pushes the remaining air (minus some of its oxygen and now containing CO2 out through the unobstructed airway.
When you manually ventilate a patient, gas no longer flows passively into the lungs under the influence of negative intrathoracic pressure. Instead, you have to provide positive pressure to inflate the lungs. Your manual breath has to lift the chest wall, push the diaphragm and abdominal contents down, and overcome initial alveolar surface tension to expand the lungs.
If the patient’s lungs are stiffer, as often occurs in bronchospasm or pneumonia, overcoming decreased lung compliance to provide an adequate tidal volume becomes more challenging.
In addition, when you squeeze the bag, the fact that the diaphragm in the supine patient is higher now acts as a disadvantage. You must use more pressure to force the diaphragms, and the abdominal contents underneath them, down and out of the way in a supine patient than you do if the patient is more upright. Your administered breath also has to lift the chest wall. If the patient is obese the weight of the abdominal wall and contents hinders ventilation.
Manual ventilation must compensate for a diaphragm resting higher in the chest, the weight of the chest wall and abdominal contents, and the compliance of the lungs. A more detailed description of manual ventilation with both self filling and free flow devices can be found here.
During both spontaneous as well as manual ventilation, exhalation is passive and depends on an open airway. If the airway is obstructed during exhalation, then air is trapped in the lungs. This leads to a phenomenon called breath stacking. Incomplete exhalation during artificial ventilation can result in residual air adding to the volume of the next inspiration with eventual unintended over inflation of the lungs.
All ventilation devices have a pressure relief, or pop-off, valve of some sort for use during manual ventilation. This includes bag-valve-mask (BMV) device as well as anesthesia machines. This pop-off valve is adjustable on most devices, allowing you to increase or decrease the inspiratory pressure needed to inflate the lungs while still allowing the patient to exhale. Some simple BMV devices have a fixed pop-off to allow ease of use, and less risk of over-pressurizing the lungs by a novice user, but less ability to manage extremes of ventilation.
We all know that giving too large a tidal volume at too high a pressure can lead to barotrauma, such as pneumothorax. with a tight mask fit, if the pop-off valve is totally closed, this acts just like airway obstruction. Inability to exhale during manual ventilation can impair ventilation and can also lead to barotrauma.
Correct Use of The Pop-Off Valve
All manual ventilation devices have a pressure relief valve. On anesthesia machines this is called an APL (Adjustable Pressure Limiting) valve. As its name indicates, the APL valve limits the amount of pressure buildup that can occur during manual ventilation. For shorthand, we often call this time of valve a pop-off valve since it allows the pressure to pop-off before it gets too high.
It’s important when adjusting any pop-off valve to watch the patient’s lung inflation – not just to make sure you are putting enough air in but also to make sure you are letting the air back out.
The pop-off valve basically changes the pressure of the tidal volume you are delivering with a good mask seal. With a good mask seal, if the pop-off is completely open, then the pressure of the delivered breath is low. If the pop-off is closed, then the pressure delivered is high. Adjusting the pop-off is a balancing act.
With healthy lungs and n open airway, inspiratory pressures of 10-20 cm H20 are common. With lung pathology such as pneumonia or bronchospasm, higher pressures, such as 40 cm H2O are occasionally needed. However use of unnecessarily high pressures for normal lungs runs the risk of injuring those lungs from that pressure. Lung injury from over pressurization is called barotrauma.
Always start with lower “pop-off” settings, by using a fairly open pop-off valve. Look at the tidal volume, feel the compliance of the chest wall as you squeeze the bag, listen for breath sounds. Use lower pop-off pressure settings if you can because lower pressures decrease the risk of barotrauma and avoid filling the stomach with air. However, if you can’t deliver an adequate tidal volume with low pressures, as demonstrated by seeing that the chest isn’t rising well or measuring a low delivered tidal volume on your anesthesia machine, then you must slowly close the pop-off valve until you can ventilate well. Gradually increase inspiratory pressure by slowly closing the pop-off and testing it.
You must always balance inflation pressure with the patient’s ability to exhale against the pop-off pressure you set. If the patient can’t exhale then you must open (loosen) the pop-off valve until they can.
As you ventilate the patient, the balance between the force needed to put air into the lungs and the ability to exhale can change. It’s very common to have to periodically change the pop-off valve settings during prolonged manual ventilation. This is especially true if patient status is changing or if you are moving the patient from place to place.
Pop-Off Valves and Endotracheal Tubes
When ventilating though a face mask, there is always the possibility of having over pressurized air escape around the edges of the mask. This acts as a bit of a safety mechanism. Once the patient is intubated, however, this ability for gas to escape is no longer present. This makes ensuring your pop-off valve is properly adjusted at all times especially important because the risk of barotrauma and pneumothorax is now much higher. You must always be alert.