Ventilating with a bag-valve-mask device requires a good mask seal against the face in order to generate the pressure to inflate the lungs. But it also requires knowledge of how to effectively use the ventilation device to deliver a breath. This article will discuss the differences in ventilation technique for self-inflating vs free-flow ventilation bags. Understanding those differences is important for you to successfully ventilate your patient.
Spontaneous vs. Manual Ventilation
The mechanics of breathing changes with the switch from spontaneous ventilation to manual ventilation.
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.
Without an adequate seal, ventilation will not occur. It’s imperative when ventilating a patient that you confirm that you are, indeed, ventilating the patient well. This starts with understanding the differences with the two types of bag-valve-mask devices in use. A bag-valve-mask device consists of a flexible bag that attaches to either a ventilation mask or endotracheal tube via some form of pressure control valve.
Squeezing the bag opens the valve, forcing air through a mask or artificial airway into the lungs. Releasing the bag allows the pressure inside the device to drop. The patient then passively exhales through the one-way valve. Typically the pop-off valve can be adjusted to release at either higher or lower pressures, allowing the provider to compensate for the compliance (ease of inflation) of the patient’s lungs. It also helps avoids over-pressurization of the lungs with potential barotrauma.
There are two types of ventilation bags, self-inflating bags and flow-inflation bags.
Self-Inflating Ventilation Bags
A self-inflating bag refills itself when you stop squeezing it. Self-inflating resuscitation bags are commonly imprecisely called by one proprietary name, the Ambu bag. Squeezing the bag inflates the lungs. Releasing the pressure allows the bag to refill with air as well as with oxygen if an oxygen source is attached.
The use of a self-inflating bag without supplemental oxygen will deliver an oxygen concentration of 21%. Most sick or injured patients need more oxygen than that. When the bag is attached to oxygen at a rate of 10-12 liters per minute you will deliver O2 levels of 40-60%. Adding a reservoir bag and running O2 at 12-15 liters per minute raises the concentration to 100%, but only if the reservoir is allowed to fill. In contrast, a flow inflation bag is filled with 100% oxygen all of the time.
When using a self-inflating bag it’s important to squeeze the bag in a manner designed to maximize oxygen concentration. When you abruptly allow the bag to refill after squeezing, it will tend to refill with room air rather than with oxygen, whose inflow time is limited. It is better to allow the bag to refill over 3–4 seconds by releasing the pressure of your hand gradually over that time period. Avoid breaking the mask seal when the bag is refilling because it will allow the bag to refill with room air rather than with oxygen.
You cannot use a self-inflating bag for effective “blow by” unless you squeeze the bag to force oxygen toward the patient.
Free-Flow Inflating Ventilation Bags
Unlike a self-inflating bag, which looks like a soft filled football when not in use, an empty free flow inflation ventilation bag looks like a deflated balloon. A flow inflation bag requires constant fresh flow of oxygen into the bag: flow-inflation bags won’t refill if the oxygen source is empty or detached. In addition you must maintain a good seal on the ventilation mask against the face, otherwise the bag deflates and you can’t ventilate.
Flow-inflation, or inflow dependent bags, while more challenging to use, are common on anesthesia machines and in other ICU type settings because they allow finer control of tidal volume, and greater ability to assess the ventilation, and provide a higher FiO2.
Because the flow-inflating bag is soft, you can easily feel lung compliance and changes in resistance. When ventilating a neonate with a 500 ml bag, extremely fine control of tidal volume is possible, even while giving tidal volumes less than 50 ml. With spontaneous ventilation, you can actually see and feel the bag partially deflate with each inhalation before it reinflates with the gas flow. The amount of deflation gives a good indication of tidal volume. To the experienced hand, you will know immediately when you have lost the seal because the bag will go flat. This is unlike a self-filling bag that may lure you into a false sense of security because it’s always full, even if the lungs are not filling well.
On the negative side, unlike self-filling bags, there must be a good seal of the mask against the face to allow a flow inflation bag to provide positive pressure. A poor seal causes the flow inflation bag to deflate like a big balloon. A novice ventilator may have difficulty maintaining the seal needed. Because flow inflation bags are dependent on an oxygen source and require more training, emergency ventilation is usually provided by self-inflating bags.
The table summarizes the difference between self-inflating and flow-inflating bags.
No matter which type of device you use to ventilate it s always important to verify that you are ventilating adequately. Watch chest rise. Look for fogging of the mask. Listen for breath sounds. Check end-tidal CO2. Keep checking as ventilation proceeds because changes in mask fit, head position, lung compliance, patient position, and your own hand fatigue can lead to worsening ventilation.
May the Force Be With You
Christine Whitten MD, author Anyone Can Intubate 5th Edition
(all illustrations copyright Christine Whitten MD)