Difference in Manual Ventilation: Self-Inflating Ventilation Bag vs. a Free Flow Inflating Bag

Manual ventilation 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 successful manual ventilation of your patient.

Spontaneous vs. Manual Ventilation

The mechanics of breathing changes with the switch from spontaneous ventilation to manual ventilation.

Spontaneous 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.

Illustration contrasting the mechanics of spontaneous ventilation showing inhalation and exhalation side by side

With spontaneous breathing, chest wall expansion and diaphragmatic contraction/relaxation change air pressure inside the thoracic cavity. If the airway is open the lungs will expand (a) and deflate (b).

Manual Ventilation

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.

Illustration showing how manual ventilation differs from spontaneous ventilation

With manual ventilation, your delivered breath must address lung compliance and overcome the weight of the abdominal contents against the diaphragm and the weight of the chest wall.

Bag-Valve-Mask Device

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.

Illustration showing the components of a Self Inflating Bag

Common parts for bag-valve-mask devices, In this case a self-inflating style bag. The reservoir bag, when present, allows near 100% inspired oxygen if allowed to fill.

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.

The table  summarizes the difference between manual ventilation with either a self-inflating or a flow-inflating bags.

Differences of self-inflating bags vs. free-flow bags.

If embedded video does not play then Follow this link to a video demonstrating the use of both types of ventilation bags using a pediatric manikin.

If embedded video does not play then Follow this link to a video clip describing effective manual ventilation.

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.

further discussion of manual ventilation can be found in these articles:

May the Force Be With You

Christine Whitten MD,
author Anyone Can Intubate 5th Edition &
Pediatric Airway Management: A Step-by-Step guide

(all illustrations copyright Christine Whitten MD)

Cover of the book Anyone Can Intubate, A Step By Step Guide to Intubation and Airway Management, 5th edition       PedsCover_626x820

Please click on the covers to see inside my books at amazon.com

7 thoughts on “Difference in Manual Ventilation: Self-Inflating Ventilation Bag vs. a Free Flow Inflating Bag”

  1. A very important topic,and well described indeed. Appreciate for your effort . However, i am concerned about the appropriate use of self-inflating bag . Often time, we see health worker using self inflating bag for neonatal resuscitation are failing do do it properly because of one of the causes; improper seal, inappropriate positioning , secretion and so on .So, modification of the device to make the breath easy is imperative.I am trying it in a small scale.

  2. What is your recommendation for a self inflating Neonatal bag and mask to be available for a home delivery if things go bad? I understand this will not be attached to an O2 source

    1. Using a self inflating bag to ventilate, even without supplemental oxygen, can be life saving because it can often restore adequate oxygenation of the patient’s blood. If one decides to have a home delivery I think having a self-inflating bag available at a home delivery would be a good precaution – but ONLY if you know how to use one. A lay person who is not trained would have a difficult time safely ventilating any patient, especially an infant. When using a self inflating bag one must both be sure one is delivering a breath (the chest rises and the patient can exhale) as well as prevent over inflation of the lungs. Over inflation of a newborn lung causing lung collapse is a significant risk if one doesn’t know how to ventilate. Midwives in most states are required to know how to ventilate/resuscitate a newborn so having a midwife in attendance in case there is an emergency is a good precaution. A good preparation for Moms and Dads (or those soon to become new Moms and Dads) is to take a basic life support (BLS) CPR class. BLS will teach how to do mouth to mouth ventilation and chest compressions for both infants and adults as well as how to deal with aspiration and airway obstruction.

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