Bag Valve Mask and Mapleson Circuits

KEYWORDS

portable ventilation devices

nonrebreathing valves

noncircle systems

BAG VALVE MASK

The bag valve mask (BVM) is known by many different names, including manual resuscitator, self-inflating bag, and so on. It is, however, best known as an Ambu bag. Ambu is the company that introduced the BVM in the 1950s. In this discussion, the device will be referred to as a BVM.

The BVM is elegantly simple in its design. There are no parts made from metal (usually), no screws, no washers, and no springs (usually) or anything else of a complicated nature. The main parts are a self-inflating bag, with two valves (one on either end of the football-shaped and -sized bag), an inlet for fresh gas, and an outlet that both ventilates the patient and allows that ventilation to be expired.

The BVM is an unsung hero in medical care. Think of all the patients worldwide who are ventilated by a BVM at some time during the day. BVMs are used by first responders, by critical care personnel, and by anesthesia personnel. Transporting critically ill patients would be much more difficult and cumbersome without BVMs.

Bag valve masks can be used in the field, the battlefield, an ambulance, a helicopter, the emergency department, the intensive care unit, and the operating room. During disasters, BVMs may be the only type of ventilator that can work because they do not require electricity. (To be complete, there are mechanical, pneumatically powered ventilators available that do not use electricity to function, but there probably aren’t that many of them in your facility, so if a disaster struck, most patients needing ventilation would be ventilated with a BVM if electrical power was interrupted.)

For anesthesia providers, the BVM also has one other important purpose: it is your spare anesthesia machine, conveniently located in a bag on your supply cart. You should never start a case, unless it is a dire emergency, without making sure you have your backup anesthesia machine in the room. Ensuring that a BVM is present should be a part of your morning checkout.

Bag Inlet Valve and Oxygen Delivery

At the end that attaches to the oxygen source (which will be called the proximal end), there is a standard clear oxygen tubing that connects to the oxygen source. This tubing is permanently joined to the proximal end of the bag. The oxygen enters the bag through a thin disc valve (bag inlet valve), which is opened by the negative pressure of the expanding self-inflating bag, as well as the pressure and flow of the oxygen from the source (Figure 19-1).

Figure 19-1 Disc valve at the proximal end of the bag valve mask. Oxygen flows through coaxial inner tubing, and on self-inflation of bag, the disc valve opens, allowing filling of the bag with oxygen.

The disc valve is what keeps the contents of the self-inflating bag from leaking out the proximal end of the bag when the bag is squeezed during ventilation. The pressure of the operator’s hand closes the disc valve back against its seating, closing the bag inlet valve.

So what happens to the oxygen flow during ventilation when the bag is squeezed? No oxygen can enter the bag because the bag inlet valve has closed plus the bag is being compressed anyway. That seems like a lot of gas flow to simply halt. The pressure of oxygen inside its tubing would increase and pop the connector off of the flowmeter or the increased pressure on both sides of the bag inlet valve would cause it to fail. Fortunately, there is a way for all the oxygen flow to be vented whenever the bag is squeezed.

At the place where the oxygen tubing enters the proximal end of the bag, there is a chamber that communicates with the oxygen tubing inlet. When the bag inlet valve is activated by someone squeezing the bag, the oxygen flow (which has not stopped) is vented out this opening. The oxygen coming from the flowmeter goes into a reservoir. The reservoir, in most cases, is a length of corrugated tubing coaxially situated with the oxygen tubing. It is open to the atmosphere, where excess oxygen and pressure are vented out the open end of the corrugated tubing.

This is important to understand because this topic will be revisited later on in the chapter. To reiterate, when the self-inflating bag is squeezed by the operator, the bag inlet valve closes off the oxygen delivery to the bag. This flow of oxygen is vented into the open corrugated tubing, which runs coaxially to the oxygen delivery tube. This reservoir of oxygen is used to increase the fraction of inspired oxygen (FiO₂), which is discussed later (Figure 19-2).

Figure 19-2 When bag is squeezed, the disc valve closes. Oxygen flow from the flowmeter continues and fills the corrugated reservoir tubing.

Self-Inflating Bag

There is really not much to say about the bag. It has two openings, one proximal and one distal. It is molded in such a way that when compressed, it rapidly returns to its normal state. The bag is textured so it is easier to grasp. Although it is not too important for anesthesiologists, the bags are made so that they can be compressed effectively while in conditions where extremes in temperatures could be seen, very hot or very cold, like a first responder would encounter. The bag, and the whole apparatus for that matter, is latex free in most cases.

The average volumes for the different sizes are 1500 to 2000 mL for an adult bag, 800 to 1000 mL for a pediatric bag, and 300 to 500 mL for a neonatal bag. (However, actual tidal volumes that can be generated are much lower than these numbers; more on that later.) The bag is filled by a combination of oxygen from the flowmeter and what is in the reservoir tubing (Table 19-1).

Table 19-1 BAG VALVE MASK VOLUME

Keep in mind, however, that many clinicians prefer to use a different kind of portable manual ventilator device when taking care of neonates and small infants (e.g., weighing <10 kg). This device is called a Jackson Rees circuit. We will discuss this circuit and the class of circuits known as the Mapleson classification later in this chapter.

Nonrebreathing Valve

The distal end, which interfaces with the patient by facemask, endotracheal (ET) tube, or supraglottic device, is called the nonrebreathing valve. It controls both inspiration and expiration. It is housed in a clear plastic housing, so the valve can be inspected for proper movement.

The housing itself is made with a standard-sized connector, which has a 15-mm internal diameter (for ET tubes and supraglottic devices) and 22-mm external diameter (for anesthesia facemasks).

There have been many different designs of nonrebreathing valves in the past. Many have had spring-loaded moving parts and rigid and flexible flaps. Readers are encouraged to refer to Dorsch and Dorsch for an excellent description of these kinds of valves.

The valve that will be discussed here is the fishmouth-flap nonrebreathing valve (Figure 19-3). It is really two valves in one, the fishmouth valve and the flap valve, but it is molded as one piece. The fishmouth part is what opens to ventilate the patient. It is easily seen when you look into the end of the device, right where you attach the mask or ET tube. The fishmouth sits in the middle of a soft plastic circular disc flap. The disc flap sits on its own valve seat, and when the bag is squeezed, the disc flap closes the expiration port. On expiration, when the operator stops squeezing the bag, the fishmouth closes, the circular flap relaxes off its valve seat, and passive expiration occurs.

Figure 19-3 Nonrebreathing valve at the distal end of a bag valve mask. When the bag is squeezed to initiate positive-pressure ventilation, the flat disc section of the fish-mouth valve is pushed onto its seat, and the fishmouth opens, allowing ventilation of patient through the fishmouth.