As an anesthesiologist, I often run to emergencies where the patient is not breathing adequately and requires intubation. However, before any intubation, a patient in respiratory distress/failure needs ventilation. Providers who have passed ACLS are often able to ventilate an apneic patient well because they have practiced on the manikin. However, I often see that providers have more difficulty trying to assist ventilation of a patient who is still breathing spontaneously.
The typical inexperienced provider will try to provide large, slow breaths just as they were taught in ACLS. Unfortunately these breaths are often out of synch with the patient’s own breathing. Squeezing the bag while the patient is exhaling means that your inflation pressure must not only overcome the diaphragm, but also reverse the passive outflow of air, the elastic recoil of the lungs, and the rebound of the chest wall combined. The vocal cords may be closed. Ventilating out of synch with the patient won’t be as effective. The breath you deliver will take the path of least resistance to enter the stomach or escape from the mask. It often makes the patient cough.
Even worse, providers will occasionally hesitate to try to assist a patient’s breathing while waiting for the intubation team because they feel they don’t know how. Delay in improving ventilation can place your patient at higher risk of complication. This is unfortunate because in many ways assisting ventilation is even easier than manually ventilating an apneic patient. Let’s see why.
Quick Review of The Mechanics of Breathing
To inhale, the muscles between the ribs (intercostal muscles) and the diaphragm contract. 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. Like liquid, air is also a fluid. 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.
While breathing, air pressure changes inside the thoracic cavity cause the lungs to expand (a) and contract (b). From Anyone Can Intubate, C. Whitten.
The lungs are elastic. As the chest wall expands, air flows into and inflates the lungs like a balloon — although in this case the balloon is composed of millions of tiny balloons like a sponge. These air sacs are called alveoli. The volume of an average breath, the tidal volume, thus generated is about 8 ml/kg and can be as high as 10-15 ml/kg with maximum expansion of the chest.
What Happens When We Lie Down?
Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of a passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in balance and the diaphragm and other respiratory muscles don’t have to do any work.
FRC represents the combined gas volumes providing most of the normal functional lung oxygen exchange. Think of it as the patient’s oxygen tank. The larger the FRC, the bigger the “tank”. A small child, who has a smaller FRC than an adult, can’t hold his breath as long without getting hypoxic because he has a smaller “tank”.
Position can change FRC. An awake adult who lies supine loses about a liter of FRC as the abdominal contents push the diaphragms upward by about 4 cm. Induction of anesthesia, and presumably unconsciousness, causes the diaphragm to move still higher, further decreasing FRC by approximately 0.4 liters.
However, the body has some built in compensation. When the diaphragms are forced upward, they become more concave in the process. During spontaneous ventilation, this more concave shape allows the muscles of the diaphragm to contract more forcefully, producing a larger tidal volume. The patient naturally takes a deeper breath as the result and compensates automatically.
This compensatory mechanism has some limits. The patient with marginal respiratory reserve or morbid obesity may feel short of breath in the supine position because he or she can’t compensate for the decreased FRC as well: too much weight pushing up from below the diaphragm. If the patient’s blood pressure will tolerate it, raising the head of the bed improves ventilation by dropping the diaphragm downward and returning some of that FRC.
The important point here is that your fully supine and compromised patient already has a functionally smaller oxygen tank, increasing their risk of hypoxia. However, if they are still breathing spontaneously their own breathing dynamics are partially compensating for it, even if they’re doing so imperfectly. You will want to take advantage of that.
What Changes With Manual Ventilation?
When you manually ventilate a patient, gas no longer flows passively into the lungs. Instead you have to provide the pressure to inflate the lungs. Your manual breath has to lift the chest wall, push the diaphragm and abdominal contents down and expand the lungs.
If the patient’s lungs are stiffer, as often occurs in bronchospasm and pneumonia, overcoming this decreased lung compliance to provide an adequate tidal volume becomes even more challenging. You must maintain an open airway and a good seal on the mask. If you don’t, air will preferentially leak around the mask or enter the stomach rather than inflate the lungs.
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. You also have to lift the chest wall. If the patient is obese the weight of the abdominal wall and contents further 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.
If ventilation is difficult and vital signs allow, placing the patient 30-45 degrees head up will drop the abdominal contents away from the diaphragm and make lung inflation easier. This can be especially helpful in the morbidly obese patient.
How to Assist Ventilation
Assisting a patient who is breathing spontaneously is actually easier than manually ventilating an apneic patient.We assist ventilation all the time as part of or anesthetic technique, both during induction of anesthesia as well as during any general anesthetic when the patient is breathing spontaneously. The important point is to work with the patient’s rate and tidal volume.
As the diaphragm contracts it begins to inflate the lungs and push the abdominal contents down. The intercostal muscles contract, expanding the chest wall. You can think of this as starting to blow up a balloon. Any balloon, once partially inflated, is easier to fill because the initial pressure is now overcome. Your manual breath is simply taking advantage of the fact that the patient has already started to fill the balloon. You are simply making the patient’s spontaneous breath incrementally bigger and deeper.
During assisted ventilation, it’s important to time your manual breath with the patient’s inhalation to take advantage of these mechanics. Watch the patient’s breathing pattern. You can often feel your ventilation bag slightly deflate as the patient inhales, although this is easier with a plenum bag (free flowing) than a self inflating bag.
Gently squeeze the bag just as the patient starts to inhale and his airway opens. Stop squeezing as the patient beings to exhale. By providing just little more pressure when the airway is open you will increase the tidal volume. It’s as easy as that.
When the patient inhales, the diaphragm pushing abdominal contents down and the chest wall expands. Timing your assisted breath to inhalation makes the patient’s breath bigger and avoids putting air in the stomach.
You will find as you get into a steady rhythm with the patient that you can progressively increase the tidal volumes. In emergency situations when the patient is in respiratory distress, the rate is often high and the tidal volume inadequate. If you try to give big slow tidal volumes you will be fighting the patient’s effort. If tidal volumes are fast and shallow, you start equally fast but slightly less shallow. As you get into the rhythm you will find that you can gradually, over every few breaths, keep increasing the tidal volume a little bit, but again using their rhythm. Is is not uncommon that as you provide increasingly better ventilation that the patient will stop breathing and let you take over. Don’t be surprised if this happens. At this point you can use whatever breathing pattern you’d like.
When Do You Assist Ventilation?
When treating any patient in distress, it’s always important to identify not just the presence of breathing but also the adequacy of breathing. Recognizing which patient needs assistance while awaiting emergency intubation and which one can wait is an important judgement call which depends on how ill and stable the patient is as well as how quickly an intubation can be performed. If you think the patient will get worse while waiting for the intubation then you should assist.
Even though the patient may still be breathing, if he is displaying serious respiratory failure with hypoxia, significant hypercarbia or dangerous respiratory acidosis, he needs your help. Keep in mind that what the patient is doing on his own is not meeting his needs. If left unchecked, his condition may progress to respiratory arrest. In general, it is better to be too aggressive than not aggressive enough. If the patient will allow you to intervene with a bag-valve mask, it generally means he needs it. And you can often improve the condition of the patient, making the patient less likely to experience complications such as hypotension from the intubation itself.
Learning how to assist ventilation is an important skill in caring for patients, and can be life-saving. It is not difficult, but it does require a thoughtful approach, attention to what the patient is doing, and patience in modifying your technique to coordinate with the patient’s own effort.
For additional discussion of how to ventilate a patient, click links:
- Exhaling During Manual Ventilation Is As Important As Inhaling
- Difference in Ventilation With A Self Inflating Ventilation Bag vs. a Free Flow Inflating Bag
- Mask Ventilation: Avoiding Hand Fatigue
- Getting A Good Mask Seal Ventilating A Patient
May The Force Be With You
Christine Whitten MD, author Anyone Can Intubate: A Step by Step Guide , 5th ed.
and
Pediatric Airway Mangement : A Step by Step Guide
Please click on the covers to look inside the books at amazon.com
Hi. I’m a final year student studying medicine in India and your article about mechanical ventilation did solve some of my doubts.
I had a doubt regarding the exhalation phase of the patient while performing bag mask ventilation. Is there an outlet for the expired air of the patient?
Also, during maintenance of ventilation as in general anasthesia,when the patients diaphragm has been anaesthetised, what is the means by which the air given by bmv is exhaled?
Waiting for your reply, thank you.
All ventilation devices, whether it is an anesthesia machine or a bag-valve-mask device have a pop-off valve of some sort. On the anesthesia machine and most bmv devices, this valve is adjustable, allowing you to change the inspiratory pressure needed to inflate the lungs while still allowing the patient to exhale. Some simple bmv devices have a fixed pop-off. You are absolutely correct in your concern that it is very important to ensure that the patient can exhale. If not, then the breaths can stack up, with greater and greater lung inflation and the risk of poor ventilation and potentially a pneumothorax. 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 are in but also to make sure you are letting the air out. Thank you so much for your question.
Hello, I am a medical student from Indonesia. I’m going to enter clinical rotation in a few months. Your article is pretty cool, I’ve got some of my questions answered in your post. But I still don’t understand about the timing to give bag mask ventilation when the patient has intrinsic respiratory drive. For example, a patient with respiratory depression, breathes spontaneously but only 6 breathes per minute. We decide to give assisted breathing using the BVM before intubation. Should we squeeze the bag every 5-6 seconds just like in ACLS despite the patient’s spontaneous breathing effort? Or should we squeeze the bag only when the patient’s breathes in (so, just 6 breathes per minute)? Sorry for my bad english, I’m really waiting for your reply. Thank you very much,doc 🙂
So sorry for the delay in responding. When first assisting ventilation, you should try to synchronize the breath you give with the breath that the patient is taking. Initially, that means that if the patient is breathing slowly and shallowly, you should try to match the rate, but provide a bigger tidal volume. You will also be able to provide additional breaths BETWEEN the breaths that the patient is taking. As you get into the rhythm of the patient’s effort it becomes easy to time these extra breaths. What you shouldn’t do is try to give a breath when the patent is exhaling. Fairly quickly over a few minutes, if the patient’s respiratory drive is depressed, the patient’s own effort will get less and less, eventually stopping, At that point you can ventilate at whatever rate and tidal volume you choose.