Airway question/BVM technique

zzyzx

zzyzx

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I have a pretty basic question about airway anatomy relating to BVM ventilations. If you bag too forcefully with a BVM, it is my understanding that the pressure inside the pharaynx will open up the esophagus, which is normally a closed tube, causing gastric insufflation. Is it also true that with too much pressure, the epiglottis will push down to close off the trachea?
 
The esophagus is not normal closed so that air cannot be "forced down it". It is "closed" at its bottom by the "cardiac sphincter" (now called the "esophageal sphincter") which should be closed but relaxes or is incompetent in a number of circumstances, including clinic death, hiatal hernia, injury or dilatation of the lower esophagus, cancer of the structures, scarring from alkali poisoning, etc.

IF the airway is sealed at the oro-nasal pharynx, there is no place for air to go once the lungs are full or the airway is blocked but down the oesophagus.

Older CPR method of three BIG inflations to start often yielded a tympanic tummy.
 
My understanding was that the risk of air entering the esophagus and stomach lied around the 18 cmH2O mark, so moderate amounts of positive pressure support could lead to issues, even in the act of spontaneous breathing.
 
ThadeusJ said:
My understanding was that the risk of air entering the esophagus and stomach lied around the 18 cmH2O mark, so moderate amounts of positive pressure support could lead to issues, even in the act of spontaneous breathing.
Click to expand...

The keys to BVM ventilation are a good seal and an open airway. It is not an easily mastered skill, especially with the one-size-fits-nobody masks that come with most BVM kits. You should know in the first one or two breaths if you have an open airway - if you don't or you're unsure, quickly place an oral or nasal airway to open it. On most patients, you should be able to easily ventilate your patient with one hand squeezing the bag. If one person is trying to maintain a seal and another is squeezing with both hands, trust me, disaster is looming.
 
Sorry, I neglected the second question.

mycrofft said:
The esophagus is not normal closed so that air cannot be "forced down it". It is "closed" at its bottom by the "cardiac sphincter" (now called the "esophageal sphincter") which should be closed but relaxes or is incompetent in a number of circumstances, including clinic death, hiatal hernia, injury or dilatation of the lower esophagus, cancer of the structures, scarring from alkali poisoning, etc.

IF the airway is sealed at the oro-nasal pharynx, there is no place for air to go once the lungs are full or the airway is blocked but down the oesophagus.

Older CPR method of three BIG inflations to start often yielded a tympanic tummy.
Click to expand...

The epiglottis will not be slammed shut by inflation pressure, the air will just go around it. Tapping it accidentally (laryngoscope, tongue depressor) without or before paralysis can cause it to sort of slam shut; hence, "dry drowning".
 
Thanks for the replies.

Yeah, it never made sense to me that air pressure inside the pharaynx could cause the epiglottis to close.

As far as air going down the esophagus, if you have someone in cardiac arrest and the cardiac sphincter is relaxed, there is no way of avoiding gastric insufflation until you drop an ET tube? That doesn't make sense to me. It seems that poor use of the BMV causes that. Can you explain the exact mechanism of how poor bagging technique (pressures greater than 15 cmH2O from too much or too rapid bagging) causes gastric insufflation? Thanks!
 
zzyzx said:
As far as air going down the esophagus, if you have someone in cardiac arrest and the cardiac sphincter is relaxed, there is no way of avoiding gastric insufflation until you drop an ET tube? That doesn't make sense to me. It seems that poor use of the BMV causes that. Can you explain the exact mechanism of how poor bagging technique (pressures greater than 15 cmH2O from too much or too rapid bagging) causes gastric insufflation? Thanks!
Click to expand...

BVM use in general will cause some amount of gastric insuflation. It's unavoidable. It's the biggest reason you avoid BVM use during pre oxygenation if possible.
 
zzyzx said:
Thanks for the replies.

Yeah, it never made sense to me that air pressure inside the pharaynx could cause the epiglottis to close.

As far as air going down the esophagus, if you have someone in cardiac arrest and the cardiac sphincter is relaxed, there is no way of avoiding gastric insufflation until you drop an ET tube? That doesn't make sense to me. It seems that poor use of the BMV causes that. Can you explain the exact mechanism of how poor bagging technique (pressures greater than 15 cmH2O from too much or too rapid bagging) causes gastric insufflation? Thanks!
Click to expand...

It doesn't make sense because it isn't true. Gastric insufflation is not an inevitable result of mask ventilation; it is an inevitable result of poorly done mask ventilation.

We can all imagine (relatively uncommon) scenarios where higher airway pressures are needed to achieve an acceptable tidal volume, and in those cases it may be impossible to avoid getting some air in the gut. But aside from those cases, it is far from unavoidable.
 
Halothane said:
It doesn't make sense because it isn't true. Gastric insufflation is not an inevitable result of mask ventilation; it is an inevitable result of poorly done mask ventilation.

We can all imagine (relatively uncommon) scenarios where higher airway pressures are needed to achieve an acceptable tidal volume, and in those cases it may be impossible to avoid getting some air in the gut. But aside from those cases, it is far from unavoidable.
Click to expand...

If the pressure required and supplied to go down pathway A (airway) is lower than that required and supplied to go down pathway B (belly), where will it go?
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In practice, I've seen a little belly inflation along with reasonable BVM and mask work, but I've also heard and smelled some pt's belch during resuscitation as well.
 
Gastric insufflation is also seen with CPAP use, which obviously occurs during spontaneous breathing in an increased pressure environment...but the explanations offered above about gas taking the path of least resistance is spot on.
 
So then what is the mechanism of how gastric insufflation occurs due to poor BVM technique? Is it that little pressure is required for the ventilated air to go down the trachea, and that more pressure is required for the ventilated air to go down the esophagus and past the cardiac sphincter? So, when a patient is ventilated with too much pressure, you get air going down the esophagus? Or is it that if you don't open the airway with an adjunct or positioning, you get air going down the esophagus? I guess it would be both of those, right?
 
zzyzx said:
So, when a patient is ventilated with too much pressure, you get air going down the esophagus? Or is it that if you don't open the airway with an adjunct or positioning, you get air going down the esophagus? I guess it would be both of those, right?
Click to expand...

That is exactly correct.

In the absence of any airway obstruction, the path of least resistance is down the trachea, so that is where gas goes when it is forced into the pharynx under positive pressure.

However, once sedation and/or paralysis occurs in the supine position, some degree of upper airway obstruction is very common. If that is not adequately corrected by positioning and adjuncts, the path of least resistance will be down the esophagus, and gastric insufflation occurs. Even if airway obstruction does not occur, the presence of high airway pressures due to overzealous BVM technique, high PEEP, or a restrictive pathology can overcome the limits of the LES.
 
Here's just one study I was able to dig up that discusses the topic:



Assessment of pulmonary mechanics and gastric inflation pressure during mask ventilation.
Authors:
Weiler N; Heinrichs W; **** W
Affiliation:
Johannes Gutenberg University Medical School, Clinic of Anesthesiology, Mainz, Germany
Source:
Prehospital & Disaster Medicine (PREHOSPITAL DISASTER MED), 1995 Apr-Jun; 10 (2): 101-5. (16 ref)

Abstract:
INTRODUCTION: Mask ventilation is a procedure routinely used in emergency medicine. Potential hazards are inadequate alveolar ventilation and inflation of the stomach with air, leading to subsequent regurgitation and aspiration. The aim of this study was to measure lung function and gastric inflation pressures during mask ventilation. METHODS: For this purpose, 31 patients scheduled for routine urological procedures were studied during induction of anesthesia. Lung function was assessed by recording respiratory flow and pressure directly at the face mask. Gastric inflation was observed with a microphone taped to the epigastric area. RESULTS: Gastric inflation occurred in 22 of the 31 patients. Mean gastric inflation pressure was 27.5 +/- 6.55 cm H2O, mean compliance was 67 +/- 24.1 ml/cm H2O, mean resistance was 17.4 +/- 6.41 cm H2O/L/sec, and the mean respiratory time constant was 1.1 +/- 0.26 seconds. CONCLUSIONS: These data suggest that inspiratory pressure be limited to 20 cm H2O, and that an inspiratory time of at least four times the respiratory time constant be allowed. Monitoring airway pressure and gastric inflation is a simple technique that may improve the safe-ty of patients during mask ventilation.
Journal Subset:
Allied Health; Biomedical; Double Blind Peer Reviewed; Editorial Board Reviewed; Expert Peer Reviewed; Peer Reviewed; USA
ISSN:
1049-023X
MEDLINE Info:
PMID: 10155411 NLM UID: 8918173
Entry Date:
20020531
Revision Date:
20091218
 
zzyzx said:
So then what is the mechanism of how gastric insufflation occurs due to poor BVM technique? Is it that little pressure is required for the ventilated air to go down the trachea, and that more pressure is required for the ventilated air to go down the esophagus and past the cardiac sphincter? So, when a patient is ventilated with too much pressure, you get air going down the esophagus? Or is it that if you don't open the airway with an adjunct or positioning, you get air going down the esophagus? I guess it would be both of those, right?
Click to expand...

Long answer: Y---E---S.:cool:

Note that if the BVM reinflates faster than the lungs expel air, the successive surplus inflation will cause a backup. As basic CPR says, the chest wall must rebound to allow cardiac refill; so the lungs must have a chance to deflate their last breath, not so much because it is out of oxygen, but the airway tree pressure is stair-stepping up.
 
So the basic principle of BVM is to "ooh sah" and go easy. It's their emergency. Not yours. You can't kill a dead man.
 
Oh, you do't get off THAT easy! :rofl:
 
zzyzx said:
So then what is the mechanism of how gastric insufflation occurs due to poor BVM technique? Is it that little pressure is required for the ventilated air to go down the trachea, and that more pressure is required for the ventilated air to go down the esophagus and past the cardiac sphincter? So, when a patient is ventilated with too much pressure, you get air going down the esophagus? Or is it that if you don't open the airway with an adjunct or positioning, you get air going down the esophagus? I guess it would be both of those, right?
Click to expand...

Yes, well said. It's also true that in unconscious folks, the lower esophageal sphincter seems to be more "plastic" than "elastic," meaning if you've already forced it open with high pressures, it doesn't bounce back; it stays open, so it'll be even easier for air to enter the stomach. (So next time you're blowing up the belly, blame the guy who bagged before you!)

The trouble is that the LES is a physiological rather than an anatomical sphincter. There's no actual valve or doorway there; it's just that normally, the autonomic nervous system squeezes down on the circular muscles near the bottom of the esophagus, keeping them shut. If they DON'T squeeze (and when you're unconscious, you lose tone everywhere), you lose this; it's an active process that's no longer being performed.
 
Brandon O said:
Yes, well said. It's also true that in unconscious folks, the lower esophageal sphincter seems to be more "plastic" than "elastic," meaning if you've already forced it open with high pressures, it doesn't bounce back; it stays open, so it'll be even easier for air to enter the stomach. (So next time you're blowing up the belly, blame the guy who bagged before you!)

The trouble is that the LES is a physiological rather than an anatomical sphincter. There's no actual valve or doorway there; it's just that normally, the autonomic nervous system squeezes down on the circular muscles near the bottom of the esophagus, keeping them shut. If they DON'T squeeze (and when you're unconscious, you lose tone everywhere), you lose this; it's an active process that's no longer being performed.
Click to expand...

Sidbar: that's why gastric reflux pts will describe routinely falling asleep then at a certain elapsed time awake with reflux and sometime aspiration (as in "My lungs were on fire"). They take that hour or so for the esoph to "fall asleep" too.

Thanks heavens the sphincter at the farthest end don't normally do that!
 
More recent paper shows pressure is even lower than originally thought:

Anesthesiology. 2014 Feb;120(2):326-34. doi: 10.1097/ALN.0000000000000094.
Real-time detection of gastric insufflation related to facemask pressure-controlled ventilation using ultrasonography of the antrum and epigastric auscultation in nonparalyzed patients: a prospective, randomized, double-blind study.
Bouvet L1, Albert ML, Augris C, Boselli E, Ecochard R, Rabilloud M, Chassard D, Allaouchiche B.

CONCLUSION:
Inspiratory pressure of 15 cm H2O allowed for reduced occurrence of gastric insufflation with proper lung ventilation during induction of anesthesia with remifentanil and propofol in nonparalyzed and nonobese patients. (Anesthesiology 2014; 120:326-34)

So, 15 cm H2O on nonobese, nonparalyzed patients.

As stated before: summon your inner zen and ventilate SLOWLY.
See page two of the November/December EPIC TBI Study newsletter. I'm too new to post a link.

But...this is easier than done.
 
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