Research and surrogate methods

Welcome back to the tasty morsels of critical care podcast.

Today we’re going to have a quick overview of the oesophageal balloon. If you’re directed to a patient in your long case who has an oesophageal balloon in, then you’re probably having a bad day. It would seem very unfair to have too many questions on this but an awareness of their existence and some cliff notes on their basic use might come in handy especially if you’re doing well and you’re in the medal type territory of the exam. Exams aside they’re a useful gateway drug into some important respiratory mechanics that are relevant to all of us.

At their most basic these are fancy NG tubes with an inflatable balloon that should end up in the lower third of the oesophagus. Inflating the balloon with a small amount of air allows you to transduce the pressure at the area the balloon lies. While that sounds straightforward there are large sections of review papers dedicated to troubleshooting placement and means of assuring the number you generate is actually accurate. I refer you to the below references for further reading.

The pressure measured is called the oesophageal pressure, often abbreviated to Pes because it seems the Americans won the spelling war on that one. Oesophageal pressure is a reasonable surrogate (with assumptions of course) for pressure within the pleural space. Once we have an estimate of pleural pressure we can subtract that from the plateau pressure displayed on the vent and we end up with a fancy number called the transpulmonary pressure. The transpulmonary pressure or Ptp is the distending pressure applied to the lung either from the muscles of spontaneous ventilation or from positive pressure ventilation from the ventilator.

Whoopdy do says the examiner – you now have another number you don’t really know what to do with. What should we use this data for, the examiner is asking? Well a short list of useful aspects you can look at with the oesophageal balloon include

• compensating for the effect of the chest wall on respiratory mechanics
• appropriate titration of PEEP
• assessing the contribution of respiratory muscle use to potential lung injury
• assessing triggering and synchrony issues

At this stage you’d be hoping the examiner is satiated and you can move on to something else but in the unlikely and terrifying event that they ask for more detail you might want to mention some of the following.

Our typical approach to safe ventilation in the passively ventilated patient is to look at driving pressures and tidal volumes. But this takes no account for the contribution of the chest wall. In the very obese patient there is a lot of flesh pressing down on the chest wall, this leads to an increasingly positive pleural pressure. It would make sense that we would need more pressure to distend the lungs in this scenario. The balloon in this scenario will allow you to set your PEEP appropriately. The Ptp at end expiration needs to sit somewhere in the 0-10cmH20 range to avoid derecruitment and in end inspiration it needs to be less than 25cmH20. This may need a lot more PEEP or less driving pressure than you’re used to giving and the balloon can help you feel safe about doing that.

In the patient weaning from the ventilator in a spontaneous mode the oesohpageal balloon can be used to make an estimate of the contribution of the patients muscular effort to the transpulmonary pressure. Your patient may be on 10/5 on a pressure support mode and you may well be lulled into a false sense of security that because the pressure numbers on the vent are modest then the pressures being exerted across the lung are also modest. What we are not measuring in this scenario is the distending pressure being applied to the lungs by the respiratory muscles, the Pmus. The balloon in this scenario can give an estimate of this as it reflects the negative pleural pressure generated by the patients inspiratory efforts allowing us to come up with a Ptp number that takes Pmus into consideration. Sometimes this might encourage you to increase the support from the vent, sometimes this might encourage you to increase the sedation depending on the context.

So given all the wonderful things the balloon can do for us why are we not doing it on everyone? A list of reasons not to use oesophagaeal balloons might include

• cost – these fancy NG tubes are pricier than you would think
• compatible software on the ventilators. These frequently don’t come as standard
• appropriate placement. These are tricky to get right and knowing that the number generated is valid is not entirely straightforward. Lots of assumptions are made
• the Pes number reflects pleural pressure only at a single location and does not take account of heterogeneity.
• the evidence base is unclear if this adds anything over doing something like simply following the high PEEP table from ARDSnet.

Interestingly several research groups (thinking the folk from Toronto or Luigi Camporata in london)  have used balloons to identify surrogate ways of measuring recruitment or estimating Pmus that we can easily measure on a standard ventilator set up. This may well be a way of bringing the important concepts of transpulmonary pressure to the bedside.

Reading:

• The Toronto Mechanical Vent Course was an excellent intro for resp mechanics for me. They offer a virtual version

• Mauri, T. et al. Esophageal and transpulmonary pressure in the clinical setting: meaning, usefulness and perspectives. Intens Care Med 42, 1360–1373 (2016).

• Yoshida, T., Grieco, D. L. & Brochard, L. Guiding ventilation with transpulmonary pressure. Intensive Care Med 45, 535–538 (2019).

• Mireles-Cabodevila, E., Fischer, M., Wiles, S. & Chatburn, R. L. Esophageal Pressure Measurement: A Primer. Respir. Care respcare.11157 (2023) doi:10.4187/respcare.11157.

• Jonkman, A. H., Telias, I., Spinelli, E., Akoumianaki, E. & Piquilloud, L. The oesophageal balloon for respiratory monitoring in ventilated patients: updated clinical review and practical aspects. Eur. Respir. Rev. 32, 220186 (2023).
• Deragned Physiology
• LITFL