Why do we do that?

A BLOG BY DR IAN RECHNER () FOR OXICM

Habit that we have got used to when ventilating patients

DOI: This is written as a commentary on what we do with patients who are on ventilators. It is not extensively literature reviewed and is more a reflection on some of the things we do in everyday life on the ICU. I am an enthusiast when it comes to ventilators, am a ventilator trainer for Hamilton Medical and my personality is aligned to processes!

Measurement of oxygenation

There often appears to be a focus on PaO2 on a blood gas.  If we take a patient with a heart rate of 60, then after 1 day there would have been 86400 heart beats or 86400 Sp02 readings (60bpm x 60minutes x 24 hours).  Can we come anywhere near this number of blood gases in a day? In addition we all know Sp02 levels change within a few minute and the oxygen dissociation curve is not fixed in position.  As time progresses we may have a device which gives us a continuous Pa02, but I suspect a long running debate on what the “magic number” might be will then develop.   As ICUs start to continuously measure end tidal C02, the same argument may apply to PaC02, with the caveat of there being a PaC02-ETC02 gap.

Patient’s weight

If the European Union brought out a law that a ventilator would only start to work once the patient’s height and sex had been entered, would lives be saved?  This would allow the patient’s ideal body weight to be calculated and would give the user a better idea of volumes delivered to that patient.  This does not stop the operator entering an erroneous height though!

Absolute vs mls/kg tidal volumes display on the ventilator

Our standard convention is to look at absolute volume on a ventilator. A tidal volume of 500mls is a “nice round number”, which we aim for. However, if your ideal body weight is 50kg then this is not good, but okay if your IBW is greater than 70kg. In addition our tidal volume alarm limits are often 300 to 800mls. Why is it not 4 to 9 mls/kg?

Tidal volumes

There is very good data in ARDS patients who are passive on a ventilator that we should use 6-8mls/kg. What do we do with patients who have COPD, patients with normal lungs or patients who are spontaneously breathing on a ventilator? The spontaneously breathing patient with a high minute volume for whatever reason makes this situation even more difficult. Do you sedate, drive the minute volume down and then control the tidal volume that way? Again we all know that our goal is to get patient’s off a ventilator so this strategy will have its faults.

Lung collapse

A CT scan will often show basal lung collapse or lung US may show something similar. If we go back to our ARDS model of 7mls/kg, if half of lung volume is lost due to collapse, this means our 7mls/kg is across a smaller total lung volume. Do we use an even smaller target tidal volume in this case or try and recruit the lung?

Fixed pressure modes of ventilation

Our targets are tidal volumes of 6-8mls/kg. When a patient is first placed on a ventilator (usually paralysed at this point), setting a fixed pressure to achieve a target tidal volume is usually pretty straight forward. However we all know that the patient’s compliance (lung water, position in bed, sedation, inflammation, passive vs active breathing) and resistance (tube biting, bronchospasm, secretions and end of ETT position) change a lot. The results of these changes to the resistance and compliance will mean an increase or decrease in tidal volume, which will either damage lungs or hypo ventilate the patient. If you know the patient’s ideal weight, have the target tidal volume presented in the mls/kg format and someone watching the ventilator closely all the time, then you can manually fix this issue. In the real world though, I feel we struggle achieving this.

Conclusion and another confession

Outcome for patients being admitted to ICUs for ventilation is improving and it is often difficult to put your finger on the reasons for this. Will the things above make any difference? I am rightly or wrongly a strong believer in ensuring the “basics” are done well. My work place has a ventilator which gives an ideal body weight, displays tidal volume in mls/kg, has a pressure control volume guarantee mode (ASV), has a recruitment tool build it and offers fully automated closed loop ventilation.

Ian Rechner is a consultant in Intensive Care Medicine and Anaesthesia in the Royal Berkshire Hospital, Reading and is the OXICM Regional Advisor for Intensive Care Medicine
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