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What is the relief pressure dial on the ventilator?

 This article refers to the workings of Smith’s Medical Pneupac ParaPac and the newer ParaPac Plus. Other transport ventilators may vary in their settings and processes. Please check what unit your ambulance trust has before following the information here.



The pressure of air inside our lungs is normally just right to not only ensure that negative pressure breathing occurs but also to prevent a shunt (where blood bypasses the lungs) (Hess & Kackmarek, 2014: 431), the oesophageal spinchter from opening, causing air to enter the stomach (Baker, 2012: 1), and a host of other potential problems. Any means of supported ventilation where the patient is not breathing for themselves produces positive pressure ventilation. Not only does this change the way air enters the patient’s lungs while they are ventilated, but it also comes with all of the above risks if any of the settings are incorrect, and more.

Many of the effects ‘are related to mean airway pressure [which is] […] the average pressure applied to the airway during the ventilatory cycle’ (Hess & Kackmarek, 2014: 420) With too ⇑ pressure comes ⇓ venous return (Hess & Kackmarek, 2014: 420) and ⇑ chance of barotrauma (Baker, 2016: 75).

To summarise, the potential negative effects are:

  • Blood bypassing the lungs (a shunt) (Hess & Kackmarek, 2014: 431)
  • Air entering the stomach (Baker, 2012: 1)
  • Venous return decreasing (Hess & Kackmarek, 2014: 420)
  • Barotrauma (Baker, 2016: 75)

Because modern ventilators are volume-controlled, preventing a different problem that occurs during pressure-controlled ventilation, there is a set tidal volume which the ventilator will always provide to the patient. To provide this volume, the pressure will constantly increase until the volume is reached (Baker, 2016: 132).

The Means of Avoiding These Problems

In ventilators, the pressure of an oxygen cylinder ‘is lowered through a reducing valve down to 30-90psi’ from 2000psi. It’s then delivered to the patient ‘to a maximum of 40cmh20 or 60cmh20’, (Baker, 2012: 18)*, depending on whether you’re in the UK or the US.

*cmh20 = centimetres of water pressure (SensorsONE, 2017)

Since we can’t have too much pressure there needs to be a way of stopping the ventilator if things get awry. The relief pressure is what this refers to. Via a safety valve (Baker, 2012), otherwise known as ‘exhaust valves’ the supply is cut and excess peak inspiratory pressure (PIP) is released into the atmosphere (Zaconeta et al, 2010), ‘prevent[ing] over-ventilation of the lungs’ (Baker, 2016: 145)

Why would the pressure get this high anyway?

  • ‘The patient circuit […] [could be] blocked or kinked’ (Baker, 2016: 142). Best to check it.
  • ‘If the lung compliance[*] is low and the airway resistance is high the pressure can rise to high levels’ because, as above, the pressure will keep rising until the set tidal volume is delivered (Baker, 2016: 142).
  • The wrong relief pressure settings.

*Compliance ‘refers to the ease of expansion of the lungs and the walls of the thorax’ (Baker, 2012: 8). Ordinarily, according to Baker (2012: 8), the value is 100 cmh20 but when ventilated it’s 40-50cmh20. Physical injuries and medical conditions may further alter this figure.

Which dial are you talking about?

This one

The Pneupac ParaPac (Smiths Medical). Photo highlights the Relief Pressure dial.

What should I set it to?

40cmh20 if you’re in the UK (Baker, 2012)

Why is the relief pressure set at 40cmh20 in the UK? Why is it 60cmh20 in the US?

Article 999 is struggling to find the answers to these questions. If you know the answers, please get it touch. It would be great to add the information to this resource.

This post is yet to be peer reviewed. Please get in touch if you have any comments.


Super Summary

Article 999 previously posted an article on positive pressure ventilation, including its risks. The aim of positive pressure ventilation is to ‘maximise the potential benefit of mechanical ventilation while minimizing the potential for harm’ (Hess & Kackmarek, 2014: 407). The relief pressure dial on the ventilator ensures that the pressure of ventilation is not so high as to cause blood bypassing the lungs (Hess & Kackmarek, 2014: 407), the oesophageal spinchter opening (Baker, 2012: 1), barotrauma (Baker, 2016: 75; Hess & Kackmarek, 2014) and more. The ventilator’s other settings ensure that the patient actually receives ventilation, but the relief pressure setting ensures that a valve drains excess air when settings, poor setup, or the patient’s lung compliance (Baker, 2016: 43) have caused the pressure to get to high.

What should I set it to?

40cmh20 (Baker, 2012)

What should I look out for if the pressure is too high?

Kinked tubing (Baker, 2016: 142) & your settings

Read more in the Detail tab.

Prefer to see a diagram? Check out the mind-map summary




Baker, D. 2012. Emergency and Transport Ventilation, an introductory guide, Smiths Medical International Limited: Luton

Baker, D.J. 2016. Artificial Ventilation: A Basic Clinical Guide. Springer International: Switzerland

Hess, D.R. & Kacmarek, R.M. 2014. Essentials of Mechanical Ventilation. McGraw Hill Education: New York

SensorsONE, 2017. cmh20 – Centimetres of Water Column at 4 deg C Pressure Unit, Available Online: (Accessed 16/11/17)

Zaconeta, C.A.M. et al, 2010. ‘Evaluation of peak inspiratory pressure and respiratory rate during ventilation of a preterm infant lung model with a self-inflating bag by paramedics of the Fire Department’, Revista Paulista de Pediatria, vol. 28. No. 1.

Not endorsed by but with thanks to Smiths Medical for providing information.



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