This blog contains dr. Federica Fusina's translations of a selection of ventilab.it (a blog created by dr. Giuseppe Natalini and friends)'s posts. You can click here for more info. Happy reading!

ARDS, permissive hypercapnia and respiratory acidosis

September 24, 2011

Original post written by Giuseppe Natalini on 24th September 2011


About ten days ago we admitted Sarah, a 45 year old woman, to ICU with ARDS secondary to Legionnaires' disease (you can see the CT scan images above).

We decided to use protective ventilation. The ventilator was set with a tidal volume of 280 ml, a breath rate of 32 breaths/minute and an external PEEP of 16 cmH2O (which was the PEEP level associated with the smallest driving pressure, i.e. associated with the lowest elastance). FIO2 was modified in order to maintain SaO2 > 90%.

Sarah is 163 cm tall and weighs 60 kg. We have to remember that tidal volume relates to ideal body weight*, which, in Sarah’s case, is 55 kg. We were therefore ventilating her with 5 ml/kg of tidal volume. With this ventilation, plateau pressure was about 30-31 cmH2O, with a transpulmonary pressure of 14-15 cmH2O. All this while using sedation, muscle paralysis and steroid therapy.


And what about her arterial blood gas results? Here they are:  pH 7.11, PaCO2 82 mmHg, PaO2 67 mmHg with FIO2 0.9. We also tried nitric oxide, pronation and lateral position (damage to the right lung was prevalent), with no improvement on ABG values or respiratory mechanics.  


In summary: Sarah was receiving protective ventilation, and with these settings we had an oxygenation which was more than sufficient. How to behave with the severe respiratory acidosis? Should we have accepted it or not?


When we want to decrease PaCO2, we have to options: 

- increasing ventilation

- using ECMO or other extracorporeal CO2 removal techniques 


Increasing ventilation, in this case, meant abandoning protective ventilation: we had already reached the upper limit with plateau pressure (for both airway and transpulmonary pressure). We have decided not to use this option. 

ECMO would, in this case, have been indicated only to treat the respiratory acidosis: PaO2 was, in fact, still sufficient.


But is respiratory acidosis really an enemy we have to fight at all costs?



Figure 1


When I started to treat my first ARDS patients, twenty years ago, we all believed the right thing to do was to strive to obtain a PaCO2 of 40 mmHg and a pH of 7.40. With terrible results.


In 1990, a revolutionary message arrived from New Zealand (1). Dr. Keith Hickling said that, for a few years, he had been ventilating patients by reducing airway pressure below 30 cmH2O (!), accepting low tidal volumes down to 5 ml/kg (!!) and not doing anything to treat moderate hypercapnia (PaCO2 up to 70 mmHg). For higher PaCO2 values he raised peak pressure up to  maximum 40 cmH2O (something similar to our plateau pressure of 30 cmH2O). The result: his patients had a mortality of 16% (the mortality predicted with APACHE II score would have been 40%)!. And his patients had “insane” PaCO2 values (up to 129 mmHg!) (see Figure 1). Permissive hypercapnia in ARDS was born.


We all know how this story ended: this strange philosophy for mechanical ventilation (which cares about lungs and not about ABG) became the standard, with a less casual attitude today towards very low pH values.


Respiratory acidosis has physiological positive and negative effects (2). In ARDS patients, it seems to have mainly a protective effect (3), as well as being protective in the early phases of sepsis (4).


Sarah does not have ARDS anymore. We are not claiming victory yet because, even if she has greatly improved, she still hasn’t won her fight for survival. The message is nonetheless clear: lung protection is much more important than arterial blood gas analysis. And hypercapnia is not necessarily an enemy: in fact, sometimes it can become an ally on which to count on.


Greetings to all.


*Let’s remember that the ideal body weight for females is calculated as: 45.5+0.91(height in centimeters-152.4). For males the formula is the same, but we have to substitute 50 to 45.5 (5).




1) Hickling KG et al. Low mortality associated with low volume pressure limited ventilation with permissive hypercapnia in severe adult respiratory distress syndrome. Intensive Care Med 1990; 16:372-7


2) Ijland MM et al. Hypercapnic acidosis in lung injury - from ‘permissive’ to ‘therapeutic’. Crit Care 2010; 14:237


3) Kregenow DA et al. Hypercapnic acidosis and mortality in acute lung injury. Crit Care Med 2006; 34:1-7


4) Curley G et al. Can ‘permissive’ hypercapnia modulate the severity of sepsis-induced ALI/ARDS? Crit Care 2011; 15:212


5) ARDS Network. Ventilation with lower tidal volumes as compared with traditional for acute lung injury and the acute respiratory distress sindrome. N Engl J Med 2000, 342:1301-8

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