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current protective lung ventilation strategies commonly involve hypercapnia . this approach has resulted in an increase in a shift in clinical paradigms regarding hypercapnia from avoidance to tolerance , with hypercapnia increasingly permitted in order to realize the benefits of low lung stretch .
the reported levels of paco2 ( mean maximum paco2 67 mmhg ) in the study conducted by hickling and co - workers reflects typical levels observed with the institution of this technique .
no consensus , however , has been established in terms of its upper limits and the optimal co2 level for mechanically ventilated patients with acute lung injury ( ali ) .
the concept of an optimal co2 concentration is essential as most physiological systems are saturable and it is therefore reasonable that an effective upper limit of co2 , a point beyond which advantages shift toward harmful effects , exists .
he was treated for pulmonary tuberculosis for the full duration and stopped att only few months back . in view of the patient 's drowsiness and hypotension ,
he was intubated in emergency and shifted to the intensive care unit ( icu ) .
his arterial blood gas ( abg ) analysis showed severe hypoxia and hypercarbia and therefore he was kept on mechanical ventilation .
sedation and paralysis was initiated . on mechanical ventilation ( mv ) , the patient 's airway pressures were very high . to prevent lung injury , the tidal volume was decreased to 150 ml with monitoring of end - inspiratory plateau pressure . because the patient was retaining co2 ,
his laboratory results included hemoglobin of 10.3 , tlc 30.1 , platelet count 413 , blood urea 16 , creatinine 0.60 , sodium 135 and potassium 4.4 .
chest x - ray on admission showed bilateral diffuse heterogeneous opacities [ figure 1 ] .
lung protective ventilation with low tidal volume was given in view of poor pulmonary compliance . during mechanical ventilation , peep was always kept at 56 and end - inspiratory plateau pressure was kept at around 30 .
other ventilator parameters include tv varying between 150 and 180 ml for the initial 48 h ; thereafter , as the pulmonary compliance improved , tv was increased .
the patient developed further rise in co2 as expected and fall in ph with maintained blood pressure and organ perfusion .
after about 24 h , the patient was started on nor epinephrine support because of falling blood pressure . during this time , his abg analysis was ph 6.96 , pco2 219 and po2 54 on mechanical ventilation setting of f 35 , tv 180 , fio2 0.6 and peep 5 , while the plateau pressure was 30 .
he was started on soda bicarbonate infusion around 36 h after admission in view of further fall in ph to 6.88 and continued need of vasopressors to raise blood pressure .
after 12 h of soda bicarbonate infusion at the rate of 50 ml / h , his blood gas analysis showed ph 7.09 , pco2 373 and po2 62 on tv 180 , f 35 , fio2 0.75 and peep 5 [ figure 2 ] .
the soda bicarbonate infusion was stopped around after 17 h. at that time , the patient 's ph rose to 7.29 , with co2 of 141 mmhg and po2 of 92 on mv settings of f 35 , tv 240 ml , fio2 0.65 and peep 6 while the plateau pressure was 30 . over the period of the next 48 h , nor epinephrine
he was weaned off from the ventilator on the 9 day of admission and shifted out of the icu on the 18 day in a stable condition .
this strategy has resulted in an increase in the clinical acceptability of elevated carbon dioxide tension in order to avoid the deleterious effects of high lung stretch .
these protective lung ventilation strategies have been demonstrated to improve survival in patients with ards . in current practice ,
mean maximum paco2 and ph associated with permissive hypercapnia are around 67 mmhg and 7.2 , respectively , and are reported to be well tolerated as long as tissue perfusion and oxygenation are preserved and there are no contraindications . in our patient , we observed a maximum documented pco2 level of 373 mmhg . while the rise in carbon dioxide tension could be contributed by soda bicarbonate infusion , it appears that a major rise was because of protective lung ventilation and underlying diseased lung as the pco2 value before starting ventilation and sodium bicarbonate infusion was 100 mmhg and 230 mmhg , respectively , and the co2 value before stopping sodium bicarbonate infusion was 141 mmhg .
several reports emphasize the potential for tolerance to extreme levels of hypercapnia , termed supercabia .
described complete clinical recovery after an episode of prolonged life - threatening status asthmaticus in an 8-year - old boy . despite maximal medical therapy involving both conventional and last resort bronchodilator therapies , a progressive respiratory acidosis developed , with a nadir ph of 6.77 and pco2 of 293 mmhg recorded 10 h after admission .
hemodynamic stability was maintained throughout and no neurologic or other organ dysfunction was detected after eventual complete resolution of status asthmaticus .
urwin et al . reported survival without adverse sequelae after extreme hypercapnia ( pco2 233 mmhg ) in an older woman with acute decompensation of chronic obstructive airway disease . to date , however , human trials studying the effect of co2 
 per se on outcome in patients with lung injury have not been performed .
ventilatory strategies involving hypercapnia are widely used in the setting of acute respiratory failure , with the aim of realizing the benefits of reduced lung stretch . no consensus , however , has been established in terms of its upper limits and the optimal co2 level for mechanically ventilated patients with ali .
it appears to be well tolerated as long as oxygenation and tissue perfusion are maintained and there is no contraindication of hypercapnia .
a 19-year - old male presented with history of massive hemoptysis . 
 the patient was kept on mechanical ventilation because of severe hypoxia . 
 lung protective ventilation with low tidal volume was given in view of very poor pulmonary compliance . 
 during the course of treatment , the patient developed a very high co2 level of 373 mmhg . 
 the patient was successfully weaned off on the 9th day without any obvious adverse consequences .