What is the highest CO2 level that human body can maintain?

My uncle (74 yrs) has been diagnosed with chronicle COPD since last several years(>20 yrs). Last week, he was admitted to ICU after having difficulties breathing. It was noticed during treatment that CO2 level was very high (>70 mmhg) and also sign of pneumonia which was spreading in the right lung. As of now, antibiotics is showing some effect and pneumonia infection is slowly coming down. Since his CO2 was hign, dr. started to put him on BiPAP, which helped to bring down to ~60mmhg with in 3 days. However, last 2 days its going up again even with BiPAP procedure and latest result shows 81.1 mmHg (pH leve is 7.29).

Also we do not have any data for his CO2 level in past years or so. In such scenario, I am just wondering what is the highest CO2 level that body can maintain.

Any help would be greatly appreciated.

Thanks,

Sanjay