Introduction
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Non-invasive positive pressure ventilation (NIPPV) has generally been accepted as beneficial to use in acute hypercapnic respiratory failure (AHcRF), but it is not still standardized how to perform it, and with which ventilation modes. In this study it was aimed to compare the efficiency of pressure control (PCV) and pressure support ventilation (PSV) modes on patients with AHcRF.

Methods
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A prospective randomized controlled study, performed in a pulmonary ICU of a university hospital. Patients who were admitted with the diagnosis of AHcRF were included in the study consecutively and they were randomized to have either NIPPV with PSV or PCV for the first 2 hours. Then NIPPV was stopped for 4 hours to achieve a basal PaCO~2~level, and after this waiting period NIPPV was continued with the other mode. The mode that leads to a greater decrease in PaCO~2~value was accepted as the successful mode.

Results
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A total of 40 patients with mean age of 69 ± 12 years were included in the study. In only two (5%) patients were endotracheal intubation and mechanical ventilation needed. Among them, only one (2.5%) patient died. After NIPPV there was a significant decrease in PaCO~2~level with both PSV and PCV modes. But when ΔPaCO~2~(PaCO~2~after NIPPV - PaCO~2~baseline) of both groups was compared, no statistically significant difference was identified. Among the 20 patients that NIPPV was started with PSV, in nine (45%) of them PSV was assessed as successful and continued; in 11 of them (55%) NIPPV was switched to PCV (*P*\> 0.05). Among the 20 patients in which NIPPV was started with PCV, in one of them NIPPV was unsuccessful ending up with intubation, in 16 of them PCV (84%) was successful (*P*= 0.051). The presence of apnea and apnea plus hypopnea with the initial mode was found as the factor affecting the assessment of PCV as the successful mode (*P*\< 0.05).

Conclusions
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Both PSV and PCV can be used efficiently in the treatment of AHcRF of various causes. But in the presence of apneas and hypopneas (either central or obstructive) PCV can be preferred instead of PSV.
