# Impact of mild hypo- and hyperventilation on cerebral oxygen supply during general anesthesia

## TL;DR

This study shows that mild hypoventilation increases brain oxygen levels more than mild hyperventilation, and this effect is the same in younger and older patients under anesthesia.

## Contribution

The study demonstrates that even mild changes in ventilation affect cerebral oxygenation similarly across age groups during anesthesia.

## Key findings

- Mild hypoventilation increased rSO2 compared to hyperventilation in both age groups.
- The change in rSO2 was similar between younger and older patients.
- Results were consistent after 15 minutes of each ventilation condition.

## Abstract

Cerebral blood flow autoregulation is affected by several physiologic and medical factors. Especially arterial carbon dioxide pressures (PaCO2) impact cerebral blood flow. Only extensive changes in end-tidal CO2 have been studied so far. The aim of this study was to evaluate the impact of mild hypo- and hyperventilation on cerebral blood flow as assessed by regional cerebral red blood cell oxygen saturation (rSO2) in two age groups.

Two groups of patients were compared under general anesthesia before the surgical procedure was started: A younger patient group (age < 40 years; YP) and older patients aged > 60 years (OP). Anesthetic management was standardized. In both groups, end-tidal CO2 was adjusted either to a low-normal value of 35–37 mmHg or a high-normal value of 43–45 mmHg for 15 min each. The sequence of these interventions was randomized. rSO2 was estimated by near-infrared spectroscopy (NIRS). The primary outcome was defined as the difference in rSO2 between hypo- and hyperventilation between the two age groups.

A total of 78 patients were included. In both groups, there was a statistically significant difference in rSO2 values after 15 min of hypo- versus hyperventilation. In the YP-group, rSO2 was 74 ± 4% after 15 min of hypoventilation and decreased to 68 ± 6% during hyperventilation (p < 0.001). In the OP-group, rSO2 was 71 ± 5% and 65 ± 6%, respectively (p < 0.001). There was no difference concerning changes in comparison of younger and elder patient groups (in both groups, Δ rSO2 = 6 ± 3%; p = 0.732).

Even mild hypoventilation increased rSO2 compared to mild hyperventilation and this difference occurred independent of age.