Randomised controlled trial to investigate the relationship between mild hypercapnia and cerebral oxygen saturation in patients undergoing major surgery

Abstract

ObjectivesThe effects of hypercapnia on regional cerebral oxygen saturation (rSO2) during surgery are unclear. We conducted a randomised controlled trial to investigate the relationship between mild hypercapnia and rSO2. We hypothesised that, compared with targeted normocapnia (TN), targeted mild hypercapnia (TMH) during major surgery would increase rSO2.DesignA prospective, randomised, controlled trial in adult participants undergoing elective major surgery.SettingA single tertiary centre in Heidelberg, Victoria, Australia.Participants40 participants were randomised to either a TMH or TN group (20 to each).InterventionsTMH (partial pressure of carbon dioxide in arterial blood, PaCO2, 45–55 mm Hg) or TN (PaCO235–40 mm Hg) was delivered via controlled ventilation throughout surgery.Primary and secondary outcome measuresThe primary endpoint was the absolute difference between the two groups in percentage change in rSO2from baseline to completion of surgery. Secondary endpoints included intraoperative pH, bicarbonate concentration, base excess, serum potassium concentration, incidence of postoperative delirium and length of stay (LOS) in hospital.ResultsThe absolute difference between the two groups in percentage change in rSO2from the baseline to the completion of surgery was 19.0% higher in both hemispheres with TMH (p<0.001). On both sides, the percentage change in rSO2was greater in the TMH group than the TN group throughout the duration of surgery. The difference between the groups became more noticeable over time. Furthermore, postoperative delirium was higher in the TN group (risk difference 0.3, 95% CI 0.1 to 0.5, p=0.02). LOS was similar between groups (5 days vs 5 days; p=0.99).ConclusionTMH was associated with a stable increase in rSO2from the baseline, while TN was associated with a decrease in rSO2in both hemispheres in patients undergoing major surgery. This resulted in a clear separation of percentage change in rSO2from the baseline between TMH and TN over time. Our findings provide the rationale for larger studies on TMH during surgery.Trial registration numberThe Australian New Zealand Clinical Trials Registry (ACTRN12616000320459).

Impact of permissive hypercapnia on regional cerebral oxygen saturation and postoperative cognitive function in patients undergoing cardiac valve replacement

“…Excluding the impacts of other relevant factors, blood dilution during CPB causes reductions in hematocrit and Hb, and insufficient blood oxygen-carrying capacity directly lowers the oxygen supply to the brain tissue. Here, Group H had a significantly higher rSO 2 (P<0.05), indicating that PHC is conducive to improving the autoregulation of the cerebral blood flow and the cerebral oxygen supply/consumption balance, which is consistent with a previous study (7). The reason may be that since CO 2 is the most effective regulator of cerebral blood flow, the PHC ventilation strategy in the patients undergoing CVR increases PaCO 2 , catalyzes the activity of neuronal nitric oxide synthase, increases nitric oxide production, causes the expansion of cerebral blood vessels, and increases cerebral blood flow and cerebral oxygen supply, thereby reducing the risk of cerebral ischemia and hypoxia and effectively protecting brain function (14).…”

“…The POCD incidence also differed significantly between the two groups at 24 h after surgery PHC is an important lung-protective ventilation strategy, as it not only increases PaCO 2 within an acceptable range, reduces tidal volume, "allows" hypercapnia, and prevents lung damage from high tension but also has protective and anti-inflammatory effects on the heart, nervous system, and many other organs (5,6). A previous study (7) showed that mild hypercapnia (PaCO 2 at 45-55 mmHg) increased rSO 2 , improved cerebral oxygenation, and reduced the incidence of POCD. The decrease in intraoperative rSO 2 is significantly associated with early POCD.…”

Alveolar Recruitment Maneuver Reduces Cerebral Oxygen Saturation and Cerebral Blood Flow Velocity in Patients During Carotid Endarterectomy

“…In addition to MAP, PaCO 2 affects rSO 2 and V-MCA. Mild hypercapnia dilated the cerebral blood vessels and increases cerebral blood flow and is associated with the increase in rSO 2 from the baseline [ 46 , 47 ]. PaCO 2 , V-MCA, and rSO 2 were higher after SM than after RM ( Table 2 ); hence, we deduced that the changes in V-MCA and rSO 2 may be associated with the hypercapnia caused by ventilation pause.…”

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“…Additional subgroup analysis established stronger evidence for targeted mild hypercarbia use. Few prospective studies [ 35 ] investigating the effects of targeted mild hypercarbia on rSO 2 during major surgery have confirmed similar effects.…”