Carbon dioxide reactivity in the evaluation of cerebral ischemia

The effect of changes in tPCO2 on the fractional tissue oxygen extraction--as measured by near-infrared spectroscopy--in neonates during the first days of life

The cerebral fractional oxygen extraction (FOE) reflects the balance between cerebral oxygen delivery (OD) and consumption (VO(2)). PCO(2) affects the cerebral blood flow (CBF): hypocapnia decreases CBF and OD and increases FOE. We recently showed that the fractional tissue oxygen extraction (FTOE) reflects FOE and hypothesized that a decrease in tPCO(2) increases FTOE. In this study we looked at the effect of changes in tPCO(2) on FTOE. We analysed 23 measurements in 13 neonates with birth weight below 1500 g and need for intensive care. Exclusion criteria were congenital malformations or cerebral complications. The tissue oxygenation index (TOI), tPCO(2), mean arterial blood pressure (MABP), heart rate (HR) and peripheral oxygen saturation (SaO(2)) were continuously recorded for 4h during the first days of life and FTOE was calculated. Over the whole group we found a significant negative (r=-0.227) correlation between tPCO(2) and FTOE and a significant positive (r=0.258) correlation between tPCO(2) and TOI. After correction for MABP these correlations remained significant. Over the whole group we found a significant positive correlation between tPCO(2) and TOI and a significant negative correlation between tPCO(2) and FTOE, which remained significant after correction for MABP. This implies that tPCO(2) influences the cerebral oxygenation independently of MABP. We therefore believe that for the interpretation of cerebral oxygenation in mechanically ventilated neonates during the first days of life continuous measurements of tPCO(2) are needed. Moreover we suggest FTOE to become a continuous parameter in the clinical setting for the non-invasive measurement of the neonatal brain oxygenation.

Hypercapnia-induced cerebral hyperperfusion: an underrecognized clinical entity

BACKGROUND AND PURPOSE

The incidence of cerebral hyperperfusion and hypoperfusion, respectively, resulting from hypercapnia and hypocapnia in hospitalized patients is unknown but is likely underrecognized by radiologists and clinicians without routine performance of quantitative perfusion imaging. Our purpose was to report the clinical and perfusion imaging findings in a series of patients confirmed to have hypercapnic cerebral hyperperfusion and hypocapnic hypoperfusion.

MATERIALS AND METHODS

Conventional cerebral MR imaging examination was supplemented with arterial spin-labeled (ASL) MR perfusion imaging in 45 patients during a 16-month period at a single institution. Patients presented with an indication of altered mental status, metastasis, or suspected stroke. Images were reviewed and correlated with arterial blood gas (ABG) analysis and clinical history.

RESULTS

Patients ranged in age from 1.5 to 85 years. No significant acute findings were identified on conventional MR imaging. Patients with hypercapnia showed global hyperperfusion on ASL cerebral blood flow (CBF) maps, respiratory acidosis on ABG, and diffuse air-space abnormalities on same-day chest radiographs. Regression analysis revealed a significant positive linear relationship between cerebral perfusion and the partial pressure of carbon dioxide (pCO(2); beta, 4.02; t, 11.03; P < .0005), such that rates of cerebral perfusion changed by 4.0 mL/100 g/min for each 1-mm Hg change in pCO(2).

CONCLUSIONS

With the inception of ASL as a routine perfusion imaging technique, hypercapnic-associated cerebral hyperperfusion will be recognized more frequently and may provide an alternative cause of unexplained neuropsychiatric symptoms in hospitalized patients. In a similar fashion, hypocapnia may account for a subset of patients with normal MR imaging examinations with poor ASL perfusion signal.

Decreased transcranial Doppler carbon dioxide reactivity is associated with disordered cerebral metabolism in patients with internal carotid artery stenosis

PURPOSE

The hemodynamic effect of stenosis of the internal carotid artery (ICA) can be assessed by measuring, with transcranial Doppler (TCD), the carbon dioxide (CO(2)) reactivity of the cerebral vessels. The aim of this study was to determine whether a decreased CO(2) reactivity is associated with a compromised cerebral metabolism, as evaluated with (1)H magnetic resonance spectroscopy (MRS).

METHODS

Sixty-six patients with unilateral or bilateral stenosis of the ICA, who were scheduled for carotid endarterectomy (CEA) and who had undergone both a TCD CO(2) reactivity test and a MRS examination, were included in this study. The ICA stenosis on one side (CEA side) was always more than 70%, and the extent of the stenosis on the contralateral side varied.

RESULTS

The CO(2) reactivity and the N-acetyl aspartate (NAA)/choline ratio were correlated in both hemispheres (r =.43; P <.001). Patients with an ICA occlusion contralateral to the CEA side are especially at risk for disordered cerebral hemodynamics and metabolism; in the contralateral hemisphere, the mean CO(2) reactivity and NAA/choline ratio were abnormal (18% and 1.52, respectively), and lactate was present in 85% of the patients. Changes indicative of disordered hemodynamics were found more often in symptomatic than in asymptomatic patients.

CONCLUSION

A decreased CO(2) reactivity appears to be associated with a disordered cerebral metabolism. Patients with severe bilateral ICA stenosis are at risk for disordered cerebral metabolism and hemodynamics. Therefore, the indication for CEA based on the degree of ICA stenosis and clinical grounds might be refined with an additional test, such as the TCD CO(2) reactivity test.

Use of transcranial Doppler sonography and acetazolamide test to demonstrate changes in cerebrovascular reserve capacity following carotid endarterectomy

OBJECTIVES

To assess the effect of carotid endarterectomy on cerebrovascular reserve capacity.

METHODS

Cerebral blood flow velocity (CBFV) and cerebrovascular reserve capacity (CVRC) were measured by transcranial Doppler sonography (TCD) and acetazolamide test in 40 patients who underwent uncomplicated unilateral carotid endarterectomy (CEA). Indication for operation was limited to stenoses > 70% as documented by angiography and/or Duplex scanning. The TCD studies were carried out 6 days (range 1-14 days) before and 8 days (range 5-12 days) after endarterectomy.

RESULTS

Before endarterectomy, resting CBFV values and CVRC in the 40 patients were significantly different between the operated (51 +/- 19 cm/s; 20 +/- 16%) and the non-operated (60 +/- 19 cm/s; 34 +/- 24%) hemisphere (p < 0.05;p < 0.01). After CEA the overall increase of resting CBFV of the operated side was highly significant with preoperative CBFV values of 51 +/- 19cm/s and postoperative values of 62 +/- 15 cm/s (p < 0.01). Cerebrovascular reserve capacity after operation was increased on both sides significantly (non-operated side: from 34 +/- 24% to 43 +/- 19%, p < 0.05; operated side: from 20 +/- 16% to 51 +/- 18%, p < 0.001), and the preoperative asymmetry was no longer present.

CONCLUSIONS

CEA has a beneficial effect on the cerebral circulation in most patients, even those who presented with asymptomatic carotid artery stenosis. Since CVRC has been assessed in the early postoperative period, our findings also suggest that cerebral vascular adaption occurs within 2 weeks after CEA.