Investigation of the cerebral haemoglobin and cytochrome signals using near infrared spectroscopy during head up tilt in patients with orthostatic hypotension

Introduction

The cardiovascular and cerebrovascular responses to head up postural change are severely compromised in primary autonomic failure (AF) because of sympathetic denervation. AF patients therefore exhibit hypotension and show signs of cerebral hypoperfusion during a head-up tilt. In this study we employed near infrared spectroscopy (NIRS) to investigate changes in haemoglobin and cytochrome oxidase redox state ([CytOx]) during periods of severe cerebral hypoperfusion defined as a drop in cerebral haemoglobin difference ([Hbdiff]=[oxyhaemoglobin]-[deoxyhaemoglobin]) of at least 10 μmol/l ¹ .

Methods

12 patients were studied and data were recorded (i) when supine and (ii) during passive head up tilt to 60 degrees. A continuous wave four wavelength NIRS system (NIRO 300, Hamamatsu Photonics KK) measured changes in cerebral tissue oxyhaemoglobin ([HbO2]) and deoxyhaemoglobin [HHb]) and [HbT]=[HbO2]+[HHb] and [CytOx] using an age corrected differential pathlength factor ² . Beat-to-beat blood pressure (BP) was measured non-invasively using a Portapres® system. Using the [Hbdiff] signal we identified the start of the head up tilt and the maximum drop and acquired 5 sequences of 10 sec mean values from the start to the maximum of the drop.

Results

8 patients demonstrated a maximum drop in the [Hbdiff] signal of at least 10 μmol/l, 6 patients (group a) demonstrated a reduction in [CytOx] and 2 patients (group b) exhibited no change, which was unrelated to the drop in the other signals. These mean changes (±SD) are shown in the figure 1.

Figure 1

Conclusions

We have shown that in some AF patients postural hypotension is associated with a reduction in [Hbdiff] accompanied by a fall in [CytOx], possibly indicating severe cerebral cellular dysoxia. The pattern of [CytOx] change shown in Group a, has previously been demonstrated using NIRS ³ . In 2 patients there is no response in the [CytOx] signal. It has been reported that large changes in chromophore concentrations, can lead to artifacts in the observed changes in low concentration chromophores, such as [CytOx] ⁴ , in the data presented here there is no evidence of crosstalk between the haemoglobin and [CytOx] signals. However individual differences in the anatomy of the different layers of the adult head (scalp/skull and brain) could introduce ‘contamination’ to the [CytOx] signal ⁵ . Measurement of [CytOx] might be a useful adjunct during monitoring of cerebral ischaemia.