Dynamic near-infrared spectroscopy assessment as an important tool to explore pulmonary arterial hypertension pathophysiology
- Marios Panagiotou1⇑,
- Ioannis Vogiatzis2,
- Zafeiris Louvaris2,
- Geeshath Jayasekera1,
- Alison McKenzie1,
- Neil Mcglinchey1,
- Julien S. Baker3,
- Alistair C. Church1,
- Andrew J. Peacock1 and
- Martin K. Johnson1
- 1Scottish Pulmonary Vascular Unit, Golden Jubilee National Hospital, Glasgow, UK
- 2Faculty of Physical Education and Sports Sciences, National and Kapodistrian University of Athens, Athens, Greece
- 3Institute of Clinical Exercise and Health Science, University of the West of Scotland, Hamilton, UK
- Marios Panagiotou, Scottish Pulmonary Vascular Unit, Golden Jubilee National Hospital, Agamemnon Street, Glasgow, G81 4DY, UK. E-mail: mariopanag{at}gmail.com
Abstract
Near-infrared spectroscopy offers a qualitative, noninvasive indication of mixed venous oxygen saturation in PAH http://ow.ly/MctV306eEYC
From the authors:
We thank S. Dimopoulos and co-workers for their particular interest in our study [1] and their contributions to this issue. The peripheral muscle hypothesis in pulmonary arterial hypertension (PAH) [2] is certainly of great and growing interest due to the potential of muscle function as a target for meaningful intervention. Accordingly, our study explored the value of quadriceps muscle oxygenation profiles in patients with PAH by means of near-infrared spectroscopy (NIRS) [1]. The satisfactory correlation between vastus lateralis muscle tissue oxygenation index (StO2) and mixed venous oxygen saturation (SvO2) both at rest and during exercise support the use of NIRS in the noninvasive investigation of patients with PAH. Importantly, they suggest that skeletal muscle oxygenation profiles reflect the pathophysiology of PAH.
We know that factors determining local muscle oxygenation are modulated by the rate of oxygen delivery and of oxygen extraction [3]. Exploration of the relative contribution of oxygen delivery and oxygen extraction in peripheral muscle StO2 in PAH would be of paramount significance, as it would help to clarify whether muscle dysfunction in PAH is a mere consequence of impaired central haemodynamics, or whether it is due to a primary, intrinsic myopathy [4]. To this end, application of NIRS technology during exercise and/or in combination with other methods, such as the occlusion technique [5], the indocyanine green dye technique [6] and histological examination, offers a very promising opportunity. Indocyanine green dye, which has long been used for the assessment of cardiac output and plasma volume, is detectable by NIRS and has been used in this way to measure regional blood flow in respiratory and locomotor muscles [7–9].
Despite the fact that our study was limited by design in the investigation of muscle function (including the use of supplementary oxygen and submaximal exercise protocol, and the absence of a control group), when prompted by S. Dimopoulos and co-workers we proceeded to further interrogation of our data. Calculations in the exercising patient group (n=10) were unfortunately not feasible due to missing data; however, interrogation of complete patient resting data sets (n=25) revealed some interesting results.
We calculated the estimated systemic oxygen delivery (DO2), the product of cardiac output and arterial oxygen content, the latter term being calculated as the product of 1.34×haemoglobin concentration×SpO2 (percentage arterial oxygen saturation measured by pulse oximetry). The systemic arteriovenous oxygen content difference (a-vO2 diff) was calculated by dividing oxygen uptake by cardiac output (the Fick principle) whereas the systemic oxygen extraction rate was calculated as the ratio of a-vO2 diff to arterial oxygen content [10].
Resting oxygen content (in mL·dL−1) was 17.2±2.9, DO2 (in mL·min−1) was 794±317, a-vO2 diff (in mLO2·dL−1) was 5.7±2.2 and oxygen extraction rate (%) was 34±13. These results do not diverge significantly from normality and do not suggest an overt skeletal muscle dysfunction at rest; however, they cannot exclude an underlying muscle impairment that may become clinically significant during exercise.
Importantly, resting StO2 correlated positively with DO2 (r=0.556, p=0.004) and inversely with oxygen extraction rate (r=−0.695, p<0.001). In a similar fashion, resting SvO2 also correlated positively with DO2 (r=0.761, p<0.001) and inversely with oxygen extraction rate (r=−0.980, p<0.001). These novel findings are important because a) they confirm a positive correlation of peripheral muscle StO2 in PAH with systemic oxygen delivery and an inverse correlation with systemic oxygen extraction rate, and b) they strengthen further the value of StO2 as a qualitative, noninvasive marker of SvO2, thus laying the foundation for further use of NIRS in the investigation of the pathophysiology of PAH. The absence of exercise data does not, of course, allow for complete extension of these results; however, our findings are still sufficient to support the use of NIRS in PAH, where routine patient assessment with right-heart catheterisation and SvO2 sampling is most often undertaken under resting conditions.
Disclosures
Supplementary Material
M. Panagiotou ERJ-02161-2016_Panagiotou
Footnotes
Support statement: Marios Panagiotou is the recipient of an ERS PAH Long-Term Research Fellowship, number 2014-3106, supported by an unrestricted grant from GSK. Funding information for this article has been deposited with the Open Funder Registry.
Conflict of interest: Disclosures can be found alongside this article at erj.ersjournals.com
- Received November 3, 2016.
- Accepted November 9, 2016.
- Copyright ©ERS 2017