Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue. It is often used non-invasively to make measurements at the skin surface. The PPG waveform comprises a pulsatile ('AC') physiological waveform attributed to cardiac synchronous changes in the blood volume with each heart beat, and is superimposed on a slowly varying ('DC') baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. Although the origins of the components of the PPG signal are not fully understood, it is generally accepted that they can provide valuable information about the cardiovascular system. There has been a resurgence of interest in the technique in recent years, driven by the demand for low cost, simple and portable technology for the primary care and community based clinical settings, the wide availability of low cost and small semiconductor components, and the advancement of computer-based pulse wave analysis techniques. The PPG technology has been used in a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease. The introductory sections of the topical review describe the basic principle of operation and interaction of light with tissue, early and recent history of PPG, instrumentation, measurement protocol, and pulse wave analysis. The review then focuses on the applications of PPG in clinical physiological measurements, including clinical physiological monitoring, vascular assessment and autonomic function.

https://iopscience.iop.org/article/10.1088/0967-3334/28/3/R01/pdf

Photoplethysmography and its application in clinical physiological measurement.

We present a method for quantifying the anaerobic capacity based on determination of the maximal accumulated O2 deficit. The accumulated O2 deficit was determined for 11 subjects during 5 exhausting bouts of treadmill running lasting from 15 s to greater than 4 min. The accumulated O2 deficit increased with the duration for exhausting bouts lasting up to 2 min, but a leveling off was found for bouts lasting 2 min or more. Between-subject variation in the maximal accumulated O2 deficit ranged from 52 to 90 ml/kg. During exhausting exercise while subjects inspired air with reduced O2 content (O2 fraction = 13.5%), the maximal O2 uptake was 22% lower, whereas the accumulated O2 deficit remained unchanged. The precision of the method is 3 ml/kg. The method is based on estimation of the O2 demand by extrapolating the linear relationship between treadmill speed and O2 uptake at submaximal intensities. The slopes, which reflect running economy, varied by 16% between subjects, and the relationships had to be determined individually. This can be done either by measuring the O2 uptake at a minimum of 10 different submaximal intensities or by two measurements close to the maximal O2 uptake and by making use of a common Y-intercept of 5 ml.kg-1.min-1. By using these individual relationships the maximal accumulated O2 deficit, which appears to be a direct quantitative expression of the anaerobic capacity, can be calculated after measuring the O2 uptake during one exhausting bout of exercise lasting 2-3 min.

Anaerobic capacity determined by maximal accumulated O2 deficit

Inert gas washout tests, performed using the single- or multiple-breath washout technique, were first described over 60 years ago. As measures of ventilation distribution inhomogeneity, they offer complementary information to standard lung function tests, such as spirometry, as well as improved feasibility across wider age ranges and improved sensitivity in the detection of early lung damage. These benefits have led to a resurgence of interest in these techniques from manufacturers, clinicians and researchers, yet detailed guidelines for washout equipment specifications, test performance and analysis are lacking. This manuscript provides recommendations about these aspects, applicable to both the paediatric and adult testing environment, whilst outlining the important principles that are essential for the reader to understand. These recommendations are evidence based, where possible, but in many places represent expert opinion from a working group with a large collective experience in the techniques discussed. Finally, the important issues that remain unanswered are highlighted. By addressing these important issues and directing future research, the hope is to facilitate the incorporation of these promising tests into routine clinical practice.

https://erj.ersjournals.com/content/erj/41/3/507.full.pdf

Consensus statement for inert gas washout measurement using multiple- and single- breath tests

We determined the incidence of exercise-induced arterial hypoxaemia and its determinants in sixteen highly trained, healthy runners who were capable of achieving and sustaining very high metabolic rates (maximal O2 uptake = 72 +/- 2 ml kg-1 min-1 or 4.81 +/- 0.13 l min-1). Arterial blood gases and acid-base status were determined at each load of a progressive short-term exercise test and repeatedly during constant-load treadmill running while breathing air and during inhalation of mildly hypoxic, hyperoxic, and helium-enriched gases. Three types of responses to heavy and maximum exercise were evident and highly reproducible within subjects. Four runners maintained arterial PO2 (Pa, O2) within 10 mmHg of resting values, another four showed 10-15 mmHg reductions in Pa, O2, and the remaining eight showed reductions of 21-35 mmHg, i.e. in all cases to a Pa, O2 of less than 75 mmHg and to less than 60 mmHg in two cases. During constant-load exercise, Pa, O2 was often maintained during the initial 30 s when hyperventilation was greatest; then hypoxaemia occurred and in most cases was either maintained or worsened over the ensuing 3-4 min. The most severe hypoxaemia during heavy exercise was associated with no or little alveolar hyperventilation (Pa, CO2 greater than 35 mmHg and PA, O2 less than 110 mmHg) and an alveolar to arterial PO2 difference [(A-a)DO2] in excess of 40 mmHg. During 3-4 min of heavy exercise alveolar PO2 (PA, O2) decreased by 20 mmHg in mild hypoxia (0.17 FI, O2; inspired % O2) and increased by 20 mmHg during mild hyperoxia (0.24 FI, O2) and 10 mmHg during the hyperventilation which accompanied normoxic helium breathing. In all cases Pa, O2 changed in proportion to PA, O2 with no consistent change in the alveolar to arterial PO2 difference [(A-a)DO2]. In view of the correction of hypoxaemia with mild hyperoxia and the very high ratios of alveolar ventilation to pulmonary blood flow (VA/QC = 4-6) achieved during heavy exercise, we think it unlikely that non-uniformity of the VA/QC distribution or veno-arterial shunt could explain the hypoxaemia observed in most of our subjects. By exclusion, we suggest that hypoxaemia may be attributed to a diffusion limitation secondary to very short red cell transit times in at least a portion of the pulmonary circulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Exercise‐induced arterial hypoxaemia in healthy human subjects at sea level.

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body’s upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10–15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures—exercise (aerobic, resistive, vibration), nutritional and pharmacological—are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986–2006)

Muscle metabolites and oxygen deficit with exercise in hypoxia and hyperoxia.

An experimental protocol was designed to investigate whether human core temperature is regulated at a “set point” or whether there is a neutral zone between the core thresholds for shivering thermo...

Core temperature "null zone".

Changes in expired nitric oxide (NO) occur in airway inflammation and have proved to be important in the monitoring of inflammatory disease processes such as asthma. We set out to develop a novel hand-held NO-analyzer with a performance comparable to the present more costly and complex chemiluminescence instruments. The new device is based on a specially designed electrochemical sensor, where we have developed a novel sampling and analysis technology, compensating for the relatively slow response properties of the electrochemical sensor technique. A Lowest Detection Limit in NO-analysis from reference gas tests of less than 3 ppb and a response time of 15 seconds together with an average precision in human breath measurements of 1.4 ppb were obtained. We also show an agreement with the existing ‘gold standard’ FENO measurement technique, within 0.5 ppb in a group of 19 subjects together with a high linearity and accuracy compared to reference gases. The new analyzer enables affordable monitoring of inflammatory airway diseases in research and routine clinical practice.

/pdf/novel-hand-held-device-for-exhaled-nitric-oxide-analysis-in-535hhjzcb0.pdf

Novel Hand-Held Device for Exhaled Nitric Oxide-Analysis in Research and Clinical Applications

Posture primarily affects lung tissue distribution with minor effect on blood flow and ventilation

An open circuit tracer gas washout method for measurement of lung volume in patients during anesthesia and intensive care is described and tested. The method employs a device for dispensing the tracer gas, sulfur hexafluoride (SF6), a fast SF6 analyzer, a pneumotachograph, and a computer. The dispensing device delivers SF6 into the airway in proportion to instantaneous inspiratory flow so that inspiratory SF6 concentration is held constant, usually at about 0.5%, regardless of the inspiratory flow pattern. The amount of SF6 present in the lungs at the end of a washin is calculated during washout from signals representing expired SF6 concentration and expired flow. From this, lung volume is derived. Accurate and reproducible results were obtained in lung model tests during ventilation with air, N2O in O2, and halothane in O2. Functional residual capacity (FRC) was measured both with SF6 washout and nitrogen washout in five mechanically ventilated patients. This gave the regression equation: FRCSF6 = 10 ml + 1.04 x FRCN2, r = 0.99. A similar close agreement was observed for total lung capacity (TLC) and residual volume (RV) measurements in eight healthy, spontaneously breathing subjects: TLCSF6 = 91 ml + 1.01 x TLCN2, r = 0.99; RVSF6 = -32 ml + 0.97 x RVN2, r = 0.95. Comparison with body plethysmography in eight healthy, sitting subjects gave the regression equation: FRCSF6 = 180 ml + 0.96 x FRCbox, r = 0.99. The median (range) for the coefficient of variation at duplicate determinations in 10 anesthetized, paralyzed, and mechanically ventilated adults was 3.0% (0.2-6.6%).(ABSTRACT TRUNCATED AT 250 WORDS)

Dag Linnarsson

Papers

Muscle metabolites and oxygen deficit with exercise in hypoxia and hyperoxia.

Core temperature "null zone".

Novel Hand-Held Device for Exhaled Nitric Oxide-Analysis in Research and Clinical Applications

Posture primarily affects lung tissue distribution with minor effect on blood flow and ventilation

Measurement of Lung Volume by Sulfur Hexafluoride Washout during Spontaneous and Controlled Ventilation: Further Development of a Method