The status of an inversely coupled oxic/sulphidic oscillator (OSO) in the whole body chemocline will determine clinical unfolding in Systemic Inflammatory Response Syndrome (SIRS)* of any aetiology.  

chemocline will determine clinical unfolding in Systemic Inflammatory Response Syndrome (SIRS)* of any aetiology.   Paul Ramesh Thangaraja,b  FRCS (Cardiothoracic Surgery) Abstract Life evolved in an euxinic world with subsequent oxic 'invasion' leading to two parallel but interconnected biospheres, hydrogen sulphide (H2S) and hydrogen peroxide (H2O2) exemplify these worlds respectively. Their concentration gradients have informational value in meromictic lakes. Similarly, it is posited, there exists a whole body chemocline in humans in which the two molecules form an inversely coupled oxic/sulphidic oscillator (OSO). The OSO is hormetic and characterised by a range of amplitudes and frequencies in health. Deviations from its baseline profile heralds the onset of SIRS before the appearance of clinical signs. Loss of oscillator status and transition to a steady state causes widespread intercellular and inter-organ communication failure presaging multiorgan dysfunction. The salient clinico-pathophysiological features of SIRS of any aetiology are emergent phenomena related to the OSO profile. Extent of recovery of organ function will mirror the recovery of the OSO profile thereby providing a tool to predict outcomes in SIRS.


INTRODUCTION
The prediction of organ dysfunction in patients with sepsis or SIRS is not possible with current risk score profiles or biomarkers. Hence they have little value in timing interventions especially ones with narrow safety margins such as initiation of steroids and extracorporeal membrane oxygenation (ECMO) therapy. There has been significant advances in the understanding of oxidant injury and sulphidic responses to it. However this information has not been harnessed to the bedside. A hypothesis that is anchored in ancient biochemical connections but providing a clinical context may help improve understanding of a complex clinical problem.

THE HYPOTHESIS
It is posited that there exists, in humans, a whole body (corporeal) chemocline. The chemocline is a composite of reactive oxygen species (an oxycline) and reactive sulphur species (a sulphidocline) exemplified by hydrogen peroxide and hydrogen sulphide respectively. H2O2 and H2S form an inversely coupled oscillator. The maximum and minimum values of their concentrations represents the amplitude, the rate of change of concentrations represents the frequency of the oscillator. This oxic/sulphidic oscillator (OSO) on account of its relatively higher permeability across membranes is a fundamental signaller of the redox status of the whole body chemocline. The oscillator occupies the bottom of an inverted pyramid of an informational cascade and has an intrinsic range of values in health. The status thus communicated is perceived intracellularly, inter-cellularly and between organs being vital to the integrative function at both organ and organismal level. The superposition of individual cellular OSO status will determine the organ level status. The superposition of the OSO status of individual organ systems will determine whole body status. The characteristic of the OSO is hormetic in nature. Lower amplitude and frequencies will signal health, higher amplitude and frequencies will signal transition to an acute disease state such as SIRS. Loss of variability (decomplexification) of the OSO meaning loss of ability to oscillate and reaching a steady state is associated with intercellular and inter-organ communication breakdown and portends organ failure.
Degree of return of intrinsic variability will determine extent of recovery of organ function.

BASIS OF THE HYPOTHESIS
The hypothesis is based on the contention that the energetics of the biosphere evolved initially in an euxinic world and that subsequent oxic 'invasion' set the stage for competing electron acceptors.
This ancient relationship between the two molecules, while modified by eons of evolutionary adaptations, has provided an inversely coupled oscillator that functions as a primordial informational unit in humans. The logical basis of the assertion can be distilled into the composite question -is there currently any indication of a corporeal chemocline, if so does it oscillate, if it does oscillate does it convey information and could it predict outcomes in SIRS?

Is there an indication for the existence of an oscillating inversely coupled chemocline?
The entire biosphere inhabits an inversely coupled oxic/sulphidic vertical chemocline extending from the atmosphere to deep hydrothermal vents. Present day meromictic lakes [1] like Proterozoic oceans [2] are characterised and stratified by similar chemoclines wherein both molecules through their role as terminal electron acceptors of the electron transport chain (ETC) fuel life. The corporeal chemocline could be conceptualised as mirroring the ones found in meromictic lakes and extending from the intravascular compartment through the extracellular fluid (ECF) space to the inside of the cell. The resistance of cell membranes to H2O2 permeability [3] and differential distribution of the sulphane sulphur pool help maintain concentration gradients. The current best estimates of the concentrations of the constituents [4][5][6][7][8] of the corporeal chemocline are given in the tabular column. The interaction of the lake with its environment results in seasonal positional changes of the oxic/sulphidic transition zone as a result of nutrient trapping, down-swelling of oxygen and upswelling of H2S [9]. Similarly the corporeal chemocline bounded by an environment that extends from the oxygen rich lungs to the sulphide rich colon communicates to the outside via the aerodigestive tract. In general there is a net movement of H2O2 from plasma towards the cell and H2S from cell towards plasma [5,10]. Fluctuations in the established rhythm could be altered by external changes in alveolar gas (eg. H2S inhalation), breakdown of the colonic barrier or increased production of H2O2 intravascularly as during an oxidative burst.

Is it a primordial informational unit?
The presence of unicellular organisms like E.proximas in meromictic lakes corresponds to H2S levels during seasonal oscillation of the transition zone [1]. Cable bacteria with exceptionally long ETCs [11]and E.coli with variable specificity ETCs [12] make the choice of electron acceptor based on availability of oxygen or sulphide. The fundamental nature of the biochemical interconnectedness of the two molecules [13]are shown in the figure.

Is the oscillator hormetic in nature and could it signal health and disease ?
The basis of this contention could be made by examining the data from various positions of the imagined chemocline -the two extreme locations, the colon and the lung and the major organ system that lies between and interacts with them both -the vascular system.
H202 at a lower concentration protects against colitis but facilitates bacterial invasion at high concentrations. [14] Serum H2S levels remained constant in healthy controls across age groups but was inversely related to severity of COPD [15].
Both molecules display clear hormesis with regard to vasodilatation and constriction in resistance arteries (16,17]. A recent interesting study highlights an important aspect of the hypothesis. Lung function from rejected lung organ donors improved in function after inserting them in pigs [18] Here it is submitted that the donor lung in an OSO that has lost its variability (the brain dead donor) was dysfunctional and its reintroduction into the normal chemocline of the pig improved function. The above interactions suggest a role for the OSO as an indicator of health and disease.

What would determine an individual patients OSO profile?
The oscillator profile of a given patient would be measured by the rate of change of H202 concentration (slope of incline), peak concentration, plateau concentration, time at plateau, rate of decrease of concentration, ability to reach baseline, difference of final concentration from baseline concentration. A similar H2S profile will also be constructed and the time lag between the two ascertained.
The OSO trajectory will need to be defined in two time frames -the immediate OSO status (minutes to less than 1 hour of the stimulus) and the delayed OSO status (probably 6-24 hours but before onset of clinical organ dysfunction). Thus the composite OSO profile would consist of three components -baseline, immediate and delayed status. A hyper and hypo-oxidant profile each with a corresponding, hyper, normal and hypo-sulphidic response implies at least six basic profiles.

Can it predict the clinical unfolding of SIRS?
Transition from physiological to a pathophysiological state has been described as a loss of intrinsic variability and progression to a steady state (decomplexification) [19]. This decomplexification, of hitherto unidentified biological oscillators, was postulated as the reason for development of multiorgan dysfunction syndrome (MODS) in sepsis [20]. H2O2 and H2S are prime candidates for the oscillator in SIRS as they reflect vigour of the oxidative burst, the rapidity of its communication through the chemocline and the sulphidic response. Baseline differences in the profile and speed of subsequent loss of variability of the signal should generate distinct OSO profiles. These distinctive profiles should correlate with extant clinical and pathophysiological features of SIRS. This implies that children, adults with and without comorbidities, immunosuppressed patients would have different OSO profiles.
In addition certain striking clinical features -the increased incidence of neurological dysfunction in non infective vs infective SIRS inspite of the same crude mortality rate, the greater incidence of SIRS with head injury than other forms of trauma, patients who develop neurological complications during sepsis having poorer short and long term outcomes, the relative protection afforded to SIRS by BMI of a particular range, the wide range in mortality (15-71%) from cardiac arrest in sepsisshould correlate with distinctive OSO profiles.

TESTING THE HYPOTHESIS
Given the questions raised about sensitivity and specificity of measurements of both molecules [5,7,21], the greatest challenge in testing the hypothesis would be the development of specific, accurate point of care devices.

Clinical studies
The profiles generated from blood levels of H2O2 and H2S in elective cardiac surgical patients would provide a practical clinical model to test the hypothesis for the following reasons: 1. Cardiopulmonary bypass (CPB) is associated with a substantial oxidative stress. Testing convergence with pathophysiological features such as immunological, vascular and bioenergetic dysfunction.

Immunological dysfunction
The dysregulated immune response in SIRS could be described as an oscillator from SIRS to CARES through a variable MARS landscape [22]. H2O2 has been shown to have both positive and negative feedback regulation of the inflammatory response [23]. IL-6 concentrations are elevated in patients who develop MODS from trauma induced SIRS [24] and H2S has been described as modifying the IL6 response in experimental studies [25].
Concomitant measurements of inflammatory biomarkers could ascertain correlation with OSO profiles.

Endothelial dysfunction (Vasoplegia and microcirculatory failure)
The endothelial response is intimately connected to the immune response with two striking features, vasoplegia and microcirculatory failure. The latter expressed as both increased microvascular permeability and functional shunting.
A familiar clinical picture is the transition from normal blood pressure to hypotension to death producing a therapeutic response of volume resuscitation, low dose pressor , high dose pressor with corticosteroids, inotropes and finally an inability to maintain pressures in spite of maximal treatment. The ability of the profiles to discern between normal, responsive and unresponsive vasoplegic patients could be studied.
The maintenance of normal range ECF profile in a given organ in the face of a changing intravascular profile indicates the robustness of the endothelial barrier. The blood brain barrier [26] and the sinusoids of the liver represent contrasting endothelial permeability under normal conditions. Microcirculation imaging and organ specific capillary transit time could be valuable adjuncts to test the association of distinctive intravascular to ECF profiles with worsening of microvascular permeability and functional shunting.

Bioenergetic dysfunction
Hyperlactemia, a feature of both post surgical SIRS [27] and sepsis [28] could be indicative of either inadequate oxygen delivery or an inability to use oxygen. H2O2 may favour aerobic glycolysis as a less energetic but less catabolic pathway designed to aid recovery [29].
H2S, the first identified inorganic substrate, can at low concentrations be an adjunct to the Krebs cycle but at high concentrations produces profound bioenergetic failure by direct inhibition of cytochrome c oxidase [30].
An inability to trigger aerobic glycolysis resulting in a blunted lactate response alongside a histopathological correlation with less recoverable forms of cell death should have distinct OSO profiles [31].

Testing predictive accuracy compared to extant risk scores
Current risk scores predict mortality based on evolving organ dysfunction and as such measure criticality. An ability to predict the onset of organ dysfunction would have value in timing therapeutic interventions (eg. steroids and mechanical support) for an optimal risk-benefit outcome. If the hypothesis holds, it should at best out perform current risk score prediction, in particular among low risk patients who die and high risk patients who survive, or at the least significantly enhance current risk score prediction.

Animal studies
The hypothesis asserts that the oscillator is a network communicator, hence it is obligatory to test it in the intact animal or at the least an intact perfused organ system that is ensconced in a setup that can simulate the potential range of the oscillator.

CONCLUSION
The hypothesis is mechanistic and reductionist in its attempt to explain a complex syndrome on the basis of a possible relationship between two simple molecules. It disregards the role of nitric oxide (NO) in the relationship. The envisaged chemocline in the intracellular region is over simplistic in its lumping together of all intracellular organelles, except the mitochondria, into the term 'cytoplasm'. NO executes many of its vasomotor actions through interactions with H2O2 and H2S [32,33].
The simplistic intracellular portion of the chemocline was influenced by Searcy's sulphur hypothesis [34] which toggles the sulphur between oxidised and reduced forms between these two locales.
Current risk scores and biomarkers are suboptimal for timing therapy. The expectation is that insight gained while testing the hypothesis could be useful in the development of a practical bedside test to time high risk interventions. Meanwhile 'that more intimate beauty' of investigating the possibility of an informational signal that extends back to the origins of life should be a reward in itself.