(134-142) Significance of venous - ISCIIIscielo.isciii.es/pdf/medinte/v32n3/revision.pdf · sular es uno de los cofactores más importantes de FMO. La oximetría venosa permite la

134 Med Intensiva. 2008;32(3):134-42

Critically ill patients are threatened or affectedby multi-organ failure (MOF). Tissue hypoxia isone of the most important co-factors of MOF.Venous oximetry allows the critical estimation ofthe global oxygen (O2) supply-demand ratio andcan be gained from mixed (SvO2) and central ve-nous blood (ScvO2). Cellular requirements domi-nate the feedback hierarchy of the O2-metabo-lism. This review describes the history and validityof haemodynamic monitoring, illustrates thephysiological background and clinical applicationof venous oxymetry and presents carbon dioxideanalysis as evidence of the usefulness of a multi-modal approach in cardio-respiratory monitoring.Variation of cardiac output, optimisation of arte-rial O2-saturation and adaptation of O2-extractionare shown to be the relevant levels of pathophy-siological adaptation as well as therapeutic inter-vention. We portray the functional equivalence ofScvO2 and SvO2 and analyse their diagnostic, ther-apeutic and prognostic significance, providingthe basis for the efficacy of venous oximetry as animportant marker of critical illness. Finally, havingdrawn an outline of current developments for thebetter understanding of the oxidative balance ofindividual organs, we stress the importance of asynoptic O2-monitoring strategy as well as theneed to use its beneficial, yet unfulfilled, clinicalpotential.

KEY WORDS: oxygen, monitoring, critical illness.

IMPORTANCIA DE LA OXIMETRÍA VENOSA ENEL ENFERMO CRÍTICO

Los enfermos críticos tienen riesgo de fracasomultiorgánico (FMO) o ya lo sufren. La hipoxia ti-sular es uno de los cofactores más importantesde FMO. La oximetría venosa permite la estima-ción crítica de la ratio de aporte-demanda del oxí-geno global (O2) y puede ser obtenida de la sangrevenosa mixta (SvO2) y central (ScvO2). Las necesi-dades celulares dominan la jerarquía de retroali-mentación del metabolismo de O2. Esta revisióndescribe la historia y validez del control hemo-dinámico, ilustra el fundamento fisiológico y apli-cación clínica de la oximetría venosa y presenta elanálisis de dióxido de carbono como prueba de lautilidad del enfoque multimodal en el control car-diorrespiratorio. Se comprueba que la variacióndel gasto cardiaco, la mejora de la saturación deO2 arterial y la adaptación de la extracción de O2son los niveles importantes para la adaptación fi-siopatológica y la intervención terapéutica.Presentamos el perfil de ScvO2 y SvO2 y analizamossu importancia diagnóstica, terapéutica y pronós-tica, proporcionando la base para la eficacia de laoximetría venosa como un importante marcadoren el enfermo crítico. Finalmente, después de pre-sentar un esquema de los desarrollos actuales pa-ra mejorar el conocimiento del balance oxidativode los órganos individuales, destacamos la impor-tancia de una estrategia sinóptica de monitoriza-ción de O2, así como la necesidad de utilizar su po-tencial clínico beneficioso, aún no aprovechado.

PALABRAS CLAVE: oxígeno, monitorización, enfermedad crítica.

OXYGEN TRANSFER FROM ATMOSPHERETO MITOCHONDRIA, AND BACK

The cardio-respiratory system provides the infra-structure for the transport of metabolic substrates to

Revisión

Significance of venous oximetry in the critically illP. BAUER, K. REINHART AND M. BAUER

Department of Anaesthesiology and Critical Care Medicine. Friedrich-Schiller-University. Jena. Germany.

Correspondence: Dr. M. Bauer.Department of Anaesthesiology and Critical Care Medicine.Friedrich-Schiller-University.07740 Jena. Germany.E-mail: [email protected]

Manuscript accepted on 19-X-2007.

(134-142) Significance of venous......qxp 28/02/2008 14:54 PÆgina 134

and from the capillary beds where cells can performeffectively. Cells are strategically well positioned atthe supply-demand interface, presenting a maximalsurface area for the exchange of metabolites, and re-present the basic functional physiological unit, asthey possess the mitochondrial machinery for energyproduction as well as all other organelles enablingthem to perform the functions of life. Oxygen (O2) isof vital importance for the efficient running of theseprocesses.

«Cellular respiration» neatly describes the mole-cular processes on the final common pathway of O2following its journey from atmosphere to mitochon-dria. On this long and complex route, O2 is movedthrough conducting airways, exchanged against car-bon dioxide (CO2) in the lungs along opposing diffu-sion gradients on either side of the red blood cellmembrane (O2-uptake), bound to haemoglobin andconvectively transported in the major blood vesselsto the tissues, there it diffuses and is distributed in thetissues. The variable fraction of O2 finally taking partin oxidative phosphorylisation to generate adenosinetriphosphate (ATP) is the effective oxygen delivery(DO2) for the cells. The arterio-venous difference ofO2 describes its extraction from DO2 depending onthe level of O2-demand (VO2).

METABOLIC AUTOREGULATIONOF THE CELL

Oxidative ATP production is a highly efficientprocess governed by metabolic autoregulation of thecell: the cumulative cellular metabolic performancesignals a level of VO2-requirement; this global VO2,in turn, is the cellular feedback parameter creating theset point for a minimal DO2 sufficient to satisfy meta-bolic demand.

Pflüger was the first to realise the feedback hierar-chy –cellular requirement > VO2 > DO2– as early as1872. He described how it was not only the task of thecardio-respiratory system to act as the logistic con-duit for substrates of metabolism, but also, and moreimportantly, that the oxygen requirement of the cellswas the paramount regulator of cardio-respiratoryphysiology: «Herein lies the predominant secret ofthe regulation of the amount of oxygen consumed bythe whole organism, determined only by the cell it-self; (…) arterial oxygen content, aortal pressure, ve-locity of blood stream, mode of respiration are all in-cidental and subordinate, they all combine theiractions in the service to the cells.»1.

OXYGEN SUPPLY AND DEMAND

Respiration is a tidal process, DO2 and VO2 haveto be constantly balanced and energy is permanentlygenerated, chemically bound in and released from itsmolecular storage form of ATP. Tissue hypoxia en-sues when this balance (DO2/VO2) is disturbed andoxygen consumption outstrips delivery. Oxidativephosphorylation stalls, cellular stress programmes

are triggered and anaerobic oxidisation occurs. Theinefficient anaerobic generation of ATP is quicklyexhausted and leads to the development of lactic aci-dosis representing, and in turn, further worsening,global cellular dysfunction. As tissue hypoxia consti-tutes one of the most important co-factors of multi-organ failure (MOF)2, it serves to illustrate the para-mount regulatory function of the cell in its strategicfrontline position.

Of all metabolic substrates, O2 has the highest per-centage of extraction and O2-reserves are exhaustedwithin a few minutes3. While this physiological factrenders O2, together with its immense biological im-portance, a very precious commodity, the phenome-non also promotes measurement of arterial and ve-nous O2-saturation as a highly effective clinical toolto monitor the cardio-respiratory system. Differentorgan systems display heterogenous oxygen extrac-tion levels, representing their differential metabolicactivity as well as their respective position in variouscapillary zones of the circulation together with theunhomogenous share of cardiac output (CO) they re-ceive per unit weight. The spectrum of venous satu-ration figures for different organs is, therefore, widealready under physiological conditions (fig. 1) andcan be further varied by pathophysiological adapta-tion.

VENOUS OXIMETRY

The two principal sources for venous oximetry atthe bedside are either a pulmonary artery catheter(PAC) or a central venous catheter, generating mixed(SvO2) and central venous saturation (ScvO2) measure-ments, respectively. ScvO2 is clinically more readilyavailable as it is less invasive. It forms a constituentpart of SvO2, rendering the true mixed venous satura-tion the more representative indicator of global tissueoxygenation. As various organs display physiologi-cal differences in DO2/VO2, and DO2 as well as VO2can both individually fluctuate acutely under patho-physiological conditions, SvO2 does not yield infor-mation on oxygen reserves or adequate tissue oxy-genation of individual organs. Furthermore, severaldisease states, such as hepatic failure or severe sepsisleading to arterio-venous shunting, limit the interpre-tation of absolute SvO2 values as indicators of tissueoxygenation. Here, normal or even supranormal SvO2levels can co-exist with severe tissue hypoxia. This isalso possible for conditions disturbing the unloadingof oxygen from haemoglobin through left shift of theoxygen dissociation curve or blockage of the respira-tory chain, such as in cyanide poisoning and, less welldefined, in distributive, for example septic shock.

In 1870, Fick explored the relationship betweenCO, global oxygen demand and oxygen extractionand stated that «total uptake or release of any sub-stance by an organ is the product of blood flow to theorgan and the arterio-venous concentration diffe-rence of the substance» (Fick’s principle)4. As O2 is inglobal cellular demand and the whole CO passes the

BAUER P ET AL. SIGNIFICANCE OF VENOUS OXIMETRY IN THE CRITICALLY ILL

Med Intensiva. 2008;32(3):134-42 135


entire cardio-respiratory system, mathematical re-arrangement of the physiological equation deducingVO2 illustrates the clinical usefulness and monitoringpotential of SvO2:

VO2 = CO × (CaO2 - CvO2) [Fick’s principle]CO = VO2/(CaO2 - CvO2) ~ CO = VO2/13.4 × Hb ×

(SaO2 - SvO2) and hence:SvO2 ~ SaO2 - (VO2/CO) × kIn other words: for a given VO2, SvO2 represents

the supply-demand ratio of oxygen. A DO2/VO2 im-balance indicates mismatch: in clinical terms, shock.Notwithstanding some methodological limitations, aswith all technology, venous oximetry provides ex-tremely useful information for the evidence-basedevaluation of the critical global oxidative balanceand, therefore, makes for a good monitor.

VALIDITY AND HISTORYOF HAEMODYNAMIC MONITORING

Sophisticated monitoring greatly enhances speedand quality of diagnosis and allows to guide and con-trol therapy, provided it is based on validated para-meters. The Latin monitor translates as “I am beingwarned/reminded”. Critical synoptic analysis of thecorrelation coefficients of the haemodynamic para-meters SvO2, mean arterial pressure (MAP) and heartrate (HR) for DO2/VO2 (fig. 2) demonstrates thatwhile the first is a reliable warning sign of oxidativeimbalance, the latter two are no reminders of oxygendelivery.

The most frequently measured monitoring parame-ters deliver the least information on oxygen transportand cellular oxygenation. In the cellular regulationframework of cardio-respiratory pathophysiology,however, evidence of O2-transport and requirementscarries the highest validity, as information on theoxidative state of the cells most closely reflects theirbioenergetic status, and those surrogate parameters,in turn, indicate most accurately what constitutes«critical» illness.

While MAP, especially if measured directly arte-rially, remains a valuable clinical parameter and firm-ly established in haemodynamic monitoring, a furtherproblem of validity arises with its use. In 1928,Jarisch pointed out that «for the development of thescience of circulation it was fateful that it is compa-ratively so awkward to measure flow, yet so easy tomeasure pressure: this is why the blood pressuremanometer gained almost fascinating influence,while most organs do not require pressure, but flowvolume»5.

The second half of the twentieth century usheredin an era of wider distribution and greater under-standing of monitoring technology for the reductionof morbidity and mortality, stimulated particularly byanaesthesiological practice in peri-operative and in-tensive care medicine. Pulse oximetry (SpO2) for thecontinuous, real-time measurement of peripheral ar-terial oxygen saturation, as well as capnography (res-piratory [end-tidal] CO2-monitoring, ETCO2) are non-invasive and have both proven so significant as tohave been promoted into the rank of obligatory moni-toring standards for the safe conduct of clinicalanaesthesia.

THE CARBON FOOTPRINT OF CELLULARRESPIRATION

CO2-monitoring offers a perspective of demandorientation not dissimilar to that of venous oximetry,as the amount of carbon dioxide produced (VCO2) foroxygen consumed are intrinsically linked through therespiratory quotient: RQ = VCO2/VO2. The RQ des-cribes the carbon footprint of oxidative phosphoryli-sation depending on the substrates used for ATP-pro-duction; for a diet of pure carbohydrates the RQ is 1as the exothermic conversion of glucose (C6H12O6)with oxygen produces equimolar quantities of carbon


136 Med Intensiva. 2008;32(3):134-42

Figure 1. Average percentage figures of physiological arterial(right) and venous (left) oxygen saturation of various organ regions:their heterogenous O2-extraction levels explain the wide venous O2-saturation spectrum.

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As the average diet in the industrialised world con-sists of a mix of carbohydrates (50%), fatty acids(30%) and proteins (20%) as sources for energy con-version, and the caloric value of fats and proteins ishigher than that of sugar, the mean RQ of a healthymale European (75 kg) at rest is approximately 0.8,that is 200 ml/min of CO2 generated for 250 ml/minof O2 consumed (basal metabolic rate).

Looking at the substrate-specific conversion for-mula for oxygen is instructive in several ways. Itsheds light on the alveolar gas exchange where, onthe one hand, water and heat serve to humidify andwarm the gases, O2 and CO2, taking part; on the other,however, CO2 and water vapour, along with nitrogen,occupy volume that cannot be used for optimisedalveolar O2-uptake (Boyle’s law of partial pressures).The disturbance of the evolutionarily old pulmonaryunit, filled with high concentration nitrogen display-ing a splinting effect, and protected by the physico-chemical properties of water on the alveolar fluid-gasinterface with low-concentration CO2 and medium-concentration O2, partially explains the detrimentallong-term consequences of the exposure of the lungsto high O2-concentrations by way of denitrogenation,with the therapeutic intent to maximize SaO2.Exceeding a threshold of the fractional concentrationof inhaled O2 (FiO2), in addition, has a negative feed-back impact on the CO. Trying to increase the effec-tive DO2 can thus paradoxically lower it.

While a synoptic analysis of O2-monitoring in-cluding capnography is, therefore, highly desirable,diet-dependency of the RQ represents only one of themethodological problems for the indirect monitoringof O2-supply-demand adequacy via CO2-monitoring.The elegant idea of gauging dysoxia via PCO2 gra-dients (ΔPCO2) provides the basis for gastro-intesti-nal tonometry, one of the few clinical tools presentlyavailable for the monitoring of tissue oxygenation6.At first regarded as a promising candidate for the ear-ly detection of dysoxia and regional hypoperfusion,critical concerns about methodological inaccuracies7

suggest a reduced, albeit still valuable role of ΔPCO2for the monitoring of the microcirculation only.However, as it cannot reliably detect anaerobic meta-bolism when flow is preserved, gastro-intestinaltonometry fails to reflect the oxygen supply dependen-cy during hypoxic episodes witnessed under conditionsof oxidative stress in the critical vascular bed of the gut.

Nevertheless, additional surrogates of impairedoxygen supply to peripheral tissues, such as ΔPCO2or plasma disappearance rate of indocyanine green(PDRicg)

8, help to widen the scope of the global oxy-genation parameter S(c)vO2, adding valuable informa-tion on individual organ regions.

LEVELS AND LEVERS OFPATHOPHYSIOLOGICAL ADAPTATION

When the variation of SvO2 (ΔSvO2) is correlated tothat of SaO2 (ΔSaO2) in the critically ill, it becomes


Med Intensiva. 2008;32(3):134-42 137

Figure 2. Critical synoptic analysis of the correlation between thehaemodynamic variables mixed venous oxygen saturation (SvO2)(A), mean arterial blood pressure (MAP) (B), as well as heart rate(HR) (C) and oxygen supply/demand (DO2/VO2) reveals that SvO2accurately reflects oxidative balance while the most frequently me-asured parameters MAP and HR deliver the least information on O2-transport and cellular oxygenation.

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138 Med Intensiva. 2008;32(3):134-42

apparent that venous saturation fluctuates much morethan its arterial equivalent over time (fig. 3), illustra-ting the functional importance of O2-extraction aswell as rendering ΔSvO2 the more specific and sensi-tive parameter to indicate adaptive changes, particu-larly in times of oxidative stress.

Optimisation of arterial O2-saturation, variation ofCO and adaptation of oxygen extraction are the threerelevant systemic mechanisms to adjust the organismto fluctuations in DO2/VO2; they present themselvesat the same time as therapeutic levers for haemody-namic management.

Without evidence of venous saturation, cardio-res-piratory monitoring, based solely on the measure-ment of heart rate and rhythm, arterial blood pressureand oxygen saturation, central venous pressure andCO, be it invasive or non-invasive, is half blind.Information on oxygen supply does not answer thedecisive question whether it is sufficient to meet cel-lular oxygen demand. While clinical monitoring thatallows the satisfactory quantification of the bioener-getic status of the cells is still some way off, it is allthe more regrettable that venous oximetry has not yetattained the position befitting a global monitoring pa-rameter of such relevance and importance.

In view of the astonishing technological progressrevolutionizing the generation, acquisition, pro-cessing, display and analysis of monitoring infor-mation, it is important not to forget the originalmeaning of «monitor» as an aid supporting the hu-man decision-making process. The first Harvardcriterion for good monitoring practice is, therefore,the constant presence of adequately qualified anaes-thesia personnel throughout the course of any

anaesthetic intervention. «Keeping a finger on thepulse» means just that.

SVO2 AND SCVO2 – NOT IDENTICAL,BUT FUNCTIONALLY EQUIVALENT

While SvO2 is the true mixed venous parameter,indicating global oxidative balance and correlatingbest with DO2/VO2, ScvO2 is its constituent part and assuch only representative of the organs of the upperbody. At the same time, ScvO2 measurements are lessinvasive and widely and quickly attainable in clinicalreality; SvO2 necessitates an intensive care environ-ment. Frequently, neither the time nor the place forPAC-dependent monitoring are available.

What is more, PAC usage, per se, has been shownnot to affect outcome in critically ill patients9, where-as the implementation of treatment strategies that incor-porate ScvO2 (or SvO2, where available) monitoring aspart of a dynamic monitoring philosophy along thelines of bioenergetic cellular integrity have succeededin reducing their mortality considerably10-14.

«Critical illness» can be defined as a state of disea-se that threatens the life of a patient by endangeringthe function of one or more vital organ systems.Tissue hypoxia is a pivotal co-factor of multi-organdysfunction syndrome (MODS), and venous oxime-try, therefore, well suited to describe this situation.Critical, in this context, is not only the biographicnadir as such, but also the narrowness of the windowof opportunity and with it the timing of therapeuticinterventions: sub-stratification analysis of haemody-namic strategy in high-risk patients15 reveals that op-timisation attempts are futile once organ failure is

Figure 3. Arterial oxygen saturation (SaO2), cardiac index (CI) and oxygen extraction, here represented by mixed venous saturation(SvO2), are the three relevant systemic mechanisms of adaptation to demand-supply-fluctuations (DO2/VO2). A) Correlation betweenthe respective variations of saturation (ΔSvO2/ΔSaO2) illustrates the functional importance and monitoring capability of SvO2 as it fluc-tuates much more than SaO2. B) Tight positive correlation of DO2 and CI marks their physiological interdependence.

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firmly established. ScvO2 serves as an early warningsign for the developing shock entity; it has the poten-tial to drag cryptic shock out into the open. It allowsthe monitoring of the critical oxidative balance in thedecisive period before both MODS and a PAC are es-tablished on the intensive care unit. Herein lies thebeauty and main advantage of ScvO2 over SvO2.

The long-running discussion about the equiva-lence of SvO2 and ScvO2 has been addressed in severalstudies16-18. These have illustrated ScvO2 and SvO2 lev-els to be numerically different, yet consistently paral-lel. Recognising a discrepancy of absolute numbers isprimarily of academic interest when their correlationis tight and, as a consequence, their physiologicaltrend patterns are equal as demonstrated in animalstudies19 as well as human clinical observations20 (fig. 4).«Mixed vs central venous oxygen saturation may benot numerically equal, but both are still clinicallyuseful»21 was the the blunt title of a recent editorialsumming up conclusively the supposed dilemma oflack of absolute congruence between ScvO2 and SvO2.

SIGNIFICANCE OF VENOUS SATURATIONMONITORING

It is of much greater value for the clinician to knowto what extent CO and SaO2 are able to meet the oxy-gen requirements of the capillary beds dictated byglobal cellular VO2 (Pflüger) than just to know thesupply side values in isolation, and nowhere more sothan in critically ill patients constantly threatened oraffected by MOF.

In addition, valuing trend lines over absolute fi-gures is an important principle of clinical measure-ment where biosensitive systems have to be calibra-ted often by multi-point analysis to detect and correctshift and drift, as well as in biostatistics where only asatisfactory quantity of data allows the description of

a phenomenon as a trend or correlation in the firstplace.

Continuous measurements display real-time trendson-line and lead, as a consequence, to more sensitiveand powerful monitoring data records than the dis-crete numbers generated by intermittent gathering ofreference points. This is of particular importance forthe close, accurate and precise monitoring of episo-dically and unforeseeably fluctuant parameters suchas DO2 and VO2. As VO2 exerts a physiological feed-back on DO2, it is instructive to measure and correlatethem both. If an increase in DO2, that is a raised CO,leads to a relevant increase in VO2, then this O2-fluxtest indicates the improvement of a previously inade-quate tissue oxygenation22.

Altogether, there can be no doubt that the benefitsof (trend analysis of continuous) ScvO2-monitoring byfar outweigh the slight, predominantly academic andclinically irrelevant discrepancy in absolute numeri-cal values when compared to SvO2.

Deduction from Fick’s principle elucidated therole of SvO2 as a functional monitoring parameter forthe critical estimation of the global balance of DO2and VO2. Three components of the equation (balance,supply, demand) on one side find their numerical ex-pression in the venous saturation on the other.

Oxidative balance in the critically ill is representedby a S(c)vO2 level of 70%. If oxygen demand outstripssupply the saturation scales are slanting towards tis-sue hypoxia - S(c)vO2 figures less than 65% are an ear-ly and urgent warning sign pointing towards devel-opment of MODS and shock. The (physio)logicalinterpretation of S(c)vO2 levels exceeding 75% is thatof a reverse imbalance with DO2 greater than VO2(fig. 5). As long as all metabolic requirements are be-ing met, this imbalance is irrelevant: supranormalityaccounts for physiological reserve. Not so in shock,though, where insufficient metabolic performance is

Figure 4. Simultaneous registration of SvO2and ScvO2; numerically different,consistently parallel, functionallyequivalent.

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the very defining, pathognomonic feature. The pa-tients’ maximal oxygen uptake and metabolism(VO2[max]) cannot fulfill the acutely increased mito-chondrial requirements in critical illness. Su-pranormal venous saturation in this context cannot beregarded as a useful or luxury reserve. Rather, it rep-resents a desperately needed, yet unused resource.

Focussing on the supply side alone does not meet ex-cessive demand requirements. The balance is notright: oxygen uptake is too low, indicating eithershunts forming at the level of the micro-circulation(effective DO2 too low) or highlighting the inabilityof the mitochondria to pass electrons through the res-piratory chain, that is a utilisation disturbance (effec-tive VO2 too low)23.

Cardio-pulmonary exercise testing (CPET) pro-vides a constructive analogy, as it is a clinically vali-dated protocol stratifying at-risk-patients into cate-gories with prognostic relevance based on theircapacity to mount a measurable response (VO2[max])to a potentially life-threatening situation of oxidativestress (major surgery). A patient presenting withsigns of MODS is, in contrast to the elective settingof CPET, a medical emergency and necessitates a ro-bust and streamlined approach. ScvO2 is currently thebest evidence-based parameter to guide and monitorthe direction of treatment strategies in critical illnesstowards the goal of an adequately raised DO2/VO2 ra-tio dynamically balanced on the level of a highermetabolic set point as dictated by the disease-inducedsurge in oxidative stress.

BETWEEN A ROCK AND A HARD PLACE:A SAFETY CORRIDOR OF ScvO2-VALUES

This is why ScvO2 is a cornerstone of an outcome-oriented algorithm such as early goal-directed thera-py (EGDT) as a constituent part of the SurvivingSepsis Campaign. Furthermore, obtaining a specimenfor blood gas analysis to establish a ScvO2 in the cri-tically ill should be a primary motivation when indi-cating the need for the placement of a central venouscatheter. What is more, persistent signs of tissue hy-poxia should then prompt continuous monitoring ofthe ScvO2, underpinning its importance as an endpoint of critical and early prognostic, diagnostic andtherapeutic significance.

In a series of 205 patients undergoing elective car-diac surgery we observed a similar hypoxic burden asindicated by increased lactate levels (> 4 mmol/l)along with comparable morbidity and mortality in pa-tients with ScvO2 measurements below 65% as well asabove 75%, respectively (own unpublished observa-tion). These data suggest a three-tiered sub-stratifica-tion along ScvO2 cut-off points gained by trend analy-sis of continuous measurements: while the welloxygen-balanced patient cohort had the best chancesof survival, both cohorts with ScvO2 values fallingoutside a «corridor of safety» of 70 ± 5% performedsignificantly worse and similarly to each other.

IMPORTANCE OF A SYNOPTICMONITORING STRATEGY OF O2IN DYNAMIC EQUILIBRIUM

Contemporary oxygen monitoring is sophistica-ted, universally available and yields important infor-mation to put intensive care for the critically ill on asound and solid evidence base as an eminent part of a


140 Med Intensiva. 2008;32(3):134-42

Figure 5. Disease-induced oxidative stress in critical illness: oxida-tive balance (ScvO2 70%) indicates an adequately raised DO2 in res-ponse to an increased VO2. Tissue hypoxia ensues when VO2 outs-trips DO2: ScvO2 < 65%, an urgent warning sign. Supranormality isa marker of poor prognosis too, as it highlights a reverse imbalance(ScvO2 > 75%): VO2 cannot be adequately up-regulated to satisfy cel-lular metabolic requirements and is outweighed by a raised DO2.

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synoptic evaluation strategy for a substrate in dy-namic equilibrium. Despite its great significance,(patho)physiological plausibility and clinical practi-cability, venous oximetry is not used to its full poten-tial yet.

Additional surrogates of impaired oxygen supplyto individual organs and functional cellular units,such as PDRicg or gastro-intestinal tonometry, aretechnically achievable and clinically available todayat least for some capillary beds. The near future willsee the introduction of more devices for the specificsurveillance of organs and their functions in relationto their individual DO2, VO2, their respective interac-tion and further important O2-dependent variables.These highly desirable instruments for prognostic,diagnostic and therapeutic management of the criti-cally ill threatened or affected by MOF allow a direc-tion of focus away from what Pflüger so poignantlydefined as «incidental and subordinate».

The challenge today is to move on from this stateof affairs and to raise understanding of the existingpossibilities of O2-monitoring and its significance forsurvival of the critically ill, characterised by a he-terogeneity of outcome which has less to do with ac-cess to high-tech medicine than with the will to con-sequently implement, at local level and into clinicalroutine, what has to be regarded as best practice at thepresent time.

Venous oximetry should play the role of the firstviolinist in the orchestrated approach necessary to dojustice to the complex and dynamic network of inter-actions that classifies oxygen metabolism and itsclinical monitoring as a symphony constantly in themaking –or breaking– in the critically ill patient.

CONCLUSION

The cardio-respiratory system provides the infra-structure for the transport of O2 from atmosphere tomitochondria, and back. Global cellular requirementdetermines effective VO2, setting the point for a mini-mally sufficient effective DO2. The cell is at the topof the feedback hierarchy of metabolic autoregula-tion. Tissue hypoxia ensues when global and/or indi-vidual organ DO2/VO2-balance are disturbed; it con-stitutes one of the most important co-factors of MOFwith highly significant morbidity, mortality and costimplications for the critically ill. Of all metabolicsubstrates, O2 has the highest percentage of extrac-tion. It is further characterised by heterogenous organsaturation levels and a quickly exhaustable reserve.This dynamic equilibrium promotes monitoring ofarterial and venous O2-saturation as a very effectiveclinical tool. Arterial O2-saturation is routinely mo-nitored invasively (SaO2) or non-invasively (SpO2); incontrast to that, venous oximetry is not used as con-sistently or effectively. There are two principalsources for venous oximetry: SvO2 and ScvO2. SvO2 isthe more representative indicator of global tissueoxygenation; ScvO2, however, is less invasive andmore readily available (they are not identical, butfunctionally equivalent). Both represent the supply-

demand ratio for O2 and, therefore, provide highlysignificant evidence for the critical synoptic analysisof cardio-respiratory monitoring data. Unfortunately,the most frequently measured haemodynamic para-meters -arterial blood pressure and HR- deliver theleast information on cellular oxygenation. Further-more, a multi-modal O2-monitoring approach eluci-dates the useful function of CO2-monitoring with itsperspective of demand orientation similar to that ofvenous oximetry; VO2 and CO2-flux (VCO2) are in-trinsically linked through alveolar ventilation in therespiratory quotient: RQ = VCO2/VO2. Optimisationof SaO2, variation of CO and adjustment of oxygenextraction (ΔS(c)vO2) are the three relevant systemicphysiological mechanisms of adaptation as well astherapeutic levers for haemodynamic management.They are half blind without evidence of venous satu-ration, as it is much more important to establish towhat extent CO and SaO2 are able to meet oxygen re-quirements than just to know the supply side valuesin isolation. In the critically ill, sustained ScvO2 levelsbelow 65% indicate ineffective DO2 while thoseabove 75% characterise an ineffective VO2, with asafety corridor of 70 ± 5% in between. ScvO2 is cur-rently the best evidence-based parameter to guide andmonitor treatment strategies of patients threatened oraffected by MOF. Obtaining ScvO2-values should be aprimary motivation when indicating the placement ofa central venous catheter. Contemporary, multi-modal O2-monitoring is sophisticated, widely avail-able and (patho)physiologically highly relevant.Despite its great clinical significance, plausibility andpracticability, it is not yet used to its full potential.Additional surrogates of impaired oxygen supply toindividual organs are promising more conclusive an-swers to the supply-demand question: micro-circula-tory insufficiency or utilisation disturbance? at themitochondrial level as the final common pathway ofO2-metabolism.

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(134-142) Significance of venous - ISCIIIscielo.isciii.es/pdf/medinte/v32n3/revision.pdf · sular es uno de los cofactores más importantes de FMO. La oximetría venosa permite la