Comparison of minimally and more invasive methods of determining mixed venous oxygenation

Smit, Marli (2013-12)

Thesis (MMed(Anes))-- Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Circulatory efficiency is the relationship between oxygen consumption and global oxygen delivery. Manipulation of circulatory efficiency has been shown to be beneficial in critically ill surgical and medical adults, and in children. Circulatory efficiency is best assessed by measuring an index of mixed venous oxygenation (content, saturation and partial pressure) and viewing this in the context of oxygen consumption. Mixed venous oxygenation has until now required intermittent sampling via a pulmonary artery catheter, or by using a pulmonary artery catheter equipped with a fibre optic bundle for continuous mixed venous oxygen saturation monitoring. However, the use of the pulmonary artery catheter is declining as it has been (correctly or incorrectly) indicted of being an “invasive” tool. Attempts have been made to estimate mixed venous oxygenation non-invasively using the “NICO” monitor[6], near infrared spectroscopy[7], skeletal muscle oxygen saturation[8], thenar muscle oxygen saturation[9] and transtracheal pulse oximetry.[4]While all of them effectively trended mixed venous oxygen saturation, their accuracy and use as a resuscitation endpoint are in doubt. Sampling central venous as a surrogate of mixed venous oxygenation is fraught with problems, particularly in sicker patients. Significant differences in oxygenation can be demonstrated between the pulmonary arterial and central venous sampling sites in shock states,[3, 10] in acutely ill post-surgical patients [11] and under varying hemodynamic conditions.[12] With the decline in the use of the pulmonary artery catheter, minimally invasive cardiac output determination is becoming increasingly popular. Apart from that their accuracy (particularly un-calibrated devices) has been questioned; they also cannot determine mixed venous oxygen saturation. To obtain a more reliable and refined, but less invasive, estimate of mixed venous oxygenation would be beneficial. The primary aim of this study was therefore to investigate whether venous oxygenation (mixed venous oxygen content, saturation and partial pressure) could be accurately predicted by minimally invasive methods of determining cardiac output and non-invasive calorimetric methods of measuring oxygen consumption. The methods compared were the current invasive gold standard represented by direct sampling of mixed venous blood and thermodilution cardiac output using a pulmonary artery catheter, with a less invasive method of calculating mixed venous saturation, the latter comprised of 4 elements: 1. Cardiac output was measured using a minimally invasive technique, namely lithium dilution (LiDCo®). 2. Oxygen consumption was measured with a non-invasive calorimetric device (M-COVX™ module manufactured by General Electric Corporation). 3. Arterial oxygen content was estimated using blood sampled via an arterial catheter. 4. These 3 variables were inputted into Fick’s equation and solved for venous oxygen content (CvO2 = CaO2 –VO2/CO). Thereafter, using the calculated venous oxygen content as well as the haemoglobin concentration, mixed venous oxygen saturation and partial pressure was estimated using an Excel® spreadsheet (Appendix G) relating oxygen saturation and partial pressure using standard oxygen dissociation curve formula, and calculating oxygen content from various haemoglobin concentrations. Analysis of the data was performed predominantly using Bland Altman analysis. LiDCo® derived cardiac output overestimated that measured using intermittent thermodilution PAC by a clinically significant average of 0.82liters/minute or 26%. The pulmonary artery catheter derived oxygen consumption underestimated that measured by the metabolic module by 52 ml/minute or 27%. Oxygen consumption was the parameter having the largest percentage error (27%) and difference between the Bland Altman upper and lower limits of agreement. The difference between oxygen consumption measured by indirect calorimetry is expected to exceed that calculated using the indirect Fick method by 20 to 30% because intra-pulmonary oxygen consumption is excluded when using this method.[ 13] However, the scatter exhibited by the calorimetry estimations of oxygen consumption was probably the major reason for the discrepancy between the calculated and measured mixed venous oxygenation variables. Despite small (12.0 to 26.3 %) differences between measurements in individual patients, venous oxygenation variables measured by the invasive and less invasive techniques were statistically different. We also considered the magnitude of these differences to be clinically significant as we were of the opinion that relying on the calculated results could adversely impact clinical decision-making. In conclusion, we could not estimate venous oxygenation accurately enough using minimally invasive methods of determining cardiac output and non-invasive methods of measuring oxygen consumption to be clinically useful.

AFRIKAANSE OPSOMMING: Sirkulatoriese effektiwiteit is die verhouding tussen suurstof verbruik en globale suurstof lewering. Manupilasie van sirkulatoriese effektiwiteit is voordelig in kritiese siek chirurgiese en mediese volwassenes, asook in kinders. Sirkulatoriese effektiwiteit word akkuraat bepaal deur gemengde veneuse suurstof (inhoud, saturasie en parsiële druk) te beskou in die lig van suurstof verbruik. Gemengde veneuse saturasie bepaling het tot onlangs afgehang van die neem van intermitterende bloedmonsters via ‘n pulmonale arterie kateter: die alternatief is deurlopende gemengde veneuse saturasie monitering met ‘n veselopties toegeruste pulmonale arterie kateter. Die gebruik van die pulmonale arterie kateter is egter aan die afneem aangesien dit (reg of verkeerd) as ‘n baie indringende moniterings apparaat beskou word. Pogings is al aangewend om gemeng veneuse saturasie te bepaal op ‘n nie-indringende manier. Onlangse studies het voorgestel dat gemengde veneuse saturasie afgelei kan word deur die gebruik van die “NICO” monitor, naby infrarooi spektroskopie, skeletspier suurstof saturasie, tenarespier suustof saturasie en transtrageale polsoksimetrie. Die meerderheid van bogenoemde was akkuraat genoeg om die tendens van die verskille in gemengde veneuse saturasie te volg. Die akkuraatheid van genoemde tegnologieë as betroubare weergawes van gemengde veneuse saturasie en hulle gebruik as eindpunt tydens resussitasie word egter in twyfel getrek. Sentraal veneuse saturasie kan ook nie as surrogaat vir gemeng veneuse saturasie gebruik word nie, veral nie in siek pasiënte nie. Groot verskille in oksigenasie bestaan tussen gemengde- en sentrale veneuse saturasie in pasiënte wat geskok is (2;8), in akuut siek postoperatiewe pasiënte en in hemodinamies onstabiele toestande. Minimaal indringende kardiale omset bepaling raak al hoe meer gewild soos die gebruik van die pulmonale arterie kateter afneem. Behalwe dat hierdie toerusting (veral die ongekalibreerde toestelle) se akkuraatheid al bevraagteken is, kan hulle ook nie gemengde veneuse saturasie bepaal nie. Dit sal dus voordelig wees om ‘n betroubare, akkurate en minimaal indringende metode te identifiseer om gemengde veneuse saturasie te bepaal. Die primêre doel van hierdie studie was om te bepaal of sirkulatoriese effektiwiteit (gemengd veneuse suurstof inhoud, saturasie en parsiële druk) akkuraat voorspel kan word met behulp van minimaal indringende metodes van kardiale omset bepaling en nie-indringende kalorimetriese metodes van suurstof verbruik. Die goue standard verteenwoordig deur ‘n direkte gemeng veneuse bloed monster en termoverdunning kardiale omset, gemeet met behulp van die pulmonale arterie kateter, is dan vergelyk met ‘n minder indringende metode. Tydens die minder indringende metode is gemengde veneuse saturasie as volg bereken is: 1. Kardiale omset is gemeet deur middel van litium verdunning (LiDCo®), ‘n minimal indringende tegniek. 2. Suurstof verbruik is gemeet deur ‘n non-indringende kalorimetriese toestel (M-COVX™ module vervaardig deur “General Electric Corporation”). 3. Arterieële suurstof inhoud is bereken met behulp van bloed verkry vanaf ‘n arteriële kannule. 4. Hierdie drie veranderlikes tesame met Fick se vergelyking is toe gebruik om veneuse suurstof inhoud te bereken (CvO2 = CaO2 –VO2 /CO). Deur gebruik te maak van die hemoglobien konsentrasie en veneuse suurstof inhoud, is gemeng veneuse suurstof saturasie en parsiële druk daarna bepaal in ‘n “Excel®”sigblad (Bylae G) wat die standaard formule vir die suurstof dissosiasie kurwe gebruik het. Analise van die data is hoofsaaklik uitgevoer deur gebruik te maak van die Bland Altman analise. LiDCo® afgeleide kardiale omset het dié gemeet deur intermitterende termoverdunning deur die pulmonale arterie kateter oorskat deur ‘n klinies relevante gemiddeld van 0.82liter/minuut of 26%. Die pulmonaal arterie kateter afgeleide suurstof verbruik het dié gemeet deur die metaboliese module onderskat met 52ml/minuut of 27%. Suurstof verbruik was die parameter met die grootste persentasie fout (27%) en verskil tussen die Bland Altman boonste en onderste limiete van ooreenkoms. Ons het verwag dat indirek kalorimetriese gemete suurstof verbruik 20 – 30% meer sou wees as suurstof verbruik bereken deur die indirekte Fick metode. Die rede hiervoor is dat intra-pulmonale suurstof verbruik 20 - 32% van heel liggaam suurstof verbruik bydra, en Fick-VO2 sluit hierdie intra-pulmonale suurstof verbruik uit. Die verspreiding soos aangetoon deur die kalorimetriese bepalings van suurstof verbruik was moontlik die hoofrede vir die onvereenigbaarheid tussen die berekende en gemete gemeng veneuse oksigenasie veranderlikes. Ongeag die klein (12 tot 26.3%) verskille tussen metings in individuele pasiënte, is die veneuse oksigenasie veranderlikes gemeet deur die indringende en minder indringende tegnieke statisties verskillend. Ons is ook van mening dat die grootte van die verskille klinies belangrik is, aangesien besluite geneem op grond van die berekende gemeng veneuse saturasie kliniese besluitneming negatief mag beïnvloed. In samevatting, kan ons nie veneuse oksigenasie akkuraat genoeg bepaal, met behulp van minimaal indringende metodes van kardiale omset en nie-invasiewe metodes van suurstof verbruik bepaling, om klinies van waarde te wees nie.

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