Browsing by Author "Smit, Marli"

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    Comparison of minimally and more invasive methods of determining mixed venous oxygenation
    (Stellenbosch : Stellenbosch University, 2013-12) Smit, Marli; Levin, Andrew I.; Coetzee, J. F.; Stellenbosch University. Faculty of Medicine & Health Sciences. Dept. of Anesthesia and Critical Care.
    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.
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    The manipulation of autophagy during early and late reperfusion : the effect on myocardial protection
    (Stellenbosch : Stellenbosch University, 2019-03) Smit, Marli; Coetzee, Andre; Lochner, Amanda; Strijdom, Hans; Stellenbosch University. Faculty of Medicine and Health Sciences. Dept. of Biomedical Sciences: Medical Physiology.
    ENGLISH ABSTRACT: Introduction: Ischemic heart disease, the leading cause of death worldwide, often has devastating effects on myocardial function. ReFestablishment of coronary flow to salvage myocardial cells results in reperfusion injury. There is an ongoing quest for effective therapeutic interventions against the deleterious effects of this phenomenon. The pharmacological manipulation of autophagy, a process dealing with the organized destruction and recycling of cellular components, is one of the latest focus areas of research on cardioprotection against reperfusion injury. The significance of the autophagic response following ischemia/ reperfusion, as well as the effect of manipulation of this process on cardioprotection is still a matter of debate. Aims: We hypothesised that autophagic induction during early reperfusion is protective while its upregulation during late reperfusion is detrimental. The primary aim was to investigate the protective effect of autophagic induction during early reperfusion and autophagic inhibition during late reperfusion. The secondary aim was to characterise autophagic flux patterns following different ischemic and reperfusion durations. Methods: Isolated hearts from male Wistar rats were perfused in working mode. The control groups consisted of two ischemic (15 and 20 min) and 5 reperfusion (10, 30, 60, 90 and 120 min) intervals. In the interventional experiments 20min of global ischemia was followed by 30 (early) or 120 min (late) of reperfusion, during which 3 MethylFadenine (3MA) and Rapamycin were used to inhibit and induce autophagy respectively. All experiments were repeated with Chloroquine, injection one hour prior to experimentation, to distinguish between steady state and autophagic flux. Western blotting was used to measure autophagic protein levels (LC3, Beclin, p62, DRP1, ULK1 and Rab9). Myocardial protection was measured assessing functional recovery and infarct size. Results: The administration of Chloroquine, 3MA and Rapamycin, in the interventional groups, was without effect on global myocardial function before initiation of ischemia. Western blotting: The control experiments demonstrated an increase in autophagic steady state and flux during reperfusion, being more pronounced following longer ischemic and reperfusion durations. Early reperfusion administered 3MA caused a reduction in conventional and alternative autophagy during early reperfusion, as well as an increase in apoptotic activity. Rapamycin failed to induce autophagy during early reperfusion, but a high dose of Rapamycin resulted in an increase in autophagic flux during late reperfusion. High dose Rapamycin, during early and late reperfusion, additionally resulted in the inhibition of the alternative autophagic pathway. Infarct size: The early reperfusion 3MA group demonstrated a significant decrease in infarct size when compared to all the other groups. Conclusions: Our experimental model can be successfully used to study autophagy, and functional autophagy can be demonstrated up to (at least) 120 min reperfusion. Early reperfusion administered 3MA has cardioprotective properties, this may be attributed to the combination of the inhibition of the conventional and alternative autophagic pathways, apoptotic induction and direct drug effects. Late reperfusion induced autophagy was without effect on cardioprotection. We were unable to convincingly induce autophagy during early reperfusion and inhibit autophagy during late reperfusion, this is mainly attributed to experimental model reperfusion duration restrictions and unexpected difficulties experienced with pharmacological manipulation of autophagy in the model used.