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Biomedical subjects

Michael G Mythen

Publications and source records attributed to Michael G Mythen.

9 recordsLinked to original sources
PubMed2006

Pulse oximetry.

PURPOSE OF REVIEW: Pulse oximetry is now a ubiquitous and essential tool of modern medicine, and while it is a relatively recent invention, the technology has rapidly matured since the first commercially available oximeters were introduced in the 1970s. This review seeks to provide an overview of the basic physical operation of the probe and discuss its limitations, sources of error and some current advances in the use of multi-wavelength probes. RECENT FINDINGS: New multi-wavelength oximeters and plethysmographic waveform analysis may expand the information that we can collect and use non-invasively. This includes distinguishing between haemoglobinopathies, monitoring volume status and volume loss, and potentially monitoring cardiac output non-invasively. SUMMARY: The pulse oximeter, like any basic tool, must be used properly. There is considerable misunderstanding and lack of education among junior clinicians as to the use and interpretation of pulse oximeters. The introduction of the pulse oximeter has demonstrated a cost saving, although the cost-benefit of new multi-wavelength probes remains unproven.

Cost-Benefit Analysis↗
PubMed2006

A head-to-head comparison of the in vitro coagulation effects of saline-based and balanced electrolyte crystalloid and colloid intravenous fluids.

Both fluid composition (e.g., type of hydroxyethyl starch) and formulation (e.g., saline or balanced salt carrier solution) may alter whole blood coagulation. We therefore enrolled 10 healthy volunteers to test ex vivo, thrombelastograph-based blood coagulation differences of eight crystalloid and colloid solutions at 20%, 40%, and 60% dilutions. Saline and lactated Ringer's solution produced a hypercoagulable state at 20%-40% dilutions. Saline, hetastarch in saline, pentastarch in saline, tetrastarch in saline, and human albumin solutions all produced a hypocoagulable state at 60% dilution. Hetastarch in saline also produced a hypocoagulable state at 40% dilution. The larger molecular weight starches produced more intense coagulation abnormalities than the medium molecular weight compounds formulated similarly (i.e., suspended in saline or balanced salt solution). The balanced salt solutions caused fewer coagulation abnormalities, especially pentastarch in balanced salt solution. This balanced salt pentastarch preparation produced the least derangement of coagulation of the colloid solutions at all dilutions, causing hypercoagulability at the lower dilutions and minimal coagulation derangement at 60% dilution. These data support the theory that smaller molecular weight hydroxyethyl starches and colloids suspended in balanced salt solutions preserve coagulation better than large molecular weight starches and saline-based colloids, as judged by thrombelastography.

Analysis of Variance↗
PubMed2005

Physiology, pharmacology, and rationale for colloid administration for the maintenance of effective hemodynamic stability in critically ill patients.

The semisynthetic colloid solutions (gelatins, dextrans, and hydroxyethyl starches) are complex drugs. Their principal role in the care of the critically ill is as plasma volume expanders, but they may also affect hemorrheology, hemostasis, and inflammatory processes. The pattern of beneficial and detrimental effects varies between products. Understanding of the physiology of plasma volume expansion, as well as the nature and magnitude of these additional pharmacological qualities, is necessary for rational prescription of these commonly used products. The composition of the solute carrier solution can influence the clinical effects of colloid solutions. A large amount of data from laboratory and small clinical studies is available to inform this choice of colloid in a variety of situations. Significant patient outcome data from large studies has until recently been lacking, and clinicians have continued to prescribe a variety of crystalloids and colloids for the maintenance of effective hemodynamic stability in critically ill patients. The recently published Saline vs Albumin Fluid Evaluation Study demonstrates that albumin has an equivalent effectiveness and safety profile to 0.9% saline as a resuscitation fluid. The choice of clinical endpoints to guide dosage (infused volume) of colloids is probably therefore more important than the choice between individual products.

Anti-Inflammatory Agents↗
PubMed2005

Low-dose endotoxin elicits variability in the inflammatory response in healthy volunteers.

Endotoxin has been implicated as a cause of sepsis, inflammation and organ dysfunction after surgery. Patients differ in their response to endotoxin, and this may account for differences in outcome. The traditional human model of endotoxin challenge (2-4 ng/kg) is not associated with significant inter-individual variability in systemic inflammation and may not be suitable for studying variability in the inflammatory response. We examined whether low-dose regimens of endotoxin cause significant variability in inflammation. Volunteers (n = 30) were randomised in a double-blinded ('double-dummy') study to one of 6 dosing regimens: saline (placebo) or Escherichia coli O:113 endotoxin as a 4 ng/kg bolus (positive control), 0.25 ng/kg (bolus), 0.25 ng/kg (30 min infusion), 0.75 ng/kg (bolus) or 0.75 ng/kg (30 min infusion). Temperature, white cell count, platelet count, C-reactive protein and cytokine changes from baseline were measured. In contrast to subjects receiving placebo, those randomised to 4 ng/kg endotoxin exhibited significant systemic inflammation during the 10-h observation period. The four low-dose regimens elicited variability in most markers of inflammation. We conclude that low-dose endotoxin elicits inter-individual variability in inflammation and could be used to test factors that may affect the human response to endotoxin.

Adolescent↗
PubMed2005

Postoperative gastrointestinal tract dysfunction.

Postoperative gastrointestinal (GI) tract dysfunction (PGID) is common and is associated with increased patient suffering and cost of care. The pathogenesis of PGID is complex and multifactorial. Traditional measures intended to reduce the incidence of PGID, such as the use of prokinetic drugs, nasogastric tube drainage, and the avoidance of early fluid and/or food intake, are apparently not beneficial. The administration of larger volumes of IV fluids to achieve predetermined increases in cardiac output has been shown in randomized trials to improve gut perfusion and reduce the incidence of PGID. A multimodal approach that includes limited surgical incision, regional local anesthesia, early mobilization, and enteral feeding has been associated with a dramatic reduction in postoperative complications, PGID, and length of hospital stay. However, none of these approaches has been validated in adequately powered multicenter prospective randomized controlled trials.

Anesthesia↗
PubMed2005

Perioperative fluid management and clinical outcomes in adults.

The administration of IV fluid to avoid dehydration, maintain an effective circulating volume, and prevent inadequate tissue perfusion should be considered, along with the maintenance of sleep, pain relief, and muscular relaxation, a core element of the perioperative practice of anesthesia. Knowledge of the effects of different fluids has increased in recent years, and the choice of fluid type in a variety of clinical situations can now be rationally guided by an understanding of the physicochemical and biological properties of the various crystalloid and colloid solutions available. However, there are few useful clinical outcome data to guide this decision. Deciding how much fluid to give has historically been more controversial than choosing which fluid to use. A number of clinical studies support the notion that an approach based on administering fluids to achieve maximal left ventricular stroke volume (while avoiding excess fluid administration and consequent impairment of left ventricular performance) may improve outcomes. In this article, we review the available fluid types and strategies of fluid administration and discuss their relationship to clinical outcomes in adults.

Adult↗
PubMed2004

Plasma volume changes associated with two hydroxyethyl starch colloids following acute hypovolaemia in volunteers.

This randomised double blind prospective study compared the effective intravascular volume expansion and maintenance, with two types of starches following induced haemorrhagic hypovolaemia. Twenty healthy male volunteers aged between 18 and 65 year were bled 10% of their total blood volume in fully monitored conditions and under the supervision of a trained specialist doctor and research nurse. The lost blood volume was replaced using one of the starch solutions. Effective intravascular volume expansion was monitored hourly using the (51)Cr radio-labelled red blood cell dilution technique, we compared the effects of two hydroxyethyl starch colloid preparations, one a high molecular weight and the other a low molecular weight preparation, on the plasma volume changes over time. The large molecular weight starch (Hextend) provided a less well-sustained volume expansion effect than the smaller one (Voluven)

Acute Disease↗
PubMed2002

The association of complication type with mortality and prolonged stay after cardiac surgery with cardiopulmonary bypass.

UNLABELLED: Outcome after cardiac surgery varies depending on complication type. We therefore sought to determine the association between complication type, mortality, and length of stay in a large series of patients undergoing cardiac surgery with cardiopulmonary bypass (CPB). Multivariate logistic regression was used to test for differences between complication types in mortality and prolonged length of stay (>10 days) while controlling for preoperative and intraoperative risk factors. In 2609 consecutive cardiac surgical patients requiring CPB, the mortality rate was 3.6%; 36.5% had one or more complications, and 15.7% experienced an adverse outcome (death or prolonged length of stay). Multivariate logistic regression demonstrated that complication type was significantly associated with adverse outcome (P < 0.001) independent of Parsonnet score and CPB time (c-index = 0.80). The development of noncardiac complications only (Group NC) and cardiac complications with other organ involvement (Group B) significantly increased mortality and hospital and intensive care unit length of stay (P < 0.001) when compared with cardiac complications only (Group C). The incidences of adverse outcome in Groups C, NC, and B were 15%, 43%, and 67%, respectively; the mortality rates were 3%, 7%, and 20%, respectively. All these intergroup comparisons were significantly different (adjusted P < 0.05). Complications involving organs other than the heart appear to be more deleterious than cardiac complications alone, underscoring the need for strategies to reduce noncardiac complications. IMPLICATIONS: Complications, particularly when they involve organs other than just the heart, increase mortality and prolong the length of hospital stay after heart surgery, independent of a patient's preoperative risk factors and the duration of cardiopulmonary bypass. Strategies aimed at preventing damage to other organs during cardiac surgery need to be improved.

Adult↗