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Biomedical subjects

A R Webb

Publications and source records attributed to A R Webb.

At least 19 recordsLinked to original sources
PubMed2001

Recognizing hypovolaemia.

Clinical shock is a common problem of the critically ill patient. Assessment of the circulating volume is important to assure adequate oxygen supply to the tissues, and hypovolaemia must be treated promptly to avoid organ dysfunction. The author reviews the diagnostic approach, focusing on clinical and "surrogate" markers of volume status. Clinical signs are often late indicators and the monitoring of hemodynamic variables through cardiac catheterism can be misleading. The fluid challenge approach provides a useful diagnostic test of hypovolaemia, and a method for titrating the dose of fluid tailored to the individual's requirement.

Blood Pressure Determination↗
PubMed2000

The appropriate role of colloids in managing fluid imbalance: a critical review of recent meta-analytic findings.

Three meta-analyses have recently been reported on the relationship between choice of resuscitation fluid and risk of mortality in critically ill patients. The relative risk of death (1.16-1.19) in two of the meta-analyses was slightly higher in colloid than crystalloid recipients; however, this observation was not statistically significant. In the third meta-analysis, 6% (95% confidence interval [CI], 3-9%) pooled excess mortality was documented in patients receiving albumin for hypovolaemia, burns or hypoalbuminaemia. The mortality difference in hypovolaemia patients (4%; 95% CI, 0-8%) was not statistically significant. A variety of serious limitations apply to the three meta-analyses, suggesting that their findings be interpreted cautiously. More than one-half of the randomized controlled trials (RCTs) included in the meta-analyses were reported prior to 1990 and hence do not reflect current practice. Each meta-analysis included only a subset of relevant RCTs, and therefore the scope of inferences to be drawn from the meta-analytic results is limited. The meta-analyses combined RCTs that were notably heterogeneous with respect to patient characteristics, type of illness, administered fluids and physiologic endpoints. Differences in illness severity, concomitant therapies and fluid management approaches were not taken into account. Very few of the RCTs were blinded. The meta-analyses do not support the conclusion that choice of resuscitation fluid is a major determinant of mortality in critically ill patients, nor do they support changes to current fluid management practice. Changes such as exclusive reliance on crystalloids would necessitate a reassessment of the goals and methods of fluid therapy. Since the effect on mortality may be minimal or non-existent, choice of resuscitation fluid should rest on whether the particular fluid permits the intensive care unit to provide better patient care.

Colloids↗
PubMed2000

Report from the meeting: Gastrointestinal Tonometry: State of the Art. 22nd-23rd May 1998, London, UK.

Gastrointestinal (GI) tonometry, the only clinically available method for the accurate diagnosis of compromised GI blood flow, has been shown to be a sensitive predictor of increased morbidity, mortality and prolonged hospitalization. The recent introduction of the Tonocap, as a means of performing automated air tonometry, has simplified the application of GI tonometry in the clinical setting. Despite this the utility of GI tonometry remains controversial. The GI Tonometry: State of the Art meeting brought together a group of clinicians with a proven track record of research, clinical interest and expertise in this field. The aim of the meeting was to come to a consensus regarding certain issues such as the past and future roles of GI tonometry and standards for its correct usage and interpretation. Finally suggestions as to further research and clinical evaluation were made within a broader discussion regarding the complexities of applying the principles of evidence-based medicine to the introduction of a new piece of medical technology.

Carbon Dioxide↗
PubMed1999

Spectral UV irradiance on vertical surfaces: a case study.

The UV spectral irradiance on horizontal and vertically oriented surfaces was measured throughout a cloudless day (18 July 1995) at Izana station, Tenerife, using a Bentham DTM300 spectroradiometer scanning from 290 to 500 nm in steps of 5 nm. Results show that irradiance measured on a horizontal surface is not proportional to irradiance on a vertical surface. The relation between the two depends upon orientation of the vertical surface, zenith angle and wavelength. At short UVB wavelengths surfaces directed toward the solar azimuth received their maximum irradiances much closer to solar noon than the maxima for longer wavelengths. Some vertical surfaces also received significantly more irradiance than the horizontal surface at long wavelengths during all but the central hours of the day, while at short wavelengths all vertical irradiances were less than the horizontal except for the measurements at the extreme ends of the day. Erythemally effective radiation followed the diurnal pattern of irradiations for short UVB wavelengths.

Humans↗
PubMed1997

Relationship between preoperative endotoxin immune status, gut perfusion, and outcome from cardiac valve replacement surgery.

STUDY OBJECTIVE: Endotoxin is a powerful trigger of systemic inflammation. Since cardiac surgery exposes patients to endotoxemia, this study was set up to define the relationship between preoperative endogenous endotoxin immune status, gut perfusion, and outcome following cardiac valve replacement surgery. DESIGN: Observational study. SETTING: University hospital. PATIENTS: Fifty-nine consecutive patients undergoing cardiac valve replacement. MEASUREMENTS AND MAIN RESULTS: Blood was assayed for IgG and IgM endotoxin core antibody (EndoCAb) levels preoperatively, immediately postoperatively, and at 4 h and 24 h postoperatively. Intraoperative gut mucosal perfusion was assessed using gastric tonometry. Complications were assessed for groups above and below the median EndoCAb value of a healthy population (100 median units micro/mL). Of the 59 patients, 12 developed at least one of a set of predefined complications. Of these 12, all had preoperative levels of IgM EndoCAb below 100 MU/mL (p<0.025). Eleven had IgG EndoCAb levels below 100 MU/mL (0.05<p<0.1). There was no relationship between the fall in gastric intramucosal pH and exposure to endotoxin as implied by the fall in unbound IgM EndoCAb levels, although the specificity of tonometry for predicting complications could be improved by considering the patient's preoperative EndoCAb status. CONCLUSIONS: Preoperative EndoCAb levels were related to poor outcome following cardiac surgery and may be used to improve the specificity of GI tonometry in predicting postoperative complications.

Adolescent↗
PubMed1997

Comparison of commonly used clinical indicators of hypovolaemia with gastrointestinal tonometry.

OBJECTIVE: The gastrointestinal tonometer, which allows measurement of gastrointestinal mucosal CO2 and subsequent derivation of gut intramucosal pH (pHi), has been demonstrated to be a sensitive predictor of outcome following major surgery. Current theory suggests that the origin of the low pH may be hypovolaemia. This study was designed to compare the temporal sequence of changes in tonometric readings with invasive blood pressure, stroke volume, heart rate, lactate and arterial blood gas measurements during progressive haemorrhage. DESIGN: Observational healthy volunteer study. SETTING: Intensive care unit at University College London Hospitals. SUBJECTS: Six healthy, medically qualified volunteers. INTERVENTIONS: After obtaining baseline measurements, the subjects were progressively bled 25% (range = 21-31%) of their blood volume over a period of 1 h in two approximately equal aliquots. Equilibration was allowed for 30 min following the bleed, after which further measurements were made and the blood was then retransfused over 30 min. MEASUREMENTS AND MAIN RESULTS: There was no consistent change in any of the haemodynamic variables other than gastric intramucosal CO2:arterial CO2 gap (PiCO2-PaCO2) after removal of the first aliquot of blood, although five of the six subjects also demonstrated a fall in pHi. After removal of the second aliquot of blood, PiCO2-PaCO2 gap and pHi continued to indicate a worsening gastric intramucosal acidosis; stroke volume, as measured by suprasternal Doppler, demonstrated a marked fall, while all other variables measured had not altered consistently or to such a degree as to elicit a clinical response or cause suspicion of a hypovolaemic state. On retransfusion, all variables returned towards baseline. CONCLUSIONS: This study demonstrates the value of tonometry as an early monitor of hypovolaemia and highlights the shortcomings of other more commonly measured clinical variables.

Adult↗
PubMed1996

Passive immunisation with IgG endotoxin core antibody hyperimmune fresh frozen plasma.

Postoperative complications have been associated with low pre-operative endotoxin core antibody (EndoCAb) status. Seventy-three patients scheduled to undergo elective cardiac surgery were screened for IgG EndoCAb levels 1 week prior to admission. Of these 73 patients, 39 had levels below the median of a healthy population (100 MU = median units/ml). Only 6 received an infusion of IgG EndoCAb hyperimmune fresh frozen plasma (FFP) due to surgical rescheduling or unavailability of compatible hyperimmune FFP. The 6 patients demonstrated significant elevation (p < 0.0001) of their IgG EndoCAb levels from a mean level of 43 MU/ml (range: 26-75) to a posttransfusion mean of 267 MU/ml (range: 227-300), which persisted until at least immediately prior to the operative period. We conclude that passive immunisation with EndoCAb hyperimmune FFP is possible prior to elective surgery and that the kinetic profile of such an administration will allow the FFP to be given the night prior to surgery and still maintain significantly elevated levels for surgery planned the following morning.

Adult↗
PubMed1996

Haemostatic changes in the pulmonary blood during cardiopulmonary bypass.

Pulmonary injury may result from the use of cardiopulmonary bypass (CPB). We investigated changes in the haemostatic system in the pulmonary vein during CPB compared with blood that circulated through the bypass circuit. Paired samples were taken from the pulmonary vein and central venous pressure (CVP) line during the peri-operative period from ten patients. Plasma levels of factor VII (P < 0.001), prekallikrein (P < 0.05), antithrombin III (P < 0.001) and heparin cofactor II (P < 0.005) were decreased in the pulmonary vein after 20 min of bypass compared with pre-operative levels. In the pulmonary vein there was a significant increase in neutrophil expressed CD11b (P < 0.001), neutrophil elastase: alpha 1-antitrypsin complexes (P < 0.001), endothelin-1(P < 0.001) and thrombin-antithrombin complexes (P < 0.001) by the end of bypass compared with pre-operative levels. There was no significant change in monocyte expressed CD11b, factor XII or C1-esterase inhibitor in the pulmonary vein for the study period. None of these variables were significantly different in the pulmonary vein compared with CVP line. In the pulmonary vein plasma levels of activated factor VII decreased following heparin administration (P < 0.001) in the majority of patients which was coincidental to an increase (P < 0.001) in tissue factor pathway inhibitor (TFPI). This increase in TFPI was significantly higher in the pulmonary vein compared with CVP line (P < 0.05) There was a decrease in neutrophil count by 20 min on CPB in both the pulmonary vein and CVP line (P < 0.001) and this did not return to pre-operative levels in the pulmonary vein. Soluble thrombomodulin levels decreased by 20 min on CPB in the CVP line (P < 0.05) but tended to increase in the pulmonary vein, although this was not statistically significant. In conclusion we found evidence of thrombin generation and possible endothelial damage together with increased neutrophil activation and adhesion molecule expression in the pulmonary vein during CPB which may play an important role in the development of post-CPB pulmonary injury.

Adult↗
PubMed1995

Perioperative plasma volume expansion reduces the incidence of gut mucosal hypoperfusion during cardiac surgery.

OBJECTIVE: To test the hypothesis that perioperative plasma volume expansion would preserve gut mucosal perfusion during elective cardiac surgery. DESIGN: Prospective randomized open study. SETTING: Teaching hospital. PATIENTS: Sixty American Society of Anesthesiology grade III patients with a preoperative left ventricular ejection fraction of 50% or greater undergoing elective cardiac surgery. INTERVENTIONS: Patients were allocated randomly to a control or protocol group. The control group was treated according to standard practices. After induction of general anesthesia, the protocol group received, in addition, 200-mL boluses of a 6% hydroxyethyl starch solution to obtain a maximum stroke volume. This procedure was repeated every 15 minutes until the end of surgery, except when the patient underwent cardiopulmonary bypass. MEASUREMENTS AND RESULTS: Cardiac stroke volume was estimated by an esophageal Doppler system, and gastric mucosal perfusion was measured by tonometric assessment of gastric intramucosal pH in all patients. Patients were followed up postoperatively until discharge from the hospital or death. The incidence of gut mucosal hypoperfusion (gastric intramucosal pH < 7.32) at the end of surgery was reduced in the protocol group (7% vs 56%) (P < .001), as were the number of patients in whom major complications developed (0 vs 6) (P = .01), mean number of days spent in the hospital (6.4 [range, 5 to 9] vs 10.1 [range, 5 to 48]) (P = .011), and mean number of days spent in the intensive care unit (1 [range, 1 to 1] vs 1.7 [range 1 to 11] days) (P = .023). CONCLUSIONS: Perioperative plasma volume expansion with colloid during cardiac surgery, guided by esophageal Doppler measurement of cardiac stroke volume, reduced the incidence of gut mucosal hypoperfusion. This group of patients also had an improved outcome when compared with controls.

Algorithms↗
PubMed1995

Maintaining blood flow in the extracorporeal circuit: haemostasis and anticoagulation.

OBJECTIVES: To review the methods and developments in maintaining extracorporeal circuits in critically ill patients. DESIGN: The review includes details of the pathophysiological processes of haemostasis and coagulation in critically ill patients, methods of maintaining blood flow in the extracorporeal circuit and methods of monitoring anticoagulation agents used. SETTING: Information is relevant to the management of critically ill patients requiring extracorporeal renal and respiratory support and cardiopulmonary bypass. CONCLUSIONS: Heparin is the mainstay of anticoagulation for the extracorporeal circuit although the complex abnormalities of the coagulation system in critically ill patients are associated with a considerable risk of bleeding. Alternative therapeutic agents and physical strategies (prostacyclin, low molecular weight heparin, sodium citrate, regional anticoagulation, heparin bonding and attention to circuit design) may reduce the risk of bleeding but expense and difficulty in monitoring are disadvantages.

Anticoagulants↗
PubMed1994

An evaluation of the microbial retention performance of three ventilator-circuit filters.

OBJECTIVES: to determine and compare the microbial retention efficiency of 3 types of heat and moisture exchange (HME) filters. DESIGN: randomised prospective study. SETTING: Intensive Care Unit at the Middlesex Hospital and the microbiology laboratory of the Department of Academic Microbiology, University College London Medical School. MEASUREMENTS AND RESULTS: An aerosol challenge to each filter using a suspension containing. S. marcescens and the bacteriophage MS2. In total 45 filters of 3 types were tested, 15 previously unused and 30 that had been used in ventilator circuits of 30 patients. The 3 types of filter generally had microbial retention efficiencies of > 99.99% for both S. marcescens and the bacteriophage MS2. CONCLUSION: The 3 types of HME filter were effective microbial filters and comparable in their microbial retention.

Aerosols↗
PubMed1994

Intra-operative gut mucosal hypoperfusion is associated with increased post-operative complications and cost.

OBJECTIVES: To determine CO and gastric mucosal perfusion in patients during elective major surgery; to seek a relationship with subsequent outcome. DESIGN: Prospective descriptive study. SETTING: University hospital. PATIENTS: 51 patients undergoing elective major surgery of an anticipated duration of greater than 2 h who were at risk of developing gut mucosal hypoperfusion and postoperative organ failure. MEASUREMENTS AND RESULTS: CO was determined by oesophageal Doppler measurement of aortic blood flow. Gastric mucosal perfusion was determined by tonometric assessment of gastric mucosal pH (pHi). Blood pressure and urine flow were measured. At the end of surgery no patients were oliguric or hypotensive. Post-operatively morbidity, mortality, duration and cost of stay in the ITU and hospital were assessed. There were 32 patients with evidence of gastric mucosal ischaemia at the end of surgery (pHi < 7.32) despite maintenance of CO. This group of patients spent a mean of 4.7 (range 0-33) days in the ITU, 14 developed major complications (7 with multiple organ failure [MOF] and 6 died. In 19 patients gut mucosal perfusion was maintained during surgery (pHi > or = 7.32); these patients demonstrated an increase in CO of 48.4% (95% confidence interval 21.3 -75.6) and spent a mean of 1.0 (range 0-4) days in the ITU. Only one developed a major complication and none died. The total cost of post-operative care for the 51 patients was estimated at pounds 356650. Mean cost per patient in the low pHi group was significantly greater at pounds 8845 (range pounds 600--pounds 42,700) compared to pounds 3874 (range pounds 2,600--pounds 9,600) in the normal pHi group. The total.cost of post-operative care for the 7 patients who developed MOF was pounds 171,450 i.e. 48% of the total cost. CONCLUSION: A low gastric pHi measured during the intraoperative period in a group of patients undergoing major (mainly cardiovascular) surgery is associated with increased post-operative complications and cost.

Adult↗