A comparison of transcutaneous Doppler corrected flow time, b-type natriuretic peptide and central venous pressure as predictors of fluid responsiveness in septic shock: a preliminary evaluation

Aortic corrected flow time (FTc) is easily measured by Doppler techniques. Recent data using transoesophageal Doppler suggest that it may predict fluid responsiveness in critical care. This use of FTc has not previously been evaluated in septic shock, nor have any studies incorporated transcutaneously measured FTc. Furthermore, no comparison has been made between FTc, plasma B-type natriuretic peptide concentration (BNP) or central venous pressure. The aim of this preliminary study was to compare FTc, BNP and central venous pressure as predictors of fluid responsiveness in septic shock patients without cardiac dysrhythmia. This was a prospective study of 10 consecutive adult septic shock patients (in sinus rhythm; 60% mechanically ventilated) treated with intravenous fluid challenge (4% albumin 250 ml over 15 minutes) in a mixed medical/ surgical tertiary intensive care unit. Mean + SD Acute Physiological and Chronic Health Evaluation II score was 21.8 +/- 12.7. Haemodynamic assessment incorporating transcutaneous aortic Doppler (USCOM) occurred before and five minutes after fluid challenge. Concurrent with initial assessment, blood samples were collected for BNP assay (ADIVA Centaur). Four patients demonstrated an increase in stroke volume > or = 15% (responders). Percent change in stroke volume strongly correlated with baseline FTc (r = -0.81, P = 0.004) but not BNP (r = -0.3, P = 0.4) or central venous pressure (r = -0.4, P = 0.2). Baseline FTc < 350 ms discriminated responders from non-responders (P = 0.047). Our data support FTc as a better predictor of fluid responsiveness than either BNP or central venous pressure in septic shock. Transcutaneous aortic Doppler FTc offers promise as a simple, completely non-invasive predictor of fluid responsiveness and should be evaluated further

Evidence-based evolution of the high stakes postgraduate intensive care examination in Australia and New Zealand

The fellowship examination for intensive care medicine in Australia and New Zealand, first held in 1979, has undergone four major periods of development and change since inception. These periods are characterised as: 1. 1979 to 1996--initiation and establishment of the exam as a relevant and comprehensive assessment process for a new specialty. 2. 1997 to 2001--revision to increase breadth of coverage, increase reliability for a growing number of candidates and ensure that each candidate received the same exam: Expansion: to incorporate assessment of CanMEDS skills (including communication, procedures and professional qualities). Lengthening: to increase the number of exposures, to ensure reliability. Quarantining of candidates: to allow the provision of a similar exam for each candidate. 3. 2002 to 2006--increasing emphasis on examiner training, standard setting and increasing feedback to candidates to improve the educational experience and guide exam preparation. Blueprinting of questions to maintain validity. 4. 2008 onwards--logistic revision to ensure feasibility for a rapidly growing number of candidates and refinement to apply modem standard setting and quality control. The exam has been regarded as a 'tough but fair' assessment in its 30 years of existence and the committee overseeing its development has aimed to continually review the process to maintain those qualities as well as reliability, validity and feasibility. The increasing number of candidates has allowed accumulation of usable statistics but has tested the feasibility of running such a labour intensive exam. To date, there have been 800 presentations to the exam with 498 successful candidates.

Equivalent Metabolic Acidosis with Four Colloids and Saline on Ex Vivo Haemodilution

Colloid infusions can cause metabolic acidosis. Mechanisms and relative severity with different colloids are incompletely understood. We compared haemodilution acid-base effects of 4% albumin, 3.5% polygeline, 4% succinylated gelatin (all weak acid colloids, strong ion difference 12 mEq/l, 17.6 mEq/l and 34 mEq/l respectively), 6% hetastarch (non-weak acid colloid, strong ion difference zero) and 0.9% saline (crystalloid, strong ion difference zero). Gelatin weak acid properties were tracked via the strong ion gap.

Four-step ex vivo dilutions of pre-oxygenated human venous blood were performed to a final [Hb] near 50% baseline. With each fluid, base excess fell to approximately −13 mEq/l. Base excess/[Hb] relationships across dilution were linear and direct (R2 ≥0.96), slopes and intercepts closely resembling saline. Baseline strong ion gap was −0.3 (2.1) mEq/l. Post-dilution increases occurred in three groups: small with saline, hetastarch and albumin (to 3.5 (02) mEq/l, 4.3 (0.3) mEq/l, 3.3 (1.4) mEq/l respectively), intermediate with polygeline (to 12.2 (0.9) mEq/l) and greatest with succinylated gelatin (to 20.8 (1.4) mEq/l).

We conclude that, despite colloid weak acid activity ranging from zero (hydroxyethyl starch) to greater than that of albumin with both gelatin preparations, ex vivo dilution causes a metabolic acidosis of identical severity to saline in each case. This uniformity reflects modifications to the albumin and gelatin saline vehicles, in part aimed at pH correction. By proportionally increasing the strong ion difference, these modifications counter deviations from pure saline effects caused by colloid weak acid activity. Extrapolation in vivo requires further investigation.

Changes in serum procalcitonin and C-reactive protein following antimicrobial therapy as a guide to antibiotic duration in the critically ill: a prospective evaluation

Serial procalcitonin is reported to be useful to titrate duration of antibiotic therapy in the non critically ill patient with pneumonia. The aim of this study was to examine the relationship between antibiotic therapy and serial serum procalcitonin concentrations in a cohort of critically ill septic patients and examine for any differences between culture positive (CP) and culture negative (CN) sepsis. Seventy-five critically ill patients with suspected sepsis were enrolled in this prospective observational study. Serial procalcitonin and C-reactive protein assays were measured on days one, three, five, seven, 10 and 14. The mean duration of antibiotic therapy was similar in the two groups (10.4 +/- 5.1 (CP) vs. 8.4 +/- 5.1 (CN) days, P = 0.09). Serum procalcitonin concentrations were significantly higher at baseline in the CP than the CN group (14.9 +/- 22.9 vs. 6.8 +/- 21.5 ng/ml, P = 0.04). During the study period, serum concentrations of procalcitonin and C-reactive protein declined in both groups. Serum procalcitonin consistently remained higher in the CP group (P < 0.05) and did not return to normal values. In the CN group, procalcitonin concentrations fell below 0.5 only on day 10. There was no significant difference in C-reactive protein profile between the two groups. Four patients in the CP group (11%) had relapse of sepsis. The mean procalcitonins in the relapsed subgroup were lower than those in the remission subgroup (P = 0.02). Therapy for proven or presumed infections was associated with declining serum procalcitonin and C-reactive protein in critically ill septic patients. The marked variability and overlap in plasma profile of these markers between CP and CN sepsis makes it difficult to define a nadir plasma concentration at which one can recommend discontinuation of antibiotic therapy.

Acid-base effects of a bicarbonate-balanced priming fluid during cardiopulmonary bypass: comparison with Plasma-Lyte 148. A randomised single-blinded study

Fluid-induced metabolic acidosis can be harmful and can complicate cardiopulmonary bypass. In an attempt to prevent this disturbance, we designed a bicarbonate-based crystalloid circuit prime balanced on physico-chemical principles with a strong ion difference of 24 mEq/l and compared its acid-base effects with those of Plasma-Lyte 148, a multiple electrolyte replacement solution containing acetate plus gluconate totalling 50 mEq/l. Twenty patients with normal acid-base status undergoing elective cardiac surgery were randomised 1:1 to a 2 litre prime of either bicarbonate-balanced fluid or Plasma-Lyte 148. With the trial fluid, metabolic acid-base status was normal following bypass initiation (standard base excess 0.1 (1.3) mEq/l, mean, SD), whereas Plasma-Lyte 148 produced a slight metabolic acidosis (standard base excess -2.2 (2.1) mEq/l). Estimated group difference after baseline adjustment was 3.6 mEq/l (95% confidence interval 2.1 to 5.1 mEq/l, P=0.0001). By late bypass, mean standard base excess in both groups was normal (0.8 (2.2) mEq/l vs. -0.8 (1.3) mEq/l, P=0.5). Strong ion gap values were unaltered with the trial fluid, but with Plasma-Lyte 148 increased significantly on bypass initiation (15.2 (2.5) mEq/l vs. 2.5 (1.5) mEq/l, P < 0.0001), remaining elevated in late bypass (8.4 (3.4) mEq/l vs. 5.8 (2.4) mEq/l, P < 0.05). We conclude that a bicarbonate-based crystalloid with a strong ion difference of 24 mEq/l is balanced for cardiopulmonary bypass in patients with normal acid-base status, whereas Plasma-Lyte 148 triggers a surge of unmeasured anions, persisting throughout bypass. These are likely to be gluconate and/or acetate. Whether surges of exogenous anions during bypass can be harmful requires further study.

A clinician's guide to predicting fluid responsiveness in critical illness: applied physiology and research methodology

Intravenous fluid administration is often used in critical care with the goal of improving haemodynamics and consequently tissue perfusion and oxygen delivery. While inotropic and vasoactive drugs are often necessary to correct haemodynamic instability, resuscitation usually begins with fluid therapy. As fluid challenge can result in clinical deterioration, the ability to predict haemodynamic response is desirable. In this way it might be possible to avoid unnecessary volume replacement in critically ill patients. Cardiac preload is a concept that accounts for the relationship between ventricular filling and stroke volume. It has been challenging to apply this concept to clinical practice. For this reason, the study of fluid responsiveness is of increasing research and clinical interest. The clinical application of predicting fluid responsiveness requires an understanding of relevant physiological principles. Furthermore, an improved understanding of these principles should assist the clinician in appraising published data, which has been characterised by significant methodological differences. This review aims to assist the clinician by detailing the physiological principles that underlie the prediction of fluid responsiveness in the critically ill. In addition, the potential importance of methodological differences in the cutrent literature will be considered.

Stability of the strong ion gap versus the anion gap over extremes of PCO2 and pH

The strong ion gap (SIG) is under evaluation as a scanning tool for unmeasured ions. SIG is calculated by subtracting [buffer base], which is ([A-]+[HCO3-), from the apparent strong ion difference, which is ([Na+]+[K+]+[Ca++]+[Mg++]-[Cl-]-[L-lactate]). A- is the negative charge on albumin and phosphate. We compared the pH stability of the SIG with that of the anion gap (AG). In normal and hypoalbuminaemic hyperlactaemic blood, PCO2 was reduced stepwise in vitro from >200 mmHg to <20 mmHg, with serial blood gas and electrolyte analyses, and [albumin] and [phosphate] measurement on completion. Respective [haemoglobin], [albumin], [phosphate] and [lactate] in normal blood were 156 (0.9) g/l, 44 (2) g/l, 1.14 (0.06) mmol/l and 1.7 (0.8) mEq/l, and in hypoalbuminaemic blood 116 (0.9) g/l, 24 (2) g/l, 0.78 (0.06) mmol/l and 8.5 (0.5) mEq/l. pH increased from < 6.85 to > 7.55, causing significant falls in [Na+] and elevations in [Cl-]. Initial and final SIG values did not differ, showing no correlation with pH. Mean SIG was 0.5 +/- 1.5 mEq/l. AG values were directly correlated with pH (normal: R2 = 0.51, hypoalbuminaemic: R2 = 0.65). Final AG values significantly exceeded initial values (normal blood: 15.9 (1.7) mEq/l versus 8.9 (1.8) mEq/l, P < 0.01; hypoalbuminaemic blood: 16.5 (0.8) mEq/l versus 11.8 (2.0) mEq/l, P < 0.05). We conclude that, unlike the AG, the SIG is not affected by severe respiratory acidosis and alkalosis, enhancing its utility in acid-base disturbances.

Acid-base and bio-energetics during balanced versus unbalanced normovolaemic haemodilution

Fluids balanced to avoid acid-base disturbances may be preferable to saline, which causes metabolic acidosis in high volume. We evaluated acid-base and bio-energetic effects of haemodilution with a crystalloid balanced on physical chemical principles, versus crystalloids causing metabolic acidosis or metabolic alkalosis. Anaesthetised, mechanically ventilated Sprague-Dawley rats (n=32, allocated to four groups) underwent six exchanges of 9 ml crystalloid for 3 ml blood. Exchange was with one of three crystalloids with strong ion difference (SID) values of 0, 24 (balanced) and 40 mEq/l. Controls did not undergo haemodilution. Mean haemoglobin concentration fell to approximately 50 g/l after haemodilution. With SID 24 mEq/l fluid, metabolic acid-base remained unchanged. Dilution with SID 0 mEq/l and 40 mEq/l fluids caused a progressive metabolic acidosis and alkalosis respectively. Standard base excess (SBE) and haemoglobin concentration were directly correlated in the SID 0 mEq/l group (R2 = 0.61), indirectly correlated in the SBE 40 mEq/l group (R2 = 0.48) and showed no correlation in the SID 24 mEq/l group (R2 = 0.003). There were no significant differences between final ileal values of CO2 gap, nucleotides concentration, energy charge, or luminal lactate concentration. SID 40 mEq/l crystalloid dilution caused a significant rise in subcutaneous lactate. In this group mean kidney ATP concentration was significantly less than controls and renal energy charge significantly lower than SID 0 mEq/l and control groups. We conclude that a crystalloid SID of 24 mEq/l provides balanced haemodilution. Bio-energetic perturbations with higher SID haemodilution may be more severe and need further investigation.

B-type natriuretic peptide concentrations and myocardial dysfunction in critical illness

B-type natriuretic peptide (BNP) is the first biomarker of proven value in screening for left ventricular dysfunction. The availability of point-of-care testing has escalated clinical interest and the resultant research is defining a role for BNP in the investigation and treatment of critically ill patients. This review was undertaken with the aim of collecting and assimilating current evidence regarding the use of BNP assay in the evaluation of myocardial dysfunction in critically ill humans. The information is presented in a format based upon organ system and disease category. BNP assay has been studied in a spectrum of clinical conditions ranging from acute dyspnoea to subarachnoid haemorrhage. Its role in diagnosis, assessment of disease severity, risk stratification and prognostic evaluation of cardiac dysfunction appears promising, but requires further elaboration. The heterogeneity of the critically ill population appears to warrant a range of cut-off values. Research addressing progressive changes in BNP concentration is hindered by infrequent assay and appears unlikely to reflect the critically ill patient's rapidly changing haemodynamics. Multi-marker strategies may prove valuable in prognostication and evaluation of therapy in a greater variety of illnesses. Scant data exist regarding the use of BNP assay to alter therapy or outcome. It appears that BNP assay offers complementary information to conventional approaches for the evaluation of cardiac dysfunction. Continued research should augment the validity of BNP assay in the evaluation of myocardial function in patients with life-threatening illness.

Active-site properties of Phrixotrix railroad worm green and red bioluminescence-eliciting luciferases

The luciferases of the railroad worm Phrixotrix (Coleoptera: Phengodidae) are the only beetle luciferases that naturally produce true red bioluminescence. Previously, we cloned the green- (PxGR) and red-emitting (PxRE) luciferases of railroad worms Phrixotrix viviani and P. hirtus[OLE1]. These luciferases were expressed and purified, and their active-site properties were determined. The red-emitting PxRE luciferase displays flash-like kinetics, whereas PxGR luciferase displays slow-type kinetics. The substrate affinities and catalytic efficiency of PxRE luciferase are also higher than those of PxGR luciferase. Fluorescence studies with 8-anilino-1-naphthalene sulfonic acid and 6-p-toluidino-2-naphthalene sulfonic acid showed that the PxRE luciferase luciferin-binding site is more polar than that of PxGR luciferase, and it is sensitive to guanidine. Mutagenesis and modelling studies suggest that several invariant residues in the putative luciferin-binding site of PxRE luciferase cannot interact with excited oxyluciferin. These results suggest that one portion of the luciferin-binding site of the red-emitting luciferase is tighter than that of PxGR luciferase, whereas the other portion could be more open and polar.

Uneven evolutionary rates of bradykinin B1 and B2 receptors in vertebrate lineages

Bradykinin acts through two receptor subtypes in mammals and generates a variety of responses including pain, inflammation and hypotension. The evolutionary history of the bradykinin system has been unclear due to shortage of information outside mammals. We describe here two receptor subtypes and the bradykinin precursor in three species of bony fish (the zebrafish Danio rerio, the Japanese pufferfish Takifugu rubripes, and the green spotted pufferfish Tetraodon nigroviridis) and chicken and analyze the relationships to mammals by a combination of phylogeny, conserved synteny and exon-intron organization. All of these species have two receptor genes located close to each other in a tandem formation, with the B2 gene 5' to the B1 gene, in chromosomal regions displaying conserved synteny between the species (albeit conservation of synteny in zebrafish is still unclear due to poor genome assembly). The evolutionary rate differs between the two genes as well as between lineages leading to differing pharmacological properties for both B1 and B2 across vertebrate classes. Also the bradykinin precursor gene was identified in all of these species in a chromosome region with conserved synteny. The tissue distribution of mRNA in T. rubripes is similar for B1 and B2, suggesting more similar regulation for the two genes than in mammals. In conclusion, the receptor tandem duplication predates the divergence of ray-finned fish and tetrapods and no additional duplicates of the receptors or bradykinin seem to have survived the ray-finned fish tetraploidization.