Martin Leuwer

Assessing fluid responsiveness by stroke volume variation in mechanically ventilated patients with severe sepsis

Background and objective: Our hypothesis was that stroke volume variation during mechanical ventilation of the lungs would allow accurate prediction and monitoring of changes in cardiac index in response to fluid loading in patients with severe sepsis. Methods: This was a prospective clinical study in a university hospital. Ten mechanically ventilated patients with severe sepsis or septic shock were given fluid loading with 500 mL 10% hydroxyethylstarch 200/0.5 over 30 min. Before and after fluid loading pulmonary arterial occlusion pressure and central venous pressure were measured. Intrathoracic blood volume index, stroke volume variation and cardiac index were measured by the transpulmonary thermodilution technique. After verifying normal distribution of the data (skewness <1.0) the paired t-test was used for statistical analysis. Results: After fluid loading stroke volume variation decreased significantly, whereas central venous pressure, pulmonary arterial occlusion pressure, intrathoracic blood volume index and cardiac index increased significantly. Changes of cardiac index in response to fluid loading were correlated to baseline values of stroke volume variation (r = 0.64, P = 0.02) and intrathoracic blood volume index (r = −0.73, P = 0.009). Changes in cardiac index were significantly correlated to percentage changes in stroke volume variation (r = −0.65, P < 0.001) and changes in intrathoracic blood volume index (r = 0.52, P = 0.002), whereas changes in cardiac index revealed no significant correlation to changes in central venous pressure (r = 0.28, P = 0.07) and changes in pulmonary arterial occlusion pressure (r = 0.29, P = 0.06). Conclusions: Measuring stroke volume variation may be a useful way of guiding fluid therapy in ventilated patients with severe sepsis because it allows estimation of preload and prediction of cardiac index changes in response to fluid loading.

Voltage-dependent block of neuronal and skeletal muscle sodium channels by thymol and menthol.

BACKGROUND AND OBJECTIVE Thymol is a naturally occurring phenol derivative used in anaesthetic practice as a stabilizer and preservative of halothane, usually at a concentration of 0.01%. Although analgesic effects have long been described for thymol and its structural homologue menthol, a molecular basis for these effects is still lacking. We studied the blocking effects of thymol and menthol on voltage-activated sodium currents in vitro as possible molecular target sites. METHODS Whole cell sodium inward currents via heterologously (HEK293 cells) expressed rat neuronal (rat type IIA) and human skeletal muscle (hSkM1) sodium channels were recorded in the absence and presence of definite concentrations of either thymol or menthol. RESULTS When depolarizing pulses to 0 mV were started from a holding potential of -70 mV, half-maximum blocking concentrations (IC50) for the skeletal muscle and the neuronal sodium channel were 104 and 149 mumol for thymol and 376 and 571 mumol for menthol. The blocking potency of both compounds increased at depolarized holding potentials with the fraction of inactivated channels. The estimated dissociation constant Kd for thymol and menthol from the inactivated state was 22 and 106 mumol for the neuronal and 23 and 97 mumol for the skeletal muscle sodium channel, respectively. CONCLUSIONS The results suggest that antinociceptive and local anaesthetic effects of thymol and menthol might be mediated via blockade of voltage-operated sodium channels with the phenol derivative thymol being as potent as the local anaesthetic lidocaine.

Structural requirements of phenol derivatives for direct activation of chloride currents via GABAA receptors

Propofol directly activates gamma-aminobutyric acid (GABA(A)) receptors in the absence of the natural agonist. This mechanism is supposed to contribute to its sedative-hypnotic actions. We studied the effects of seven structurally related phenol derivatives on chloride inward currents via rat alpha1beta2gamma2 GABA(A) receptors, heterologously expressed in HEK 293 cells in order to find structural determinants for this direct agonistic action. Only compounds with the phenolic hydroxyl attached directly to the benzene ring and with aliphatic substituents in ortho position to the phenolic hydroxyl activated chloride currents in the absence of GABA. Concentrations required for half-maximum effect were 980 microM for 2-methylphenol, 230 microM for 2,6-dimethylphenol, 200 microM for thymol, and 23 microM for propofol. Drug-induced chloride currents showed no desensitisation during the 2-s application. These results show that the position of the aliphatic substituents with respect to the phenolic hydroxyl group is the crucial structural feature for direct GABA(A) activation by phenol derivatives.

Propofol Blocks Human Skeletal Muscle Sodium Channels in a Voltage-Dependent Manner

Propofol decreases muscle tone in the absence of neuromuscular blocking drugs. This effect probably cannot be attributed solely to central nervous depression. We studied the effects of propofol on heterologously expressed skeletal muscle sodium channels. Our hypothesis was that the decrease in muscle tone may partly be attributed to an interaction of propofol with sarcolemmal sodium channels. Cells were voltage clamped and whole-cell sodium inward currents were recorded in the absence and presence of propofol. When depolarizing pulses to 0 mV were started from a holding potential close to the normal resting potential of muscle (−70 mV), or when a 2.5-s prepulse inducing slow inactivation was applied before the test pulse at −100 mV, a significant reduction in the peak current amplitude was achieved by 10 and 5 &mgr;M propofol, respectively (P < 0.001). Half-maximum blocking concentrations with these protocols were 23 and 22 &mgr;M. Blocking potency increased at depolarized membrane potentials with the fraction of inactivated channels; the estimated dissociation constant Kd from the inactivated state was 4.6 &mgr;M. These results suggest that propofol significantly blocks sarcolemmal sodium channels at clinically relevant concentrations while maintaining potentials close to the physiological resting potential.

Tramadol, fentanyl and sufentanil but not morphine block voltage-operated sodium channels

&NA; Lidocaine‐like sodium channel blocking drugs provide pain relief either by interrupting impulse conduction in neurons when applied locally in high concentrations or, when given systemically, by suppressing high‐frequency ectopic discharges due to preferential drug binding to inactivated channel states. Lidocaine‐like actions of opioids have frequently been demonstrated clinically. However, drug binding to resting and inactivated channel conformations has been studied systematically only in the case of meperidine. The aim of this in vitro study was to investigate the effects of four currently used opioids on heterologously expressed neuronal (NaV1.2) voltage‐gated sodium channels. Block of sodium currents was studied at hyperpolarized holding potentials and at depolarized potentials inducing either fast‐ or slow‐inactivation. Sufentanil, fentanyl and tramadol but not morphine reversibly suppressed sodium inward currents at high concentrations (half‐maximum blocking concentrations (IC50) 49 ± 4, 141 ± 6 and 103 ± 8 &mgr;M) when depolarizations were started from hyperpolarized holding potentials. Short depolarizations inducing fast‐inactivation and long prepulses inducing slow‐inactivation significantly (*p ≤ 0.001) increased the blocking potency for these opioids. 15% slow inactivated channels reduced the respective IC50 values to 5 ± 3, 12 ± 2 and 21 ± 2 &mgr;M. These results show that: (1) Sufentanil, fentanyl and tramadol block voltage‐gated sodium channels with half‐maximum inhibitory concentrations similar to the IC50 reported for meperidine. (2) Slow inactivation – a physiological mechanism to suppress ectopic activity in response to slow shifts in membrane potential – increases binding affinity for sufentanil, fentanyl and tramadol. (3) Morphine has no such effects.

The Nonpsychotropic Cannabinoid Cannabidiol Modulates and Directly Activates Alpha-1 and Alpha-1-Beta Glycine Receptor Function

Loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord plays a key role in the development of chronic pain following inflammation or nerve injury. Inhibitory postsynaptic transmission in the adult spinal cord involves mainly glycine. Cannabidiol is a nonpsychotropic plant constituent of Cannabis sativa. As we hypothesized that non-CB receptor mechanisms of cannabidiol might contribute to its anti-inflammatory and neuroprotective effects, we investigated the interaction of cannabidiol with strychnine-sensitive α1 and α1β glycine receptors by using the whole-cell patch clamp technique. Cannabidiol showed a positive allosteric modulating effect in a low micromolar concentration range (EC50 values: α1 = 12.3 ± 3.8 μmol/l and α1β = 18.1 ± 6.2 μmol/l). Direct activation of glycine receptors was observed at higher concentrations above 100 μmol/l (EC50 values: α1 = 132.4 ± 12.3 μmol/l and α1β = 144.3 ± 22.7 μmol/l). These in vitro results suggest that strychnine-sensitive glycine receptors may be a target for cannabidiol mediating some of its anti-inflammatory and neuroprotective properties.

Resuscitation from septic shock with capillary leakage: hydroxyethyl starch (130 kd), but not Ringer's solution maintains plasma volume and systemic oxygenation.

There is evidence suggesting that early fluid resuscitation is beneficial in the treatment of septic shock. The question as to which solution should be used remains controversial. Using a porcine septic shock model, we tested the effects of a new synthetic colloid hydroxyethyl starch (HES 130 kD) and a crystalloid regimen with Ringer’s solution (RS) on plasma volume (PV) maintenance as well as on systemic and regional hemodynamics. Fourteen anaesthetized mechanically ventilated pigs received 0.75 g kg−1 body weight of feces into the abdominal cavity to induce sepsis. They were randomly allocated to receive 6% HES 130 kD (n = 5) or RS (n = 5) and were compared with nonseptic controls receiving 6% HES 130 kD (n = 4). The infusion rate was titrated to maintain a central venous pressure of 12 mmHg. PV was determined by chromium-51-tagged erythrocytes and hematocrit. Albumin escape rate (AER) was calculated using iodine-125-labeled albumin. Arterio-intramucosal pCO2 gap, systemic hemodynamics, and oxygenation were obtained before and 6 h after induction of sepsis. AER increased in the HES (+38%) and RS groups (+38%) compared with control. PV was reduced in the RS group (−39%), but was maintained in the HES group (−1%). After 6 h of sepsis, HES 130 kD-treated animals had a significantly higher cardiac output (166 ± 28 mL min−1 kg−1 vs. 90 ± 18 mL min−1 kg−1, P < 0.05), and a significantly higher mixed-venous oxygen saturation (65% ± 8% vs. 40% ± 14%, P < 0.05) than RS animals. In this porcine septic shock model with concomitant capillary leakage syndrome, resuscitation with HES 130 kD but not RS could maintain PV and preserve systemic hemodynamics and oxygenation.

Evaluation of noninvasive determinants for capillary leakage syndrome in septic shock patients

AbstractObjective: Capillary leakage syndrome (CLS) is a frequent complication in sepsis, characterized by loss of intravasal fluids leading to generalized edema and hemodynamic instability despite massive fluid therapy. In spite of its importance no standardized diagnostic criteria are available for CLS. Design: Prospective clinical study. Setting: 1800-bed university hospital Patients: Six septic shock patients with CLS were compared to six control patients. Measurements and results: CLS was clinically determined by generalized edema, positive fluid balance, and weight gain. Plasma volume was measured by indocyanine green, red blood cell volume by chromium-51 labeled erythrocytes, and colloid osmotic pressure before and 90 min after the administration of 300 ml 20% albumin. Extracellular water (ECW) was measured using the inulin distribution volume and bioelectrical impedance analysis. Red blood cells averaged 20.2±1.0 ml/kg body weight in CLS patients and 23.3±4.1 in controls. ECW was higher in CLS patients than in controls (40.0±6.9 vs. 21.7±3.7 l; p<0.05). ECW of inulin was correlated with that measured by bioelectrical impedance analysis (r=0.74, p<0.01). The increase in colloid osmotic pressure over the 90 min was less in CLS patients than in controls (1.1±0.3 vs. 2.8±1.3 mmHg; p<0.05). Conclusion: These results suggest that measurements of an increased ECW using bioelectrical impedance analysis combined with a different response of colloid osmotic pressure to administration of albumin can discriminate noninvasively between patients with and those without CLS.

Plasma endotoxin in horses presented to an equine referral hospital: Correlation to selected clinical parameters and outcomes

REASONS FOR PERFORMING STUDY Endotoxaemia is frequently presumed on the basis of clinical signs in horses with colic. OBJECTIVE Measurements of plasma endotoxin (LPS) are rarely made in clinical cases and there is little information on the correlations between this variable, clinical variables and outcomes. OBJECTIVES To measure LPS levels in plasma of horses presented to the Philip Leverhulme Equine Hospital on admission and daily for up to 4 days and to relate LPS levels to selected clinical parameters, such as heart rate and packed cell volume, and outcomes. METHODS Blood samples were collected and stored at -20°C prior to assay of the plasma using a validated kinetic chromogenic Limulus amoebocyte lysate (LAL) assay. Clinical parameters and outcome variables were collected from hospital records. Associations were determined by Chi-squared test and logistic regression analysis. RESULTS Daily blood samples were collected from 234 horses. LPS was detected in 26.5% of the study population and in 29% of those horses presented for colic. Horses providing samples with detectable LPS were more likely to die whilst in the hospital than those that did not (P = 0.045). Horses presenting with colic were more likely to have detectable LPS in their plasma than noncolic cases (P = 0.037), although LPS was detected in the plasma of 8 out of 42 noncolic horses. A horse that did not meet the study definition of clinical endotoxaemia was 10 times less likely to provide a positive LPS sample (OR 0.10, 95% CI: 0.05-0.22). CONCLUSIONS The proportion of horses providing samples with detectable LPS was similar to other studies. POTENTIAL RELEVANCE LPS was detected in the minority of horses presented with colic. Increased levels of LPS positively correlated with packed cell volume and with risk of mortality in colic cases.

High‐affinity blockade of voltage‐operated skeletal muscle and neuronal sodium channels by halogenated propofol analogues

Voltage‐operated sodium channels constitute major target sites for local anaesthetic‐like action. The clinical use of local anaesthetics is still limited by severe side effects, in particular, arrhythmias and convulsions. These side effects render the search for new local anaesthetics a matter of high interest.