Anesthesia and analgesia
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Anesthesia and analgesia · Oct 1998
Protamine reversal of heparin affects platelet aggregation and activated clotting time after cardiopulmonary bypass.
Bleeding after cardiopulmonary bypass (CPB) is related to multiple factors. Excess protamine weakens clot structure and decreases platelet function; therefore, an increased activated clotting time (ACT) after protamine reversal of heparin may be misinterpreted as residual heparin anticoagulation. We evaluated the effects of protamine, recombinant platelet factor 4 (rPF4), and hexadimethrine on ACT in blood obtained after CPB. In addition, we examined the effect of protamine on in vitro platelet aggregation. Incremental doses of protamine, rPF4, and hexadimethrine were added to heparinized blood from CPB, and ACTs were performed. Incremental concentrations of protamine were added to heparinized platelet-rich plasma, and aggregometry was induced by adenosine diphosphate (ADP) and collagen. The mean heparin concentration at the end of CPB was 3.3 U/mL. Protamine to heparin ratios >1.3:1 produced a significant prolongation of the ACT that was not seen with rPF4 and was observed only with 5:1 hexadimethrine to heparin ratios. ADP-induced platelet aggregation was reduced with protamine administration > or =1.3:1. Excessive protamine reversal of heparin prolongs ACT and alters ADP-induced platelet aggregation in a dose-dependent manner in vitro. Additional protamine administered to treat a prolonged ACT may further increase clotting time, reduce platelet aggregation, and potentially contribute to excess bleeding after CPB. ⋯ We found that excess protamine prolonged the activated clotting time and altered platelet function after cardiopulmonary bypass, whereas heparin antagonists, such as recombinant platelet factor 4 and hexadimethrine, exhibited a wider therapeutic range without adversely affecting the activated clotting time. Approaches to avoid excess protamine or use of alternative heparin antagonists after cardiopulmonary bypass may be beneficial.
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Anesthesia and analgesia · Oct 1998
Reliability of the transient hyperemic response test in detecting changes in cerebral autoregulation induced by the graded variations in end-tidal carbon dioxide.
The transient hyperemic response (THR) in the middle cerebral artery (MCA) after the release of brief compression of the ipsilateral common carotid artery has been used to study cerebral autoregulation. We conducted the present study to evaluate the reliability of THR to detect changes in cerebral autoregulation induced by graded variations in PETCO2. Seven healthy adult volunteers were recruited. Fifteen THR tests were performed on every volunteer: three at baseline PETCO2, three each at PETCO2 of 7.5 mm Hg and 15 mm Hg above the baseline, and then three each at PETCO2 of 7.5 mm Hg and 15 mm Hg below the baseline. Transient hyperemic response ratio (THRR) and strength of autoregulation (SA) were calculated using established formulae. Both THRR and SA were highly sensitive (96%) in detecting the changes in cerebral autoregulation induced by graded changes in PETCO2. The within-individual variability of SA was significantly smaller than that of THRR at all levels of PETCO2. ⋯ This study demonstrates the reliability of the THR test, when used for repetitive measurements, in detecting changes in cerebral autoregulation induced by graded changes in PETCO2. This test may provide a simple and noninvasive method of evaluating changes in cerebral autoregulation within an individual.
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Anesthesia and analgesia · Oct 1998
Fundamental properties of local anesthetics: half-maximal blocking concentrations for tonic block of Na+ and K+ channels in peripheral nerve.
Local anesthetics suppress excitability by interfering with ion channel function. Ensheathment of peripheral nerve fibers, however, impedes diffusion of drugs to the ion channels and may influence the evaluation of local anesthetic potencies. Investigating ion channels in excised membrane patches avoids these diffusion barriers. We investigated the effect of local anesthetics with voltage-dependent Na+ and K+ channels in enzymatically dissociated sciatic nerve fibers of Xenopus laevis using the patch clamp method. The outside-out configuration was chosen to apply drugs to the external face of the membrane. Local anesthetics reversibly blocked the transient Na+ inward current, as well as the steady-state K+ outward current. Half-maximal tonic inhibiting concentrations (IC50), as obtained from concentration-effect curves for Na+ current block were: tetracaine 0.7 microM, etidocaine 18 microM, bupivacaine 27 microM, procaine 60 microM, mepivacaine 149 microM, and lidocaine 204 microM. The values for voltage-dependent K+ current block were: bupivacaine 92 microM, etidocaine 176 microM, tetracaine 946 microM, lidocaine 1118 microM, mepivacaine 2305 microM, and procaine 6302 microM. Correlation of potencies with octanol:buffer partition coefficients (logP0) revealed that ester-bound local anesthetics were more potent in blocking Na+ channels than amide drugs. Within these groups, lipophilicity governed local anesthetic potency. We conclude that local anesthetic action on peripheral nerve ion channels is mediated via lipophilic drug-channel interactions. ⋯ Half-maximal blocking concentrations of commonly used local anesthetics for Na+ and K+ channel block were determined on small membrane patches of peripheral nerve fibers. Because drugs can directly diffuse to the ion channel in this model, these data result from direct interactions of the drugs with ion channels.
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Anesthesia and analgesia · Oct 1998
Infraclavicular brachial plexus block: parasagittal anatomy important to the coracoid technique.
Infraclavicular brachial plexus block is a technique well suited to prolonged continuous catheter use. We used a coracoid approach to this block to create an easily understood technique. We reviewed the magnetic resonance images of the brachial plexus from 20 male and 20 female patients. Using scout films, the parasagittal section 2 cm medial to the coracoid process was identified. Along this oblique section, we located a point approximately 2 cm caudad to the coracoid process on the skin of the anterior chest wall. From this point, we determined simulated needle direction to contact the neurovascular bundle and measured depth. At the skin entry site, the direct posterior insertion of a needle will make contact with the cords of the brachial plexus where they surround the second part of the axillary artery in all images. The mean (range) distance (depth along the needle shaft) from the skin to the anterior wall of the axillary artery was 4.24 +/- 1.49 cm (2.25-7.75 cm) in men and 4.01 +/- 1.29 cm (2.25-6.5 cm) in women. Hopefully, this study will facilitate the use of this block. ⋯ We sought a consistent, palpable landmark for facilitation of the infraclavicular brachial plexus block. We used magnetic resonance images of the brachial plexus to determine the depth and needle orientation needed to contact the brachial plexus. Hopefully, this study will facilitate the use of this block.