102
Anästhesie (Subject) / Physiologie (Lesson)
FMH
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- Aktionspotential, Herzmuskulatur The action potential of non-pacemaker cardiac cells (atrial, Purkinje and ventricular fibres) differs from that of pacemaker tissue (sinoatrial and atrioventricular nodes). Non-pacemaker cells have a resting membrane potential of -80 mV to -90 mV. During phase 0, there is massive influx of sodium ions via fast response sodium channels. This is terminated at a transmembrane potential of +20 mV by the cessation of sodium influx and by the influx of chloride ions (phase 1). The influx of calcium via slow calcium channels starts during phase 0 and continues during phase 2 (plateau phase) when there is an accompanying efflux of sodium ions. During phase 3, slow repolarisation is achieved by efflux of potassium ions and the inactivation of calcium ion influx, until the normal resting membrane potential is reached. During the absolute refractory period (phases 0 - 2 and part of 3) the cell does not respond to any stimulation. In the relative refractory period the cell will respond to a stimulus of sufficient intensity.
- Hypoxic pumonal vasoconstriction Hypoxic pulmonary vasoconstriction optimises O2 uptake in atelectasis, pneumonia, asthma, and adult respiratory distress syndrome. During single-lung anaesthesia, hypoxic pulmonary vasoconstriction helps maintain systemic oxygenation. The mechanism of hypoxic pulmonary vasoconstriction involves a redox-based O2 sensor within pulmonary artery smooth muscle cells. Pulmonary artery smooth muscle cells mitochondria vary production of reactive O2species in proportion to PaO2. Hypoxic withdrawal of these redox second messengers inhibits voltage-gated potassium channels, depolarising the pulmonary artery smooth muscle cells. Depolarisation activates L-type calcium channels, increasing cytosolic calcium and triggering hypoxic pulmonary vasoconstriction. Some anaesthetic agents attenuate this response with the potential for causing further deterioration in ventilation perfusion mismatch. Agents that inhibit HPV Ether Halothane Desflurane (>1.6 MAC) Agents with minimal or no effect on HPV Thiopentone Fentanyl Desflurane (1MAC) Isoflurane (<1.5MAC) Sevoflurane (1MAC) Propofol.