Early afterdepolarisations, occurring before the cell has fully repolarised, are particularly likely to be seen when action potentials are prolonged, and arise due to reactivation of calcium and sodium channels that would normally switch off until the next heartbeat is due. Under the right conditions, reactivation of these currents, facilitated by the sodium-calcium exchanger, can cause further depolarisation of the cell. The early afterdepolarisations triggering arrhythmias in long QT syndrome tend to arise from the Purkinje fibres of the cardiac conduction system. Early afterdepolarisations may occur as single events, but may occur repeatedly leading to multiple rapid activations of the cell.Some research suggests that delayed afterdepolarisations, occurring after repolarisation has completed, may also play a role in long QT syndrome. This form of afterdepolarisation originates from the spontaneous release of calcium from the intracellular calcium store known as the sarcoplasmic reticulum, forcing calcium out of cell through the sodium calcium exchanger in exchange for sodium, generating a net inward current.While there is strong evidence that the trigger for Torsades de Pointes comes from afterdepolarisations, it is less certain what sustains this arrhythmia. Some lines of evidence suggest that repeated afterdepolarisations from many sources contribute to the continuing arrhythmia. However, some suggest that the arrhythmia sustains through a mechanism known as re-entry. According to this model, the action potential prolongation occurs to a variable extent in different layers of the heart muscle with longer action potentials in some layers than others.
the size and shape of the tongue and palate relative to the size of the mouth. the teeth, especially noting the presence of prominent maxillary incisors, any loose or damaged teeth, or crowns.Many classification systems have been developed in an effort to predict difficulty of tracheal intubation, including the Cormack-Lehane classification system, the Intubation Difficulty Scale (IDS), and the Mallampati score. The Mallampati score is drawn from the observation that the size of the base of the tongue influences the difficulty of intubation. It is determined by looking at the anatomy of the mouth, and in particular the visibility of the base of palatine uvula, faucial pillars and the soft palate. Although such medical scoring systems may aid in the evaluation of patients, no single score or combination of scores can be trusted to specifically detect all and only those patients who are difficult to intubate. Furthermore, one study of experienced anesthesiologists, on the widely used Cormack–Lehane classification system, found they did not score the same patients consistently over time, and that only 25% could correctly define all four grades of the widely used Cormack–Lehane classification system. Under certain emergency circumstances (e.g., severe head trauma or suspected cervical spine injury), it may be impossible to fully utilize these the physical examination and the various classification systems to predict the difficulty of tracheal intubation. A recent Cochrane systematic review examined the sensitivity and specificity of various bedside tests commonly used for predicting difficulty in airway management.