HOW EVOKED POTENTIALS BEHAVE , WHEN BRAIN SUFFERS FROM ISCHEMIA ❓

🕶Cerebral ischemia slows neurotransmission and neuronal energy metabolism, resulting in decreased amplitude and increased latency of specific peaks. 

🕶For SSEPs, a 50% reduction in amplitude and/or a 10% increase in latency [changes in the central conduction times, namely, the interpeak latencies between the N14 and N20 peaks] of SSEP signals from the baseline are generally accepted to be a significant change 

🕶A 50% reduction on SEP amplitude has been shown to occur when cerebral blood flow decreases below 14 mL/100 g/min 

🕶MEP have less well-defined warning criteria as compared to SSEPs; however, increased stimulus thresholds and/or decreased MEP amplitudes in relation to dramatic events (i.e., clip application) are indicative of pending neurologic insult. 

🕶For BAEP, an increase in latency of more than 1 msec, particularly in wave V, is considered to be clinically significant. 

🕶Unlike EEG monitoring the evoked potential tests can detect subcortical functional status by way of perforating branches such as the anterior choroidal and medial striate arteries 

Reference: Anesthesiology Research and Practice Volume 2014, Article ID 595837, Controversies in the Anesthetic Management of Intraoperative Rupture of Intracranial Aneurysm, Tumul Chowdhury, Andrea Petropolis,Marshall Wilkinson, Bernhard Schaller  Nora Sandu and Ronald B. Cappellani

ADENOSINE INDUCED TRANSIENT ASYSTOLE FOR INTRACRANIAL ANEURYSM CLIPPING SURGERY

▶️Adenosine alters electrical conduction at the atrioventricular (AV) node and has a negative chronotropic effect on the sinoatrial node. 

▶️Adenosine acts on cardiac A1 receptors to reduce cAMP activity, which decreases inward calcium conductance and diminishes pacemaker current, resulting in bradycardia, AV nodal blockade, and sinus pauses. 

▶️It has a very short half-life time (less than 10 seconds) and is rapidly taken up by the vascular endothelium and erythrocytes. 

▶️The effect on heart rate is seen within 10 to 20 seconds after administration

▶️ There is a relative hypotension period, 1 minute after asystole

▶️Multiple doses are usually required for very large and complex aneurysms to obtain repeated episodes of asystole

▶️ Bebawy et al. recommended an initial dose of between 0.3 and 0.4 mg/kg ideal body weight (IBW) 

▶️ Powers et al. gave a standard 6 mg dose initially escalated to 6 mg more than the previous dose (e.g. 6 mg, 12 mg, 18 mg, 24 mg ; repeating the dose as often as every 1 to 2 minutes) until 30 to 40 seconds of asystole was reached.

▶️ Guinn NR et al reported the median dose of intravenous adenosine resulting in bradycardia greater than 30 seconds was 30 mg. The median dose of adenosine resulting in hypotension greater than 30 seconds was 15 mg, and greater than 60 seconds was 30 mg

▶️Luostarien et al. reported that the median dose for a single bolus was 12 (6–18) mg, whereas the median total dose for multiple boluses was 27 (18–89) mg

▶️ Hypotension ( most often transient , sometimes requiring vasopressors), atrial fibrillation etc were reported as side effects 

▶️ Adenosine-induced flow arrest briefly reduces cerebral perfusion pressure and reduces the turgor of the aneurysm, thereby facilitating the clip ligation 

▶️ The adenosine dose will achieve approximately 45 seconds of controlled systemic hypotension and a bloodless surgical field.

▶️ Periods of flow arrest have to be carefully coordinated with the surgeon such that necessary working time is available for aneurysm dissection and clip placement

Reference: Adenosine-Induced Transient Asystole

Gavin W. Britz, Methodist Debakey Cardiovasc J. 2014 

Oct-Dec; 10(4): 220–223.

Adenosine-induced transient asystole for intracranial aneurysm surgery: a retrospective review. Guinn NR, et al. J Neurosurg Anesthesiol. 2011.

DRAW-OVER VAPORISERS IN A NUTSHELL FOR EXAMS

🏺They are placed inside the breathing system and rely on a negative pressure downstream from the vaporiser to create the flow required to entrain the agent. This negative pressure is generated either by the patient’s own inspiration or by a self-inflating bag

🏺So they must have a very low resistance to flow to avoid additional resistance to the patient’s breathing. 

🏺Goldman vaporiser, the Oxford miniature vaporiser (OMV) and the Epstein MacIntosh vaporiser (EMV) etc are draw-over vaporisers. The triservice apparatus, used by the military, incorporates two OMVs

🏺 They are simpler, lightweight, smaller and less expensive. 

🏺 As it is not possible to calibrate for the large range of tidal volumes created by the patient/ self-inflating bag, they are inaccurate 

🏺So they are not generally used in hospitals, and are reserved for ‘in-the-field’ use, where portability is required. 

Reference: Al-Shaikh B, Stacey S. Essentials of Anaesthetic Equipment, 2nd edn. Edinburgh: Churchill Livingstone, 2002 . Davis PD, Kenny GNC. Basic Physics and Measurement in Anaesthesia, 5th edn. Oxford: Butterworth–Heinemann, 2003 .

Physics For Anesthesiologist ( #PFA ) : #IMPEDANCE

🖊Impedance is a term that is commonly used in the world of #electrophysiology and #BiomechanicalEngineering. 

🖊The chance of getting an electric shock is high when you have wet hands because the impedance of the skin is lower than when it is dry. 

🖊Thoracic impedance increases during inspiration.

🖊When applying electric current to the chest during #defibrillation, less energy may reach the heart during the inspiratory phase than during the expiratory phase because of this phenomenon, thereby decreasing the possible success of defibrillation. 

🖊So better to attempt defibrillation during the expiratory phase of mechanical ventilation.

🖊Where the #resistance of a circuit is dependent on the frequency of the current through it, the term impedance is used. 

 🖊The unit of impedance is therefore the same as that of resistance (the ohm), but the symbol Z is used to differentiate it from the symbol used for resistance (Ω).

🖊In case of a capacitor, as the frequency of the current increases, the current passes through the circuit more easily, i.e. the resistance of the capacitor falls with increasing current frequency. 

🖊In contrast, the resistance of an inductor rises as the frequency of the current increases.

#PhysicsForAnesthesiologist , #anesthesiologist , #anesthesia , #biomedical

Davis PD, Kenny GNC. Basic Physics and Measurement in Anaesthesia, 5th edn. Oxford: Butterworth–Heinemann, 2003; pp. 149–64 . Ewy GA, Hellman DA, McClung S, Taren D. Influence of ventilation phase on transthoracic impedance and defibrillation effectiveness. Crit Care Med 1980; 8: 164–6