Ct1 Basic Monitoring


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Introductory notes for CT1 anaesthetic trainees

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Ct1 Basic Monitoring

  1. 1. Basic Physiological Monitoring CT1 Education Series
  2. 2. Monitoring Standards - AAGBI <ul><li>The anaesthetist must be present and care for the patient throughout the conduct of an anaesthetic. </li></ul><ul><li>Monitoring devices must be attached before induction of anaesthesia and their use continued until the patient has recovered from the effects of anaesthesia. </li></ul><ul><li>The same standards of monitoring apply when the anaesthetist is responsible for a local /regional anaesthetic or sedative technique for an operative procedure. </li></ul><ul><li>Monitoring devices “essential to safe conduct of anaesthesia” </li></ul>Recommendations for standards of monitoring during anaesthesia and recovery (4th Ed.) AAGBI 2007
  3. 3. Monitoring: Induction & Maintenance <ul><li>Pulse oximeter </li></ul><ul><li>Non invasive blood pressure monitor </li></ul><ul><li>Electrocardiograph </li></ul><ul><li>Airway gases: oxygen, carbon dioxide and vapour </li></ul><ul><li>Airway pressure </li></ul><ul><li>The following must also be available </li></ul><ul><li>A nerve stimulator whenever a muscle relaxant is used </li></ul><ul><li>A means of measuring the patient’s temperature </li></ul>
  4. 4. Monitoring: Recovery <ul><li>Clinical observations must be supplemented by the following: </li></ul><ul><li>Pulse oximeter </li></ul><ul><li>Non-invasive blood pressure monitor </li></ul><ul><li>The following must also be immediately available </li></ul><ul><li>Electrocardiograph </li></ul><ul><li>Nerve stimulator </li></ul><ul><li>Means of measuring temperature </li></ul><ul><li>Capnograph </li></ul>
  5. 5. ASA Monitoring Standards <ul><li>“ Qualified anesthesia personnel shall be present in the room throughout the conduct of general anesthetics, regional anesthetics, and monitored anesthetic care.” </li></ul><ul><li>“ During all anesthetics, the patient’s oxygenation, ventilation, circulation and temperature shall be continually evaluated.” </li></ul>Oxygenation: FiO2 analyser Pulse oximetry Ventilation: Capnography Disconnect alarm Circulation: ECG Blood pressure Pulse oximetry Temperature: Temp. probe
  6. 6. Pulse Oximetry - Terminology <ul><li>SaO 2 </li></ul><ul><ul><ul><li>true fractional oxygen saturation </li></ul></ul></ul><ul><ul><ul><li>= HbO 2 /(HbO 2 + Hb +COHb + MetHb + SfHb + COSfHb) </li></ul></ul></ul><ul><ul><ul><li>i.e. requires co-oximeter capable of detecting abnormal haemoglobins </li></ul></ul></ul><ul><ul><ul><li>Generally obtained from co-oximeter in ABG machine </li></ul></ul></ul><ul><li>SpO 2 </li></ul><ul><ul><ul><li>functional oxygen saturation measurement </li></ul></ul></ul><ul><ul><ul><li>= HbO 2 /(Hb + HbO 2 ) </li></ul></ul></ul><ul><ul><ul><li>over-estimates saturation in presence of abnormal Hb </li></ul></ul></ul><ul><ul><ul><li>most 2-wavelength pulse oximeters </li></ul></ul></ul><ul><ul><li>Terms often (mis)used interchangeably </li></ul></ul>
  7. 7. Pulse Oximetry Basics <ul><li>Probe emits 2 wavelengths of light : </li></ul><ul><ul><ul><li>660nm (red) for Hb & 940 nm (infra-red) for HbO 2 </li></ul></ul></ul><ul><li>Sensor detects light absorbed at each frequency </li></ul><ul><ul><li>Systolic absorption at 940nm increases due to HbO 2 inflow </li></ul></ul><ul><ul><li>Detects pulsatile flow (photoplethysmography) </li></ul></ul><ul><ul><li>Cancels out absorption during non-pulsatile flow i.e. calculates increase in absorbance over baseline </li></ul></ul><ul><li>Algorithm relates absorbance change (ratio) to saturation </li></ul>Pulsatile:static absorbance ratio of 1:1 = SpO2 85%
  8. 8. Factors Altering Accuracy <ul><li>Methaemoglobin </li></ul><ul><ul><ul><li>at low concentrations SpO 2 overestimates SaO 2 </li></ul></ul></ul><ul><ul><ul><li>at higher concentrations SpO 2 falls as MetHb absorbs light at 660nm and 940nm equally </li></ul></ul></ul><ul><ul><ul><li>As MetHb approaches 35% the absorbance ratio = 1:1 so SpO 2 reads 85% (Huge MetHb absorbance at 660nm) </li></ul></ul></ul><ul><li>Carboxyhaemoglobin </li></ul><ul><ul><ul><li>absorbs light at 660nm (like Hb) but NOT 940nm </li></ul></ul></ul><ul><ul><ul><li>SpO2 tends towards 100% as concentration rises </li></ul></ul></ul><ul><ul><ul><li>False high SpO 2 </li></ul></ul></ul><ul><li>Dyes produce falsely low SpO 2 </li></ul><ul><ul><ul><li>Methylene blue> indocyanine green> indigo carmine </li></ul></ul></ul><ul><ul><ul><li>NOT bilirubin </li></ul></ul></ul><ul><li>No effect with HbF, HbS, SfHb, flourescein </li></ul>
  9. 9. ECG <ul><li>Conventional 3-lead system </li></ul><ul><ul><ul><li>leads I, II, III </li></ul></ul></ul><ul><ul><ul><li>Lead II parallels atrial & ventricular depolarisation waves and best for P wave detection + good QRS </li></ul></ul></ul><ul><li>Modified 3-lead system </li></ul><ul><ul><ul><li>Move left arm leads to V5 position </li></ul></ul></ul><ul><ul><ul><li>Allows monitoring for anterior ischaemia in Lead I </li></ul></ul></ul><ul><li>5-lead system </li></ul><ul><ul><ul><li>Four limb leads + V5 </li></ul></ul></ul><ul><ul><ul><li>V5 is 75% sensitive for ischaemic events </li></ul></ul></ul><ul><ul><ul><li>V5 + II is 80% sensitive </li></ul></ul></ul><ul><ul><ul><li>V5 + V4 is 90% sensitive </li></ul></ul></ul><ul><ul><ul><li>V5 + V4 + II is 94% sensitive </li></ul></ul></ul>
  10. 10. NIBP Measurement <ul><li>Korotkoff method </li></ul><ul><ul><ul><li>SBP = point at which sounds appear (phase 1) </li></ul></ul></ul><ul><ul><ul><li>Sound changes quality as cuff deflated (phases 2&3) </li></ul></ul></ul><ul><ul><ul><li>Sudden muffling at phase 4, disappears at phase 5 </li></ul></ul></ul><ul><ul><ul><li>DBP = phase 5 (AHA) or phase 4 </li></ul></ul></ul><ul><li>Oscillometric method </li></ul><ul><ul><ul><li>cuff inflates until oscillations cease = SBP </li></ul></ul></ul><ul><ul><ul><li>Cuff deflated until maximal oscillation = MAP </li></ul></ul></ul><ul><ul><ul><li>DBP cannot be measured (but can be calculated) </li></ul></ul></ul><ul><li>Cuff width should be 40% of arm circumference </li></ul><ul><li>Cuff too small = falsely high BP </li></ul><ul><li>Cuff too large = falsely low BP </li></ul>
  11. 11. Invasive BP Measurement <ul><li>When/why? </li></ul><ul><ul><ul><li>rapid changes in BP anticipated or pharmacological BP manipulation </li></ul></ul></ul><ul><ul><ul><li>Repeated blood sampling </li></ul></ul></ul><ul><ul><ul><li>NIBP not possible (access/anatomy) </li></ul></ul></ul><ul><ul><ul><li>Additional monitoring e.g. PiCCO </li></ul></ul></ul><ul><li>Zeroed to atmosphere with transducer at right atrium </li></ul><ul><li>May be zeroed at brain level depending on patient position </li></ul>
  12. 12. Invasive BP - access & waveforms <ul><li>Radial artery superficial and easily palpable </li></ul><ul><li>Can use brachial, femoral, dorsalis pedis </li></ul><ul><ul><li>Brachial artery is close to median nerve </li></ul></ul><ul><ul><li>SBP may be high with dorsalis pedis </li></ul></ul><ul><ul><li>Ulnar artery is main source of hand perfusion </li></ul></ul><ul><li>Transducer has low compliance diaphragm </li></ul><ul><li>For every 15 cm in height change in transducer position there is 10mmHg change in pressure reading </li></ul>
  13. 13. Capnography <ul><li>Graphical display of instantaneous [CO 2 ] against time </li></ul><ul><li>Often measured by infrared spectrography (CO 2 absorption band) </li></ul><ul><li>Exhaled gas sampled at 50-500 ml/minute </li></ul><ul><li>Time delay due to sampling rate and tubing length </li></ul><ul><li>System measures exhaled CO 2 </li></ul><ul><li>Resulting waveform has 4 phases & gives useful information </li></ul>Phase I - exhalation of deadspace gas Phase II - transition from deadspace to alveolar gas Phase III - exhalation of alveolar gas Phase IV - Beginning of inspiration
  14. 14. Capnography <ul><li>Two-types of capnograph </li></ul><ul><ul><li>Sidestream: measurement unit located in main machine with exhaled gas sampled through tube </li></ul></ul><ul><ul><li>Mainstream: cuvette with CO 2 sensor placed between ET tube and breathing circuit. Sensors are heated to minimise water vapour. They can be bulky. </li></ul></ul>1 2
  15. 15. Capnography Traces In oesophageal intubation any CO 2 in stomach is gone in 3 breaths or so and trace disappears
  16. 16. Temperature Monitoring <ul><li>Core temperature falls about 1-1.5 o C in first hour of anaesthesia unless measures taken to prevent this </li></ul><ul><li>Hypothermia has (adverse) effects on outcome </li></ul><ul><li>Monitoring sites: </li></ul><ul><ul><li>PA (gold standard core temp) </li></ul></ul><ul><ul><li>tympanic membrane </li></ul></ul><ul><ul><li>bladder (esp. when good UO) </li></ul></ul><ul><ul><li>oesophagus </li></ul></ul><ul><ul><li>nasopharyngeal </li></ul></ul><ul><ul><li>oropharyngeal </li></ul></ul><ul><ul><li>axilla </li></ul></ul><ul><ul><li>Skin temperature </li></ul></ul>Good correlation Poor correlation