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Depth of Anaesthesia

Sign & Symptom of light anaesthesia
  1. Movement response (not possible with N-M blocking drugs)
  2. Increase BP, Increase pulse, sweating, laryngospasm
  3. Tachypnea, breath holding, coughing, laryngospasm
  4. Eye movements, eye lash response
  5. Patient reflexes but may be preserved with ketamine
  6. Electrophysiological measures


Desynchronization (20-60 fast rhythm)
  1. Persistent evoked responses (auditory is best guide)
  2. Contractility of lower esophagus
  3. EEG changes during anaesthesia



Inhalational agents(subanaesthetic)

Inhalational agents(1-2 MAC)

Barbiturates(small dose)


Benzodiazepine (small dose)


Etomidate(small dose)


Nitrous oxide




Mild hypercapnia

Marked hypercapnia

Sensory stimulation

Hypothermia (AIIMS Nov 06)



BIS monitor.

A Bispectral Index (BIS)

A Bispectral Index (BIS) monitor is a neurophysiological monitoring device which continually analyses a patient's electroencephalograms during general anaesthesia to assess the level of consciousness during anaesthesia. The "depth of anaesthesia" is commonly used as a surrogate for "the likelihood of forming experiences or memory", known as surgical awareness. The bispectral index is a statistically based, empirically derived complex parameter. It is a weighted sum of several electroencephalographic subparameters, including a time domain, frequency domain, and high order spectral subparameter. The BIS monitor provides a single dimensionless number, which ranges from 0 (equivalent to EEG silence) to 100. A BIS value between 40 and 60 indicates an appropriate level for general anesthesia, as recommended by the manufacturer
  1. Cardiovascular
    1. Pulse rate
    2. Blood pressure non invasive
      For invasive BP radial Artery is most commonly cannulated but Allen’s test is mandatory before radial artery cannulation to see the patency of ulnar artery
      Normal values = Collateral supply to the radial side with in 7 Sec.
    3. ECG – lead II for arrhythmias & lead V5 for ischemia are preferred
    4. Central venous pressure: Normal CVP 6-8 mm Hg
    5. Pulm. A catheterization (Swan Ganz catheter is used)
Uses of Pulmonary A catheterization
  1. Cardiac chambers pressure monitoring. PAP PCWP
    1. RA – 0.8 mm Hg
    2. RV – 15-30 / 0-8 mm Hg
    3. PA – 15-30 / 5-15 mm Hg
    4. PCWP (Pulmonary Capillary Wedge Pressure )
      1. Normal is 4 to 8 mmHg PCWP,
      2. Represents left atrial pressure
      3. Pulmonary edema develops if PCWP is more than 25 mmHg
    5. LA – 4-12 mmHg
    6. LV – 100-140/60-90
  2. For mixed venous oxygen saturation
  3. For cardiac output. Cardiac index
Transesophageal ECHO cardiology:
  1. Most sensitive for wall motion abnormality (ischemia) and air embolism
  2. Today TEE is the most sensitive monitor for cardiovascular monitoring.
  3. A transesophageal echocardiogram, or TEE is an alternative way to perform an echocardiogram. A specialized probe containing an ultrasound transducer at its tip is passed into the patient's esophagus. This allows image and Doppler evaluation which can be recorded.
  4. It has several advantages and some disadvantages compared to a transthoracic echocardiogram (TTE).
  1. The advantage of TEE over TTE is usually clearer images, especially of structures that are difficult to view transthoracicly (through the chest wall).
  2. The explanation for this is that the heart rests directly upon the esophagus leaving only millimeters that the ultrasound beam has to travel.
  3. In adults, several structures can be evaluated and imaged better with the TEE, including the aorta, pulmonary artery, valves of the heart, both atria, atrial septum, left atrial appendage, and coronary arteries. TEE has a very high sensitivity for locating a blood clot inside the left atrium
  1. TEE requires a fasting patient, (the patient must follow the ASA NPO guidelines(i.e. usually not eat or drink anything for eight hours prior to the procedure)
  2. Requires a team of medical personnel
  3. Takes longer to perform
  4. May be uncomfortable for the patient
  5. May require sedation or general anesthesia
  6. has some risks associated with the procedure (esophageal perforation)
Clinical uses
  1. In addition to use by cardiologists in outpatient and inpatient settings, TEE can be performed by a cardiac anesthesiologist to evaluate, diagnose, and treat patients in the peri-operative period.
  2. Most commonly used during open heart procedures, it can be used in the setting of any operation if the patient's status warrants it. TEE can only be performed by a cardiologist or cardiac anesthesiologist certified in TEE by the National Board of Echocardography.

Pulse oximeter

Pulse oximetry is a non-invasive method for monitoring a person's O2 saturation.

Based on BEER- LAMBERTS law
  1. Oxygen saturation (SpO2) measured by pulse oximeter (normal 97-98%)
  2. A typical pulse oximeter utilizes an electronic processor and a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. 
Limitations:- Erroneously low readings may be caused by
  1. Hypoperfusion of the extremity being used for monitoring (often due to a limb being cold, or from vasoconstriction secondary to the use of vasopressor agents);
  2. Incorrect sensor application;
  3. Highly calloused skin; or movement (such as shivering), especially during hypoperfusion .
  4. Pulse oximetry also is not a complete measure of circulatory sufficiency. If there is insufficient blood flow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.
  5. Since pulse oximetry only measures the percentage of bound hemoglobin, a falsely high or falsely low reading will occur when hemoglobin binds to something other than oxygen:
    1. Hemoglobin has a higher affinity to carbon monoxide than oxygen, and a high reading may occur despite the patient actually being hypoxemic. In cases of carbon monoxide poisoning, this inaccuracy may delay the recognition of hypoxia (low blood oxygen level).
    2. Cyanide poisoning gives a high reading, because it reduces oxygen extraction from arterial blood. In this case, the reading is not false, as arterial blood oxygen is indeed high in early cyanide poisoning.
    3. Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s.
Capnography is a valuable monitor of the pulmonary, cardiovascular and anesthetic breathing systems.
Luft developed principle of capnography in 1943
Physics – Infrared method
Types – Sidestream Mainstream

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO2) in the respiratory gases.
  1. The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO2, and an indirect monitor of the CO2 partial pressure in the arterial blood. 
  2. In healthy individuals, the difference between arterial blood and expired gas CO2 partial pressures is very small, and is probably zero in children. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.
  3. Capnography directly reflects the elimination of CO2 by the lungs to the anaesthesia device. Indirectly, it reflects the production of CO2 by tissues and the circulatory transport of CO2 to the lungs.

A normal capnograph demonstrating the three phase

Phase 1- dead space
Phase 2- mixture of dead space and alveolar gas
Phase3- alveolar gas plateau
Alpha angle- 100 to 110
Beta angle – 90

Extra Edge:
  1. End tidal CO2 = 32 - 42 mmHg measured by capnography
  2. PCO2 (Partial pressure of CO2 = 35 - 45 mmHg measure by ABG) normal difference between ETCO2 and PCO2 is 4 mm Hg(normal shunting)
  3. The shape of a capnogram is identical in all humans with healthy lungs. Any deviations in shape must be investigated to determine a physiological or a pathological cause of the abnormality
    1. Capnography – the measurement of carbon dioxide (CO2) in exhaled breath.
    2. Capnometer – the numeric measurement of CO2.
  4. Capnogram – the wave form. Five characteristics of capnogram should be evaluated=
    1. Frequency
    2. Rhythm
    3. Height
    4. Baseline
    5. Shape
  5. End Tidal CO2 (ETCO2 or PetCO2) - the level of (partial pressure of) carbon dioxide released at end of expiration.
  6. Capnography provides an immediate picture of patient condition. Pulse oximetry is delayed
    1. Its main development has been as a monitoring tool for use during anaesthesia and intensive care. .
    2. The plot may also show the inspired CO2, which is of interest when rebreathing systems are being used.
    3. During anaesthesia, there is interplay between two components: the patient and the anaesthesia administration device (The critical connection between the two components is either an endotracheal tube or a mask, and CO2 is typically monitored at this junction.
    4. The analysis is rapid and accurate, but the presence of nitrous oxide in the gas mix changes the infra-red absorption via the phenomenon of collision broadening. This must be corrected for.
  7. Clinical Uses of Capnography
    1. Monitoring Ventilation
      ETCO2 Less Than 35 mmHg = "Hyperventilation/Hypocapnia"
      ETC02 Greater Than 45 mmHg = "Hypoventilation/Hypercapnia"
    2. Confirming, Maintaining, and Assisting Intubation
    3. Measuring Cardiac Output During CPR
    4. ETCO2 in Asthma, COPD
      Bronchospasm will produce a characteristic “shark fin” wave form, as the patient has to struggle to exhale, creating a sloping “B-C” upstroke. The shape is caused by uneven alveolar emptying.
    5. to diagnose hypermetabolic state- malignant hyperthermia( end tidal co2 will rise alarmingly)
    6. Diagnosis of embolism -end tidal co2 will fall alarmingly
    7. FLAT CAPNOGRAM- THIS can be seen in following conditions- accidental extubation, disconnection of anaesthesia tubing, mechanical ventilation failure and complete brochospasm.

Blood gas analysis

  1. Usually sample is taken from radial artery preferably in glass syringe (heparinized)
Neuromuscular monitoring
Neuromuscular monitoring, also known as train of four monitoring, is a technique used during recovery from the application of general anesthesia to objectively determine how well a patient's muscles are able to function. It involves the application of electrical stimulation to nerves and recording of muscle response using, for example, an acceleromyograph
Ulnar nerve stimulation
Patterns of nerve stimulation
  1. Single twitch stimulation.
  2. Train-of-four stimulation. 2 Hz stimuli every 0.5 sec. repeated after 10-12 sec. Most useful for maintenance and differentiate between depolarizing and non-depolarizing block
  3. Tetanic stimulation.
  4. Post-tetanic count stimulation.
Train-of-Four Stimulation (TOF)
This is the most widely used mode of nerve stimulation. Four supramaximal stimuli of 2Hz (four stimuli every 0.5 sec) are applied over 2 second interval which are repeated every 10-12 second. The response is observed as TOF count or TOF ratio.

TOF ratio


 0.4 or less

Patient is unable to lift head or arm
Tidal volume may be normal
Vital capacity and inspiratory force will be reduced


Patient is able to lift head for 3 seconds
Open eyes widely
Stick out tongue
Vital capacity and inspiratory forces are still reduced

 0.7 -0.75

Patient can normally cough sufficiently
Can lift head for at least for 5 seconds
Grip strength may be as low as about 60% of control

 0.8 or more

Vital capacity and inspiratory forces are normal
Patient may still have diplopia or facial weakness


Core temp > rectal temp> surface temp.
Sites for core temp
  1. Pulmonary artery most accurate
  2. Tympanic membrane most accurate for brain temp
  3. Nasopharynx (best for brain temp)
  4. Lower esophagus (most commonly used site and best)
  5. Oral cavity
  6. Axilla
  7. Rectal
  8. Bladder
  9. Skin temperature


Most common thermal abnormality during Anaesthesia is hypothermia
  1. Reasons of hypothermia
    1. Because of vasodilatation by anaesthetics heat transfer from core to skin
    2. Evaporation
    3. Decrease Room temp
    4. Cold fluids
  2. Hypothermia
    1. mild 28 deg to 35 deg celsius
    2. Moderate 21 deg-27 deg Celsius
    3. Profound < 20 deg Celsius
      Oxygen consumption & BMR reduced by 7% with each degree oC fall in temperature
      Brain protection for 10 min at 30 deg C & 60 min at 15 deg C
  3. Rx of intraoperative hypothermia
    1. Warm intravenous fluids
    2. Blankets
    3. Forced air by Bair Hugger – most effective & most commonly used method
  4. Uses of hypothermia / induced hypothermia
    1. Brain protection
    2. Protection against tissue ischemia during cardiac surgery
Evoked potentials
  1. Evoked responses, measure the electrophysiologic responses of the nervous system to a variety of stimuli. In theory, almost any sensory modality can be tested; however, in clinical practice, only a few are used on a routine basis. The EPs most frequently encountered are the following
  2. Visual evoked potentials (VEPs; these include both flash and checkerboard types)
  3. Short-latency somatosensory evoked potentials (SEPs)
  4. Short-latency brainstem auditory evoked potentials (BAEPs)
  5. The brainstem auditory evoked potential (BAEP), or brainstem auditory evoked response (BAER), measures the functioning of the auditory nerve and auditory pathways in the brainstem.
Recent advances-
Entropy monitoring-
  1. It is a relatively new method of assessing anaesthetic depth.
  2. It relies on a method of assessing the degree of irregularity in electroencephalogram (EEG) signals.
  3. The founding principle behind this theory is that the irregularity within an EEG signal decreases with increasing brain levels of anaesthetic drugs.
  4. If we relate the irregularity to the entropy within the signal, then an entropy scale can be assigned.
  5. The signal is captured via a forehead mounted sensor, in a similar way employed by bispectral index (BIS).
  6. Entropy monitors produce two numbers (RE - Response Entropy, SE- State Entropy) that are related to frequency bandpass used.
  7. Response Entropy incorporates higher frequency components that include that of electromyogram activity. The reason for using higher frequency bandpass in response entropy is to allow faster response from the monitor in relation to clinical state.
  8. Entropy scores relate to clinical levels of anaesthetic depth. Most anaesthetic drugs are detectable by entropy monitoring, a notable exception being nitrous oxide.

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