Monitoring in Anesthesia

• This is a measure of the adequacy of the anesthetic agent.
• Inadequate anesthesia can be signaled by:
  • Facial grimacing or movement of arm or leg.
• But with muscle relaxants (full paralysis), it can be signaled by:
  • Hypertension, tachycardia, tearing, or sweating.
• Excessive anesthesia can be signaled by:
  • Cardiac depression - bradycardia, and Hypotension.
  • May result in hypoventilation, hypercapnia, and hypoxemia.

Clinically monitored by patient color (with adequate illumination) Defined as the presence of 5 gm/dL of deoxygenated hemoglobin (deoxyHb).

Pulse Oximetry:

• Allows analysis of oxygenation.
• Depends on differences in light absorption between oxyHb and deoxyHb.
• Red and infra-red light frequencies transmitted through a translucent portion (finger-tip or earlobe).
• Microprocessors then analyze the amount of light absorbed by the 2 wavelengths, comparing measured values, then determining concentrations of oxygenated and deoxygenated forms (oxy- and deoxy-).
• After all data is processed, oxygen saturation can be calculated.
• Pulse plethysmograph (visual analysis of pulse waveform) is used, along with an audible form (auditory assessment of oxygenation status).
• Pulse oximetry (SpO2) measures oxy-, deoxy-, met-, and carboxyHb.
• Carbon monoxide (CO) poisoning can give an overestimation of the true O2 saturation (SaO2), especially in burn victims.
• Inaccurate measurements may be seen in poor tissue perfusion (shock or cold extremities), movement, dysrhythmias, or when electrical interference is present (surgical cautery unit), ambient light, and with dyes like methylene blue.
• Carbon monoxide (CO) poisoning can give an overestimation of the true O2 saturation (SaO2), especially in burn victims.
• Inaccurate measurements may be seen in poor tissue perfusion (shock or cold extremities), movement, dysrhythmias, or when electrical interference is present (surgical cautery unit), ambient light, and with dyes like methylene blue.
• Normal SaO2 is >90%.

Clinically, monitored by pulse palpation, heart auscultation, and monitoring intra-arterial pressure.

Quantitatively using ECG signals and arterial blood pressure measurements every 5 min.

Monitoring: Electrocardiogram (ECG)

• A 3 or 5 lead electrode system is used for ECG monitoring in the OR.
• The 3 lead system has electrodes positioned on the right arm, left arm, and chest position (placed in the left anterior axillary line at the 5th interspace, referred to as V5). Lead 2 is usually monitored by this system.
• The 5 lead system adds a right leg and left leg electrodes, which allows monitoring V1, V2, V3, AVR, AVL, AVF, and V5.

Electrocardiography can be used to monitor:

• Cardiac activity
• Arrhythmia: Lead II
• Myocardial ischemia: ST segment analysis
• Electrolyte imbalance
• Pacemaker function

Monitoring: Blood Pressure (BP)

Methods of BP measurement:

1. Palpation: Simplest method of BP measurement, estimating the SBP, is by palpating the return of arterial pulse as the cuff is deflated.
2. Auscultation of the Korotkoff sounds on deflation (providing both SBP and DBP)

Mean Arterial Pressure (MAP) = DBP + 1/3(SBP – DBP)

3. Automated non-invasive BP measurements (oscillometry).

METHODOLOGY: A microprocessor-controlled oscillometer allows automatic inflation of the BP cuff at preset time intervals. A pressure transducer receives these readings and digitalizes them. This technique gives rapid, accurate (±9 mmHg) measurements of SBP, DBP, MAP, and HR.

LIMITATIONS: Errors occur due to movements, arrhythmias, or BP fluctuations due to respiration. 3–5 minutes intervals are recommended to prevent compressive peripheral nerve injury due to repeated rapid measurements.

4. Invasive BP measurements (Arterial BP):

Indications:

• Rapid moment-to-moment BP changes
• Frequent blood sampling
• Major surgeries (cardiac, thoracic, vascular)
• Circulatory therapies: vasoactive drugs, deliberate hypotension
• Non-invasive BP not possible: burns, morbid obesity
• Major trauma

The radial artery at the wrist is the most common site for an arterial catheter.

Alternatives are ulnar, femoral, brachial, and dorsalis pedis.

Perform Allen’s test before proceeding.

Complications:

• AV-fistula
• Aneurysm
• Hematoma
• Infections
• Thrombosis
• Embolization
• Hemorrhage

Central Venous Pressure (CVP)

• Catheter inserted into the SVC providing an estimate of the right atrial pressures and intravascular volume.
• CVP provides an estimate of:
• Intravascular volume
• RV preload

Central Venous line and Pressure (CVP)

• Indications:
• CVP monitoring
• Advanced Cardiopulmonary disease + major operation
• Secure vascular access for drugs
• Secure access for fluids + traumatic pts
• Aspiration of entrained air: sitting craniotomies
• Sites of insertions:
• Right internal jugular vein
• Usual site
• Predictable anatomic location, high success rate, short straight course to SVC.
• Subclavian vein:
• Better patient comfort v. Int. Jug.
• Higher Risk of pneumothorax-2%
• External jugular:
• 20%: cannot access central circulation

Complications of Central lines (jugular):

• Bleeding
• Injury to surrounding structures such as carotid artery
• Pneumothorax
• Arrhythmia

• Clinically: monitored by observing chest excursions, reservoir bag displacement, and breath sounds over both lungs.
• Quantitatively: ETCO2 analysis.
• Arterial blood gas analysis for assessing both oxygen and ventilation.

Capnography and EtCO2

• Capnometry: is the numerical measurement of CO2 concentration during inspiration and expiration.
• Capnogram: refers to the display of the CO2 concentration waveform sampled from the patient’s airway during ventilation.
• Capnography: is the continuous monitoring of a patient’s capnogram.

End-tidal CO2 monitoring is standard for all patients undergoing GA with mechanical ventilation.

• It is an important safety monitor and a valuable monitor of the patient’s physiologic status, and it has been an important factor in reducing anesthesia-related mortality and morbidity.
• Indications:
• Confirming successful intubation
• Acts as an indirect estimate of PaCO2
• Aids in diagnosis of PE and malignant hyperthermia
• Recognition of a partial airway obstruction
• Can serve as a disconnection alarm
• Management of head injured patients

Measurement of ETCO2

• Sample the patient’s respiratory gases near the airway.
• Pass infra-red light through the gas
• Measure the light after passage through the gas
• The difference represents infra-red absorption by the gas
• This is a measure of the amount of gas in the sample

• Potential heat loss or risk of hyperthermia necessitates continuous temperature monitoring
• Normal heat loss during anesthesia averages 0.5 -1 C per hour, but usually not more than 2 -3 C
• Temperature below 34C may lead to significant morbidity
• Hypothermia develops when thermoregulation fails to control the balance of metabolic heat production and environmental heat loss
• Normal response to heat loss is impaired during anesthesia
• Those at high risk are elderly, burn patients neonates, spinal cord injuries

Causes of Hyperthermia

• Malignant hyperthermia
• Endogenous pyroxenes (IL1)
• Excessive environmental warming
• Increases in metabolic rate secondary to:
  • Thyrotoxicosis
  • Pheochromocytoma

Monitoring Sites

• Tympanic membrane
• Oesophagus
• Rectum
• Nasopharynx