Monitoring During Anesthesia Essentials

Overview

• Lecture topic: Monitoring during Anesthesia by Asst. Prof. Güçcan Berkel, MD (Marmara University, Faculty of Dentistry).
• Purpose: Explain why and how vital functions are monitored during anesthesia.
• Emphasis: Monitors warn clinicians but cannot replace the anesthetist.

Basic Principles of Monitoring

• Anesthesia affects CNS, cardiovascular, and respiratory systems; suppresses automatic functions.
• Objective: Keep vital signs and physiological parameters within normal values.
• Ideal monitor properties:
  • Noninvasive, minimal intervention, no physiological disturbance.
  • Trustworthy, easy to collect and interpret data.
  • Easy-to-use, portable, low cost, minimal required technical support.

Standard Anesthetic Monitoring (ASA)

• Four basic areas: Oxygenation, Ventilation, Circulation, Body Temperature.
• Standard devices: pulse oximeter, ECG, noninvasive blood pressure device, temperature monitor.
• ASA published basic anesthetic monitoring guidelines (first 1986, revised 1996).

Oxygenation

• Goal: Ensure adequate inspired oxygen and arterial oxygenation throughout anesthesia.
• Inspired gas:
  • Measure oxygen concentration in breathing system with oxygen analyzer.
  • Use low-oxygen concentration limit alarm.
• Blood oxygenation:
  • Use quantitative measures (pulse oximetry) during all anesthetics.
  • Audible pulse tone and low-threshold alarms required.
  • Visual assessment of color needs adequate illumination.
• Critical values:
  • SpO2 ≈ 90% corresponds to PaO2 ≈ 60 mmHg (threshold for organ oxygen risk).

Pulse Oximetry

• Noninvasive measure of arterial oxygen saturation (SaO2) using two-wavelength light.
• Sites: finger, ear lobe, nasal bridge.
• Analyzes only pulsatile signals.
• Typical SaO2 normal range: 95–97%.

Ventilation

• Goal: Ensure adequate patient ventilation during all anesthetics.
• Methods:
  1. Qualitative clinical signs: chest excursion, reservoir bag, breath sounds.
  2. Continuous monitoring of expired CO2 (capnography) is required; measuring expired gas volume is encouraged.
  3. Verify correct airway device placement by identifying CO2 in expired gas.
  4. Mechanical ventilation requires devices to detect breathing system disconnections with audible alarms.
  5. During regional/local anesthesia without sedation, use clinical observation; with moderate/deep sedation add expired CO2 monitoring unless invalid.
• Capnography:
  • ETCO2 normal: 35–45 mmHg.
  • Typical gradient: PaCO2 − ETCO2 ≈ 5–10 mmHg.
  • Low ETCO2 can indicate hyperventilation, cardiac arrest, airway obstruction, disconnection, or ventilation errors.
  • High ETCO2 can indicate hypoventilation, rebreathing, or exhausted (dirty) soda lime.

Respiration Devices

• Respirometer: measures respiratory rate, tidal volume, and minute ventilation.
• Gas analysis: mass spectrometer or gas chromatography can measure O2, CO2, N2O, and volatile agents (halothane, isoflurane, sevoflurane).
• FiO2 measurement recommended for inspired oxygen concentration.

Circulation

• Goal: Ensure adequate circulatory function for organ perfusion.
• Continuous ECG display from start of anesthesia until preparing to leave anesthetizing location.
• Arterial blood pressure and heart rate determined and evaluated at least every 5 minutes.
• Additional circulation monitoring options: pulse palpation, heart auscultation, intra-arterial pressure tracing, peripheral pulse ultrasound, pulse plethysmography/oximetry.
• Blood pressure basics:
  • MAP relates to cardiac output × systemic vascular resistance.
  • Noninvasive BP by pneumatic cuff is common; cuff width should be ~40% of arm circumference and bladder encircle ≥50% of arm.
  • Korotkoff sounds: phase 1 = systolic; phases 4 or 5 = diastolic.
• BP components clues:
  • MAP (organ perfusion), DAP (coronary perfusion), SAP (myocardial oxygen use).

Invasive Arterial Pressure Measurement

• Direct arterial pressure via cannulation gives continuous, accurate readings and blood sampling.
• Risks: hemorrhage, infection, distal ischemia.
• Common arterial sites:
  • Radial artery (preferred, collateral flow) — perform Allen test.
  • Brachial (easy, but prone to kinking).
  • Axillary (risk to nearby nerves).
  • Femoral (higher risk: pseudoaneurysm, plaque).
  • Dorsalis pedis or posterior tibial possible for lower limb access.

Central Venous and Pulmonary Artery Catheters

• Central venous catheter: inserted from peripheral vein to SVC or right atrium (sites: arm veins, IJV, EJV, subclavian, axillary, femoral).
• Used with saline manometer or pressure transducer for CVP monitoring.
• Pulmonary artery catheter (Swan-Ganz) measures pressures including PCWP (normal 5–10 mmHg) and can help calculate cardiac output.
• Indications: cardiac diseases, severe pulmonary disease, complex hemodynamic monitoring.

Cardiac Monitoring (ECG)

• ECG monitors rate, rhythm, and can detect acute electrolyte changes and ischemia.
• Standard 3-lead placement: right shoulder, left shoulder, left lower chest. Lead II monitors rate/rhythm well.
• Alternate placement for ischemia detection: move electrodes to monitor lead equivalent of CM5 (better for myocardial ischemia).
• Heart rate norms: 60–100 bpm (bradycardia <60, tachycardia >100). Athletic/resting rates may be 40–50 bpm.

Cardiac Output

• Cardiac output (CO) = stroke volume × heart rate.
• CO distribution described (arm/leg, lungs, etc.) — invasive monitoring used when detailed data required.

Body Temperature

• Goal: Maintain appropriate body temperature during all anesthetics.
• Monitor when significant temperature changes are intended, anticipated, or suspected.
• Sites: mouth, rectum, skin, esophagus, tympanic membrane.
• Hypothermia defined as <36°C; hyperthermia >37°C.
• Effects:
  • Hypothermia decreases O2 consumption; may protect heart/brain from ischemia.
  • Postoperative shivering increases O2 consumption fivefold, risks myocardial ischemia and angina.

Acid-Base and Blood Gases

• Normal arterial blood gas values:
  • PaO2: 80–100 mmHg.
  • SaO2: 0.95–0.97 (95–97%).
  • pH: 7.35–7.45.
  • PaCO2: 35–45 mmHg.
  • HCO3−: 22–28 mEq/L.
  • Base excess (B.E.): −3 to +3.

Neuromuscular Monitoring

• Monitor effectiveness of neuromuscular blocking agents.
• Use nerve stimulators and electromyography (e.g., facial nerve EMG) to assess block depth and recovery.

Central Nervous System Monitoring

• Clinical signs of consciousness level: sedation, deep sleep, general anesthesia, coma, awakening.
• Monitoring techniques:
  • Electroencephalography-derived indices (BIS, entropy).
  • Evoked potentials.
  • Cerebral blood flow measures and transcranial Doppler.
  • Intracranial pressure monitoring when indicated.

Renal Function and Urine Monitoring

• Urine output target: >1 mL/kg/h.
• Evaluate urine color, clarity, and volume.
• Urine monitoring indicated for CHF, renal or hepatic insufficiency, shock, cardiac/renal surgery, craniotomy, or suspected postoperative oliguria.

Other Monitoring Methods and Laboratory Data

• Transesophageal echocardiography (TEE).
• Intracranial pressure monitoring.
• Mixed venous oxygen saturation.
• Regular laboratory tests as indicated (electrolytes, blood gases, hemoglobin, coagulation).

Summary Table: Main Monitors and Purpose

Key Terms and Definitions

• Anesthesia monitoring: Continuous observation of physiologic parameters affected by anesthesia.
• ETCO2: End-tidal CO2, indicator of ventilation.
• SpO2: Peripheral oxygen saturation measured by pulse oximeter.
• PCWP: Pulmonary capillary wedge pressure; reflects left atrial pressure.
• CO: Cardiac output = stroke volume × heart rate.
• Hypothermia: Core temp <36°C; Hyperthermia: >37°C.

Action Items / Practical Points

• Always ensure pulse oximeter and capnography in use for general anesthesia.
• Confirm airway device placement by detecting exhaled CO2.
• Use appropriately sized BP cuff and check BP at least every 5 minutes.
• Consider invasive monitoring (arterial line, central line, PAC) for high-risk or complex cases.
• Monitor temperature when significant changes are likely and manage hypothermia to avoid shivering-related complications.
• Use neuromuscular monitoring whenever neuromuscular blockers are administered.
• Remember: the anesthetist remains the primary monitor and decision-maker; devices only assist.