Jeff Ko, DVM, MS, DACVA
2/25/03

Goal of anesthetic monitoring

• The overall goal of monitoring anesthetized animals is to ensure adequate tissue perfusion with oxygenated blood.

Why should we monitor anesthetized patients? 

• Monitoring circulation, oxygenation and ventilation in the anesthetized patient allows the veterinary anesthetist to identify problems early, institute treatment promptly, and thus avoid irreversible adverse outcomes.

What parameters should be monitored?

• The American College of Veterinary Anesthesiologists (ACVA) developed a set of guidelines for monitoring anesthetized patients.
• The ACVA recommends that circulation, oxygenation, and ventilation of the anesthetized patient should be monitored.
• The objective of monitoring the circulatory system is to ensure adequate blood flow to tissues during all anesthetic procedures.
• The objective of monitoring oxygenation in the anesthetized patient is to ensure adequate oxygen concentration in the patient's arterial blood.
• To ensure adequate ventilation, the ventilatory function of the anesthetized patient needs to be monitored.
• In addition, the depth of anesthesia and patient core body temperature should also be monitored during general anesthesia.

How to monitor the anesthetized patient?

• Subjective and objective monitoring of anesthetized patients
  • Subjective monitoring involves using the anesthetist's visual, touch, and auditory senses to assess the patient's vital signs. 
• Circulation (cardiovascular function)
• Subjective methods:
  • The subjective assessments of an anesthetized patient's circulatory system have been published by the ACVA.
  • These include palpation of peripheral pulse, palpation of the heart beat through the chest wall, and auscultation of heart beat.
  • Use of a regular stethoscope, esophageal stethoscope, or other audible heart sound monitor aids the anesthetist in assessing the "presence", "absence", "regularity" or "irregularity" of a patient's heart beat.
  • Palpation provides a subjective feeling of "presence" or"absence"; "strong"or "weak"; "regular" or "irregular" peripheral pulse.
  • Assessing capillary refill time provides a subjective assessment of tissue perfusion.  A prolonged capillary refill time suggests poor tissue perfusion.
• Objective methods used to clinically assess circulation in the anesthetized patient include using
• Electrocardiography (ECG)
  • An accurate diagnosis of cardiac arrhythmia requires electrocardiography.
  • A continuous assessment of the electrocardiogram will allow detection of any cardiac arrhythmia, enable the anesthetist to evaluate the patient's progress, and guide the treatment of the arrhythmia.
  • One of the limitations of an ECG is that it displays the patient's heart rate, not the pulse rate. This is because electrocardiography only monitors the electrical activity of the heart (heart rate), not the mechanical activity (pulse rate).
  • No other circulatory information (i.e., blood pressure, stroke volume and cardiac output) is provided by the ECG.  
• Arterial blood pressure:
  • Measurement of arterial blood pressure provides information regarding the adequacy of blood flow to the patient's tissue. 
  • If the anesthetized patient has a mean arterial blood pressure below 60 mmHg, organ and tissue perfusion is inadequate.
  • In addition to mean arterial blood pressure, systolic and diastolic blood pressure provide useful information concerning peripheral vascular resistance, stroke volume, and intravascular volume.
  • The systolic blood pressure is determined by a combination of peripheral vascular resistance, stroke volume, and intravascular volume whereas diastolic blood pressure primarily arises from peripheral vascular resistance.
  • Measurement of systolic and diastolic blood pressures allows one to estimate pulse pressure, which is the difference between systolic and diastolic pressures. 
  • Clinically, the pulse pressure allows the anesthetist to estimate the stroke volume.  When palpating the peripheral pulse, a progressively weakening pulse may correlate with decreased pulse pressure and reduced stroke volume.
  • Normal systolic blood pressure range for anesthetized dogs and cats is between 90 mmHg and 120 mmHg.  Diastolic blood pressure ranges between 55 mmHg and 90 mmHg.
  • Arterial blood pressure may be monitored using non-invasive methods such as a Doppler ultrasound probe coupled with a pressure cuff and sphygmomanometer or an automated oscillometric device (e.g., Dinamap, Criticon, Tampa, FL).
    • The advantage of automated oscillometric detector over Doppler ultrasonic detector is that the oscillometric is automated and requires neither experience nor labor of the operator.
    • The measurement intervals may be adjusted from continuous to 120 minute measurement intervals.
    • The disadvantage of oscillometric detector is decreased accuracy or unreliability when used on a hypotensive or a small patient (less than 5 kg body weight).
    • The advantage of the Doppler technique is that it can be applied to any size animal (from a rat to a horse) and is relatively inexpensive.
    • The disadvantage of the Doppler technique is the requirement for operator experience.
  • Arterial blood pressure can also be measured invasively by placing a catheter into an artery and connecting this catheter to a pressure transducer and monitor/recorder.
    • Invasive blood pressure measurment is the 'gold standard' against which all other methods are compared.
    • The invasive method provides a continuous, beat to beat assessment of the patient's blood pressure, is more accurate in hypotensive situations, and provides access to an arterial blood sampling site for blood gas analysis.
    • However, invasive blood pressure monitoring is less practical in the clinical setting requiring more expensive equipment, demands skills in placement of an catheter, and potentially increases the risk of infection.
• Oxygenation (cardiorespiratory function)
  • Subjective methods:
    • Clinically, the presence of pink mucous membranes in an anesthetized patient is subjectively indicative of acceptable oxygenation.  However, oxygenation is either difficult or not possible to assess in anemic patients or patients with peripheral vasoconstriction.  These patients usually have pale mucous membranes. 
  • Objective methods for evaluating a patient's oxygenation include
    • Using blood gas analysis for P_aO₂ (partial pressure of oxygen in the arterial blood), hemoximetry, and pulse oximetry. 
    • Hemoximetry and pulse oximetry both analyze oxyhemoglobin although there are several differences.  Hemoximetry uses four wavelengths of light to identify and calculate the percentage of each of the four types of hemoglobin (oxyhemoglobin, reduced hemoglobin, methemoglobin, and carboxyhemoglobin) in the blood.  A blood sample is required for hemoximetry analysis. 
    • Because methemoglobin and carboxyhemoglobin do not contribute to functional oxygen transport, pulse oximetry ignores these two hemoglobin species and uses only the two wavelengths to detect oxyhemoglobin and reduced hemoglobin.  It therefore indicates the percent of hemoglobin that is saturated with oxygen.
    • Pulse oximetry provides a non-invasive, continuous detection of pulsatile arterial blood in the tissue bed, calculates the percentage of oxyhemoglobin present in the arterial blood, and provides the pulse rate of the monitored patient. 
    • Proper functioning of the pulse oximeter may be affected by many situations.  These include motion artifact (e.g, shivering), ambient light, poor peripheral blood flow from hypotension and vasoconstriction, electrical noise from electrocautery, increased carboxyhemoglobin and methemoglobin levels and dark skin color.
    • Pulse oximetry has rapidly become standard care in anesthetized human patients and has gained widespread popularity in veterinary anesthesia. 
      • Normal pulse oximeter readings in anesthetized animals should be 99-100%.  
      • Hemoglobin oxygen saturation (SpO₂) of 90% corresponds to P_aO₂ of 60 mmHg.  In the clinical setting, P_aO₂ (as measured by the blood gas analysis) can be estimated using pulse oximetry. Because there is a liner relationship, as PaO₂ = SpO₂ minus 30 for pulse oximeter readings between 75 and 90 % (see the following figure of hemoglobin dissociation curve).  For example, a pulse oximeter reading of 85% approximates a P_aO₂ of 55 mmHg (85-30=55) in the dog.  When pulse oximeter reading falls outside of the 75-90% range, the liner relationship between pulse oximeter reading and P_aO₂ no longer valid.
      • Using arterial blood sample, the partial pressure of the oxygen tension can be measured with blood gas analyzer. Normal PaO₂ in the anesthetized animal that is breathing 100% oxygen should be greater than 200 mmHg (it can be as high as 650 mmHg).
      • Hypoxemia (insufficient oxygenation of arterial blood) is considered to be present in the anesthetized patient, when
        • the P_aO₂ is less than or equal to 60 mmHg or
        • the SpO₂ reading is less than 90%.   

Hemoglobin Dissociation Curve

• Ventilation (respiratory function)
  • Subjective evaluation of an anesthetized patient's ventilation efficiency may be done by observing chest wall movement or rebreathing bag excursion when the patient is connected to an anesthesia machine.  Auscultation of breathing sound via an esophageal stethoscope or an audible respiratory monitor provides only respiratory rate and the absence or presence of respiration.
  • Objective evaluation and monitoring of ventilation efficiency requires respirometry, blood gas analysis, or capnometry.
    • A respirometer (or respirometry)
      • is an instrument that measures the amount of volume of expired gases
      • is usually placed between the expiratory limb of an anesthetic machine and the anesthetic breathing hose.
      • Alternatively, a respirometer connected to a face mask may be used to assess ventilation efficiency in a non-intubated anesthetized patient.
      • Respirometry assesses tidal volume and minute volume in the anesthetized patient.  Minute volume is the product of respiratory rate per minute and tidal volume of the patient (MV = TD x RR/min).
      • Respiratory rate and/or tidal volume reduction resulting in minute volume reduction and reflect the patient's depressed ventilatory function.
    • Arterial Blood Gas Analysis:
      • It measures arterial blood pH, bicarbonate, partial pressures of oxygen and CO_2.
      • PaCO₂, which is the partial pressure of CO₂ in the patient's blood, can be determined in the same way as PaO₂, using a sample collected from the femoral artery or other peripheral artery. 
      • Normal values in the anesthetized patient are between 35 mmHg to 45 mmHg.  PaCO₂ may be used to assess and monitor the anesthetized patient's ventilation efficiency. 
      • Although more accurate than respirometer, PaCO₂ measurement requires an expensive blood gas analyzer and samples may be difficult to collect.
    • Capnometry
      • It is the measurement and numerical display of CO₂ concentration, in the expired air, during the respiratory cycle (inspiration and expiration).
      • It is most commonly used to monitor end-tidal CO₂ and has become standard care for intraoperative monitoring of ventilation efficiency in human anesthesia.  End-tidal CO₂ (ET CO₂) is the partial pressure of CO₂ at the end of exhalation.
      • Normally end-tidal CO₂ reflects the partial pressure of CO₂ in the alveoli.  The measurement of end-tidal CO₂ is useful for determining optimal minute ventilation, hypoventilation, airway disconnection or airway obstruction.
      • This technique is finding increased use in veterinary anesthesia as it is a relatively inexpensive, continuous method to document ventilation adequacy and limits the need for invasive procedures such as arterial blood gas analysis. 
      • Capnometry can be performed using either a capnometer or a capnography. 
        • A capnometer measures and displays the readings without a graphic presentation.
        • Capnography uses a graphic recorder with display of CO₂ concentration on a screen or paper.
        • The waveform that is generated by the capnograph instrument is called a capnogram. 
        • A capnograph presents a graphic shape of exhaled CO₂ gas, end-tidal CO₂ concentration and also indicates the respiratory rate of a patient.
        • A capnometer, on the other hand, presents only the end-tidal CO₂ concentration and respiratory rate without a graphic shape.
      • There are four distinct phases of the capnogram (see figure of capnogram).
        • Phase I is the inspiratory baseline.  This phase represents fresh gas flow, anesthetic plus oxygen, past the CO₂ sensor during inspiration. The baseline should have a value of zero otherwise the patient is rebreathing CO₂.
        • Phase II is the expiratory upstroke.  This represents the arrival of CO₂ at the sensor just as exhalation begins.  It is usually very steep.
        • Phase III is the expiratory plateau, which represents exhaled CO₂.  The peak of this exhaled CO₂ is called the end-tidal CO₂.
        • Phase IV is the inspiratory downstroke.  This is the beginning of the inhalation and the CO₂ graphic curve falls steeply to zero.
      • End-tidal CO₂ concentrations between 35 and 45 mmHg are considered normal in anesthetized animals.
      • There is only one normal shape of capnogram.  Clinically any deviations from this shape should be investigated.

The Normal Capnogram

A-B: exhalation of CO2 free gas contained in dead space at the beginning of exhalation

B-C: respiratory upstroke, representing the emptying of connecting airways & the beginning of emptying of alveoli

C-D: Expiratory (or alveolar) plateau, representing of emptying of alveoli - due to uneven emptying of alveoli, the slope continues to rise gradually during the expiratory pause

D: End-tidal CO2 level - the best approximation of alveolar CO2 level

D-E: Inspiratory downstroke, as the patient begins to inhale fresh gas

E-A: Inspiratory pause, where CO2 remains at 0

• Clinical implication of capnogram:
• In the anesthetized patient, normal end-tidal CO₂ concentrations together with normal capnogram appearance indicate normal function of patients of metabolism, circulation, ventilation, and of the anesthetic machine.
• This is because CO₂ is produced by tissue metabolism, carried to the lungs by the blood, and subsequently exhaled by alveolar ventilation.
• Increases in end-tidal CO₂ concentrations may be due to impaired alveolar ventilation (anesthetic induced respiratory depression), increased metabolism (malignant hyperthermia or sepsis), or the addition of CO₂ to the circulatory system as a result of rebreathing CO₂.
• Rebreathing CO₂ can be due to soda lime exhaustion, incompetent expiratory valve on the anesthetic machine allowing exhaled CO₂ to be re-inhaled (even with normal function of soda lime), or intravenous bicarbonate injection.
• Decreased or abolished ET CO₂ concentrations may be due to hyperventilation, low cardiac output (low blood volume deliver to the lungs), respiratory arrest (no alveolar ventilation), or cardiac arrest (no circulation).
• Capnogram monitoring in anesthetized patients, also provides vital information regarding the patient's airway patency.  A depressed or absent capnogram may be due to a dislodged endotracheal tube, misplaced endotracheal tube (i.e., esophageal intubation), obstructed endotracheal tube or airway, a leak around endotracheal tube cuff or, disconnection of the endotracheal tube from the anesthetic machine.  Qualitative analysis of capnogram morphology provides detecting and diagnosing patient and anesthesia machine abnormalities.
• The drawback of capnometer is that it lacks the capnogram, and therefore, cannot provide a qualitative analysis and precise diagnosis for the morphologic changes of exhaled CO₂.
• Pulse oximetry, capnography and non-invasive blood pressure monitoring represent three current non-invasive monitoring techniques available to use in the anesthetized patient. When pulse oximetry and capnography are used together, a beat-to-beat and breath-to-breath non-invasive cardiorespiratory monitor is provided.
• Ultimately, the real challenge in the clinical setting is how to use all the available monitoring information and make the best decision to treat the anesthetized patient appropriately.   

Abnormal Capnograms

The capnogram shows a rebreathing of CO2 due to a possibility of incompetent expiratory valve or exhaustion of soda lime.

This capnogram shows a kinked endotracheal tube. Note the prolonged expiratory upstroke (A) and a slanted expiratory plateau (B) due to the slower gas flow rate during these two phases.

ACVA Postion Statement:
Suggestions for monitoring anesthetized veterinary patients.
(JAVMA, Vol. 206, No. 7, 936-937.)

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© 1996-2003, Oklahoma State University College of Veterinary Medicine, all rights reserved Last modified February 2, 2006 by Ron Mandsager, DVM, DACVA Questions? Comments? Contact me at aerrane@okstate.edu

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