Monitoring Anesthesia: New Techniques And Their Limitations

William W. Muir, III, DVM, PhD, DACVECC, DACVA
The Ohio State University, Columbus, Ohio

            Let’s assume (big assumption) that the monitor being used is reliable, accurate and dependable.  The first question we have to ask ourselves is what is it that the monitor monitors and of what clinical value (relevance) is this information.  For example, many if not most of the respiratory rate monitors currently being sold to veterinarians monitor the change in temperature of the respired gases (since exhaled gas is warmer than inhaled gas) during a single breath in order to determine respiratory rate.  What is being monitored then is air temperature not air flow.  There are several problems with this type of monitor, the most obvious being that even though this monitor may provide a digital display of “respiratory rate” it does not indicate whether or not an adequate amount of gas (enough to maintain normocarbia and oxygenation) is flowing to sustain normal ventilation.  A similar argument could be levied against the routine use of an EKG machine if it is being used to monitor “cardiovascular status”.  The EKG machine provides a continuous display of the hearts electrical activity from which heart rate and rhythm can be ascertained.  It provides no real indication of hemodynamic function particularly in sick, debilitated or anesthetized patients.  Indeed some patients may be suffering from pulseless electrical activity (normal or near normal EKG but very weak or no peripheral pulse) or electromechanical dissociation (EMD; normal or near normal EKG very poor or no cardiac contractile activity) and may still display a normal EKG.

            Monitors that provide the most clinically relevant information regarding the cardiorespiratory systems are those that record and potentially store and trend data that indicate (directly or indirectly) physical function.  Said another way and in the context of currently available equipment they usually detect pressure, flow or volume changes.  A reasonable monitor for the respiratory system, for example, would measure volume-flow over time thereby allowing the calculation of tidal volume and minute volume.  Moniotors that display pressure and vomue changes simultaneously, and some do, offer the ability to determine whether or not the lung is stiff (low compliance) or is getting stiffer with time and therefore requiring greater inflation pressures to expand and adequately ventilate. An End-tidal CO2 (ETCO2) monitor can be purchased to indirectly assess adequacy of ventilation although such devices are relatively expensive.  Similarly a reasonable monitor of the cardiovascular system would detect arterial blood pressure (systolic, diastolic, mean) and/or blood flow (cardiac output).  The use of devices (doppler, oscillometric, plethysmographic) that noninvasively or indirectly  assess arterial blood pressure (NIBP) has become routine at most colleges of veterinary medicine and at many privately owned veterinary hospitals.  Technical limitations aside these devices (when working properly) provide the clinician with excellent information regarding patient hemodynamic status. Most of the commercially available NIBP machines also provide heart rate and many are combined with pulse oximeters and other noninvasive monitoring modalities (ECG, ETCO2).  Interestingly, but not surprisingly the use of an appropriately placed esophageal stethoscope can indirectly provide important and physiologically relevant information regarding both respiratory and cardiovascular function.  The intensity, duration and quality (frequency spectra) of both breath and heart sounds can be used to determine frequency (rate) and suggest the adequacy of breathing and cardiac contraction.  Breath and heart sound monitors therefore are excellent indicators of cardiorespiratory function when used properly.  Electronic stethoscopes (Welch-Allyn, Hewlet-Packard) offer the potential to amplify and broadcast heart and breath sounds while logging a digital record for future review and analysis.

            The clinical use of pulse oximeters has received a great deal of attention in recent years.  What is pulse oximetry, what are pulse oximeters and why is it said “that no monitor of oxygen transport has had a greater impact on the practice of anesthesiology than the pulse oximeter?”.

            Pulse oximetry is a noninvasive method of monitoring the oxygen carried by hemoglobin in arterial blood vessels.  The instrument that is used to make this measurement is a pulse oximeter of which there are currently more than 15 manufacturers.  The pulse oximeter provides continuous and noninvasive estimates of arterial hemoglobin saturation, which is directly related to the arterial oxygen content and since the measurement is dependent on the sensing of an arterial pulse the device also displays heart rate.  Pulse oximeters use a light source and a photodetector to distinguish oxygenated hemoglobin (O2 Hb) in small arteries from deoxygenated or reduced hemoglobin (RHb) in veins.  The key point to remember is that the pulse oximeter does not detect or measure the Hb concentration or PCV and therefore cannot be used by itself to determine whether or not the patient is adequately oxygenated.  Said another way the patient could have a SaO2 of 100% and still be hypoxic if Hb were acutely reduced to less than 5-7 g/dl (PCV=15=21).

Limitations of Pulse Oximetry

Because pulse oximeters are designed under the assumption that arterial pulsations (local changes in arterial blood volume) are the only source of variations in light absorbance anything that alters light absorption may lead to erroneous readings.  Furthermore, and because pulse oximeters only emit light at two wavelengths to detect O2 Hb and RHb any light absorbing species other than O2 Hb and RHb (carboxyhemoglobin, COHb; methemoglobin, MetHb) are not accounted for and when present produce erroneous readings (Table 1).

Table 1.  Common Sources of Error in Pulse Oximetry
  1.    Low Pulse Pressure
    Hypotension (Drugs, Shock)
    Hypovolemia
    Tachycardia
    Hypothermia (vasoconstriction)
   
  2.    Increased venous pulsations
    Right heart failure
    Tricuspid regurgitation
    Tourniquets (automatic indirect arterial blood pressure devices)
   
  3.    Abnormal pulses
    Cardiac arrhythmias
   
  4.    Motion - Artifact
    Movement
    Shivering
   
  5.    Dyshemoglobins
    Carboxyhemoglobin (carbon monoxide)
    Methemoglobin (Benzocaine, nitrates, nitrites, metoclopfamide, sulfa drugs)
   
  6.    Dyes and Pigments
    Methylene blue
    Hyperbilirubinemia
   
  7.    Anemia (Hb, 5 g/dl)
   
  8.    External light sources (fluorescent light, heat lamps, surgical lamps)
   
  9.    Electrocautery units
   
  10.  Poor probe positioning
           

            As stated or implied several times during this discussion, tissue (peripheral) pulsations are required in order for the pulse oximeter to distinguish between light absorption by arterial blood versus background absorption (venous blood, tissues).  Pulse oximeters may provide erroneous and unreliable results or cease to function when peripheral pulsations (pulse pressure) is reduced or absent.  Pulse pressure (systolic pressure - diastolic pressure) is reduced during hypotension, hypovolemia, arrhythmia and hypothermia (Table 1).  Hypotension and hypovolemia produce low arterial blood pressure and weak arterial blood pressures.  Tachycardia whether supraventricular or ventricular in origin limit the time for arterial pressure decay thereby producing small pulse pressures regardless of the mean arterial pressure.  Finally hypothermia that causes intense vasoconstriction will limit tissue blood flow and peripheral tissue pulsations.  Similarly, movement artifacts and increased venous pulsations lead to artificially lowered arterial oxygen saturation because the pulse oximeter interprets changes in light absorption as resulting from an arterial pulse.

            The presence of other light absorbing species of hemoglobin other than O2 Hb and RHb (methemoglobin, carboxyhemoglobin) will cause the pulse oximeter to overestimate arterial oxygen saturation.  This occurs because the pulse oximeter interprets MetHb and COHb as though they were composed mostly of O2 Hb.  One potential cause of MetHb in dogs and cats if the use of cetacaine® (benzocaine) sprays prior to endotracheal tube placement.  It should be noted that fetal hemoglobin does not affect the accuracy of pulse oximeters.  The light absorption characteristics of fetal hemoglobin are basically the same as adult hemoglobin.

            It should be intuitively obvious based upon the previous discussion that extraneous blood borne dyes (methylene blue) or substances (bilirubin lipide) that can affect arterial blood light absorption may artificially alter pulse oximeter SpO2 values.  Although the presence of methylene blue in blood is know to potentially lower SpO2 values to as low as 1% the effects of hyperbilirubinemia or hyperlipidemia on SpO2 values are poorly described and controversial.

            Anemia is a potential cause of erroneous SpO2 values.  Hemoglobin values of less than 5 g/dL are interpreted as though a low state of perfusion exists (Table 1).  The pulse oximeter produces unreliable SpO2 values because readings are dependent upon light absorption by Hb.  Of greater clinical importance than marginally inaccurate SpO2 values, however, is the fact that the pulse oximeter may display an SpO2 value of 100 percent despite marked decrease in CaO2  because of low Hb values.

            Light sources, electrocautery units and poor sensor positioning are all potentially responsible for variable and erroneous pulse oximeter SpO2 readings.  All artificial “white” light sources emit wavelengths in the red and infrared range that can be sensed by the photodetector.  Electrocautery units produce electrical interference (electromagnetic radiation) and poor position often times mimics poor perfusion states.  Regardless of these limitations recent technological advances in LED programming photodetector location and probe design have markedly improved the dependability and versatility of most pulse oximeters in use today.

The Physical Characteristics and Clinical Applications

The design and versatility of most commercially available pulse oximeters has come a long way since their introduction into human medicine.  Many medical equipment manufacturing companies have incorporated pulse oximetry into their anesthetic, surgical, and recovery hemodynamic monitoring devices.  These monitors, however, are large, expensive, and relatively immobile.  Current day pulse oximeters are about twice the size of a VCR remote, battery operated and rechargeable.  Future trends in the design of pulse oximeters will probably include the incorporation of temperature sensing devices and electrocardiographic leads in probes designed for esophageal use.  One particular advantage of rectal or esophageal probes is the elimination of artificial light sources as a potential source of erroneous SpO2 values.  There is no question that pulse oximetry has become “the minimum standard of care” for many human anesthesiologists.  The same may become true in veterinary medicine based on the ability of a pulse oximeter to detect hypoxemia.  One review aptly noted “metaphorically speaking the pulse oximeter is a century standing on the edge of the cliff of desaturation.  It gives no warning when we are approaching the edge, it only tells us when we have fallen off.”  Indeed the continuous digital readout of both SpO2 and heart rate values with temperature and ECG will markedly improve anesthetic monitoring in many veterinary practices.

End Tidal Carbon Dioxide Monitoring

            End tidal CO2 analysis is a non-invasive method of continuously approximating arterial carbon dioxide partial pressure (PaCO2).  Combined with pulse oximetry end-tidal CO2 monitoring can be considered as an alternative to arterial blood gas analysis in order to assess a patients’ ventilatory and acid/base status where blood gas analyzers are not available.  While blood gas analysis is the “gold standard” for PaCO2, it has the disadvantage of being invasive, expensive and providing only intermittent information.  When used in combination with blood gas analysis to determine alveolar dead space, capnography can be a powerful diagnostic tool in assessing the extent of ventilation-perfusion abnormalities.

            End-tidal CO2 is the partial pressure of carbon dioxide in the expired air obtained at the end of expiration.  This value approximates that of alveolar air, assuming that:

1. 

Capillary blood and alveolar gas CO2 are in equilibrium

2. 

End-tidal CO2 approximates the time weighted average of the ventilation weighted PaCO2

3. 

V/Q mismatch does not exist

4. 

Tidal volumes are large enough to displace dead space

5. 

Fresh gas flow is low enough to prevent dilution, and sample aspiration is low enough as to not entrain air or interfere with patient ventilation

 

End-tidal CO2 is a product of three major determinants:

1. the rate of CO2 production by the tissues
2. the rate of exchange of CO2 from the blood to the alveoli
3. the rate of CO2 removal by alveolar ventilation [(tidal volume - dead space)] x frequency
 

            End tidal CO2 can provide information regarding (1) metabolism, (2) circulation, and (3) ventilation.  Most capnographs used in clinical monitoring use infrared absorption spectroscopy to determine the concentration of carbon dioxide in the expired air.  Infrared light, at wavelengths absorbed by carbon dioxide, is passed through the expired gas sample and the concentration is determined according to the Lamber-Beer law.  Capnography is a much more straight forward technology than pulse oximetry in that the sample is removed from the patient and is not subject to corrections for patients background interference.

            End-tidal CO2 monitors are available with and without capnographic waveform displays.  The former are more expensive, but provide the user with more information by allowing for the visualization of the capnogram. End-tidal CO2 monitors can be very accurate and provide important clinically relevant information regardless of several articles published in refereed veterinary journals suggesting otherwise.  The key is to know how to use them. End-tidal CO2 that are determined during spontaneous breathing are frequently inaccurate.  The ETCO2 should be determined at the end of exhalation after a controlled (pressure and volume) breath.  This means that the rebreathing bag should be squeezed to deliver an inspiratory pressure of 20 cmH­2O and tidal volume or approximately 15 m/kg and the ETCO2 determined at the end of the breath. 

End Tidal Inhalant Anesthetic Monitors

            End-tidal inhalant anesthetic monitors are used to determine the inhalant anesthetic concentration in the anesthetic circuit.  Most monitors dispaly both inhaled and exhaled inhalant anesthetic concentration. End-ttdal inhalant anesthetic monitors are helpful for determining whether or not the anesthetic vaporizer is delivering enough or to much anesthetic and therefore why the patient is waking up or may accidentally  be receiving an overdose of inhalant anesthetic.  Given the multiple inhalant anesthetics available today and the various anesthetic drugs used to produce or supplement anesthesia the use of end-ttdal inhalant anesthetic monitors offers another dimension in monitoring and patient safety during inhalant anesthesia.

Point of Care Laboratory Testing: Hand-held Blood Gas and Clinical Chemistry Analyzers

Point of care laboratory testing offers the obvious and much needed advantage of being able to perform pH, blood gas (PO2, PCO2) and many clinical chemistry determinations at patient side.  This means that accurate, reliable and rapidly obtained laboratory data is immediately available to the clinician from which critical decisions can be made.  Two units (i-STAT; IRMA) have been evaluated in small animal and equine practice.  Both have performed satisfactorily and are capable of determining pH, blood gases and derived related variables (HCO3).  They both can determine Na, K and Ca+2.  The i-STAT offers the additional advantage of being able to determine additional blood chemistry values including BUN, glucose, SpO2, anion gap, Hb and others.  Both instruments are comparably priced (approximately $5000) and the “chips” into which small volumes of blood are injected in order to make the determinations cost from $2-$13.

Summary

            Lots of different monitors provide the ability to monitor one, several or all of the variables described above.  Current technologies allow all of the above variables to be monitored simultaneously with a touch screen monitor for less than $10,000.00.  The important issue is to select a monitor that can be integrated into your practice situation and to select a monitor that provides an immediate indication when something is wrong.  This generally means monitoring respiratory rate, ECG and blood pressure.