i) Continual monitoring of oxygenation, ventilation, circulation, and temperature.

  ii) Frequency of mandatory monitoring varies between each category, but never exceeds five minutes.

(1) If not used, a reason should be recorded on the patient record.

  iii) The following are all specifically mandated.

(1) Oxygen analyzer with a low inspired concentration limit alarm during general anesthesia

(2) Quantitative assessment of blood oxygenation

(3) Ensuring adequate ventilation during all anesthetic care including verification of expired oxygen (when possible), quantitative measurement of tidal volume, and capnography in all general anesthetics.

(4) Qualitative evaluation of ventilation is required during all other care.

(5) Ensure correct placement of endotracheal tube or laryngeal mask airway via expired carbon dioxide (CO₂).

(6) Alarms for disconnects when a mechanical ventilator is used

(7) Continuous display of ECG

(8) Determination of arterial BP and heart rate at least every 5 minutes.

(9) Adequacy of circulation is to be determined by quality of pulse either electronically, through palpation, or auscultation

(10) The means to determine temperature must be available and should be employed when changes in temperature are anticipated or intended.

2) Oxygen analyzer

    a) Most modern anesthesia machines monitor both inspired and expired concentrations of O₂.

    b) This is essential during anesthesia because it is possible to deliver a hypoxic gas mixture when mixing O₂, air, nitrous oxide, and/or volatile anesthetic agents.

1) Pulse oximetry

    a) Provides quantitative analysis of the patient’s saturation of hemoglobin with O₂.

4) Carbon dioxide (CO₂)

    a) Inspired and expired CO₂ should be monitored.

    b) Expired CO₂ is frequently displayed through capnography with a displayed value correlating to the peak expired CO₂ of each breath.

    c) Capnography

  i) Provides qualitative and quantitative information regarding expired CO₂.

  ii) Quantitatively, this is useful to ensure the endotracheal tube is within the respiratory tract as well as to ensure adequate cardiac output.

    d) Inspired CO₂

  i) Monitored to ensure that the CO₂ absorber of the anesthesia machine is adequately removing all CO₂ from the circuit.

  ii) If inspired CO₂ is greater than zero, changing of the absorbent should be considered. The color of absorbent turns blue when its capacity is exhausted.

5) Multiple expired gas analysis

    a) While not an essential monitor according to the ASA, this allows determination of the percent inspired and expired of the volatile agents and nitrous oxide.

    b) This allows the ability to better determine the delivery of an adequate anesthetic without over or under dose.

6) ECG

    a) The minimum of three leads is to be used, although five leads are used for most adults (3).

    b) Consideration must be taken for the surgical field and patient positioning.

  i) Lead placement is commonly altered for cases involving the chest, shoulders, back, and neck.

    c) Five Lead ECG

  i) Includes the right arm (RA), left arm (LA), right leg (RL), left leg (LL), and V.

  ii) The five lead arrangement can be used to display I, II, III, aVR, aVL, aVF, and/or V (Fig. 8-1).

    d) Three lead ECG

  i) Includes the RA, LA, and LL leads and can be used to display leads I, II, and/or III (Fig. 8-2).

(1) A three lead ECG can be modified to display V5 by moving the LA lead to the V5 position in the fifth intercostal space at the anterior axillary line (Fig. 8-3).

Figure 8-3 Modification of Three Lead ECG

Modification of the three lead ECG allows display of both leads II and V5. Lead V5 is monitored through the display of lead I in this configuration.

    e) The most commonly monitored leads are II and V5.

  i) II is best used to monitor rhythm because it provides the best visibility of the P wave.

  ii) V5 monitors for anterior and lateral ischemic events.

  iii) Most anesthesia machines will allow you to change how many and which leads are displayed.

(1) Some will show all leads simultaneously, although this often removes all other monitors from the display while viewing all leads.

(2) If an arrhythmia or ischemic event appears to be present, temporarily viewing all leads simultaneously may be helpful for diagnostic purposes.

7) Arterial blood pressure (BP)

    a) BP can be monitored invasively or non-invasively.

    b) Non-invasive methods

  i) Include palpation, ausculatation, Doppler probe, oscillometric cuff, or tonometry.

  ii) Palpation, auscultation, Doppler, and tonometry are rarely employed and are not discussed here.

    c) Automatic oscillometric

  i) The cuff is able to sense oscillations in cuff pressure which correlate with arterial pulsation.

  ii) The cuff is inflated beyond the point at which oscillation ceases and then is slowly deflated.

  iii) The oscillation in cuff pressure begins at systolic pressure, peaks at mean arterial pressure and disappears once again at diastolic pressure.

  iv) Due to the speed at which the cuff deflates, all three values are mathematically derived, with MAP being the most accurate (Fig. 8-4).

Figure 8-4 Illustration of Oscillometric Cuff Pressure Measurement Methodology with Corresponding Cuff Measurements with Various Common Sources of Artifact

Reproduced from Greenberg SB, Murphy GS, Vender JS. Standard monitoring techniques. In: Barash PG, Cullen BF, Stoelting RK, et al., eds. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2009:702, with permission.

  v) Placement

(1) Each cuff is labeled with an arrow pointing to where arterial pulsation is felt best.

(2) The cuff is then placed on the arm over the brachial artery, forearm over the radial artery, or thigh/calf over the popliteal artery.

  vi) Patient positioning

(1) When monitoring non-invasive pressure, consideration must be taken of patient position.

  vii) Invasive BP monitoring

8) Temperature

    a) Temperature changes should be anticipated and expected under any general anesthetic and therefore any general anesthetic requires temperature measurement.

  i) Very brief procedures may be an exception, but the availability of temperature monitoring should be recorded.

    b) The temperature may be measured from many locations including skin, nasopharynx, esophageal, bladder, rectal, or a pulmonary arterial catheter.

    c) Core temperatures obtained from a pulmonary catheter, esophageal stethoscope, or rectal probe are preferable sources.

References

1. Standards for Basic Anesthetic Monitoring,” last amended October 25, 2005. http://www.asahq.org

2. Greenberg SB, Murphy GS, Vender JS. Standard monitoring techniques. In: Barash PG, Cullen BF, Stoelting RK, et al., eds. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2009:702.

3. Morgan GE, Mikhail MS, Murray MJ, eds. Patient monitors. In: Clinical Anesthesiology. 4th ed. New York, NY: McGraw Hill, 2006:117–154.

Pulse Oximetry

Rachel Boggus, MD

1) Basic Concepts

    a) Pulse oximetry measures the amount of oxyhemoglobin using the Lambert-Beer law

  i) The Lambert-Beer law is a mathematical means of expressing how light is absorbed by matter

    b) There are two main types of oximetry. Fractional oximetry and functional oximetry

  i) Fractional oximetry. Oxyhemoglobin/(oxyhemoglobin + deoxyhemoglobin + methemoglobin + carboxyhemoglobin)

  ii) Fractional oximetry measures the arterial oxygen saturation (SaO₂)

(1) Can only be measured by an arterial blood sample

  iii) Functional oximetry

(1) Functional oximetry gives you the SpO₂

(2) Can be measured noninvasively by a standard pulse oximeter

2) How pulse oximetry works

    a) A pulse oximeter emits two wavelengths of light: red (660 nm) and infrared (940 nm)

  i) Deoxyhemoglobin absorbs more light in the red band

  ii) Oxyhemoglobin absorbs more light in the infrared band

    b) Sensors in the oximeter detect the amount of red and infrared light absorbed by the blood

    c) Photoplethysmography is then used to identify pulsatile arterial flow (alternating current [AC]) and non-pulsatile flow (direct current [DC])

    d) The ratio of AC/DC at both 660 and 940 nm is measured using the equation: (AC/DC)₆₆₀/(AC/DC)₉₄₀

    e) The pulse oximeter calculates the SpO₂ by taking the above equation and using an algorithm built into the software to derive the SpO₂

  i) The calibration to derive SpO₂ from the (AC/DC)₆₆₀/(AC/DC)₉₄₀ ratio was made from studies of healthy volunteers

1) Accuracy of the pulse oximeter

    a) If the SpO₂ is between 70% and 100% the pulse oximeter is accurate to within 5%

  i) It is not accurate below 70% because calibration of the pulse oximeter involved healthy volunteers whose SpO₂ did not routinely reach levels <70%

  ii) For the relationship between SaO₂ and PaO₂ (see Fig. 9-1) (7)

  iii) It is also not as accurate below 70% because there is more deoxyhemoglobin present

(1) The absorption spectrum of deoxygenated hemoglobin is very steep at 600 nm in the red range so small changes in the amount of deoxyhemoglobin can cause very wide variances in SpO₂ (Fig. 9-2)

Figure 9-2 Light Absorption with Oxygenated and Deoxygenated Hemoglobin

Oxygenated hemoglobin (red line) absorbs more infrared light and allows more red light (vertical red line) to pass through. Deoxygenated hemoglobin (purple line) absorbs more red light and allows more infrared light (vertical purple line) to pass through. Adapted from red and infrared light absorption. http://www.oximeter.org/pulseox/principles.htm accessed March 8, 2010

    b) Ear probes may be more accurate than finger probes because the probe is closer to the heart

    c) Pulse oximetry is not as accurate in low amplitude states (Table 9-1)

  i) Low perfusion makes it difficult for the pulse oximeter to distinguish a true signal from background noise

4) Dyshemoglobinemias

    a) Pulse oximetry only accurately measures oxyhemoglobin and deoxyhemoglobin—all other forms of hemoglobin are not accurately measured

  i) Carboxyhemoglobin is measured as 90% oxyhemoglobin and 10% deoxyhemoglobin

(1) Thus, when there are high amounts of carboxyhemoglobin it will overestimate the SpO₂

(2) This is an important consideration in patients exposed to smoke or fires

  ii) Methemoglobin absorbs equal amounts of red and infrared light so the SpO₂will read 85%

(1) Methemoglobin is formed when iron goes from it’s +2 ferrous form to the +3 ferric state

(2) The ferric state of iron displays a left shift on the oxygen dissociation curve and releases oxygen less easily

(3) Methemoglobinemia can be caused by many drugs (Table 9-2)

  iii) Patients with sickle cell anemia presenting in a vasoocclusive crisis can have an inaccurate SpO₂ reading

  iv) High levels of bilirubin do not alter SpO₂ readings

5) Other Limitations

    a) IV dyes

  i) Methylene blue, indocyanine green, and indigo carmine all cause transient decreases in the SpO₂ lasting anywhere from 30 seconds to 20 minutes

    b) Dark skin pigmentation—melanin inhibits passage of light through tissue

    c) Motion artifact—pulse oximeter unable to measure peaks and troughs of waveform correctly

    d) Fluorescent light—fluorescent light emits wavelengths in the 660 nm region which interferes with the red band of the pulse oximeter

    e) Nail polish—inhibits passage of light through nail.

References

1. Huch A, Huch R, Konig V, et al. Limitations of pulse oximetry. Lancet 1988;1:357–358.

2. Juban, A. Pulse Oximetry. Crit Care. 1999;3(2):R11–R17.

3. Schnapp LM, Cohen NH. Pulse oximetry-uses and abuses. Chest 1990;98:1244–1250.

4. Tremper KK, Barker SJ. Pulse oximetry. Anesthesiology. 1989;70:98–108.

5. Welch JP, DeCesare MS, Hess D. Pulse oximetry: instrumentation and clinical applications. Respir Care. 1990;35:584–601.

6. Wukitisch MW, Peterson MT, Tobler DR, et al. Pulse oximetry: analysis of theory, technology, and practice. J Clin Monit. 1988;4:290–301.

7. Martin, Lawrence. All you really need to know to interpret arterial blood gases. 2nd ed. Lippincott Williams & Wilkins, 1999.

8. “Red and infrared light absorption.” Oximetry.org.Web. Mar 8, 2010. http://www.oximeter.org/pulseox/principles.htm.

Capnography

Jessica Spellman, MD

1) Definitions (2)

    a) Capnometry

  i) Measurement and numeric representation of CO₂ concentration or partial pressure in respiratory gases during inspiration and expiration

    b) Capnogram

  i) Concentration time display of CO₂ concentration sampled at a patient’s airway

    c) Capnography

  i) Continuous monitoring of a patient’s capnogram during the respiratory cycle

2) Methods of capnometry (3)

    a) Sidestream capnometers

  i) Sample tubing connected to the airway that continually aspirates respiratory gases at a rate of 50 to 400 mL/min to a measurement chamber

  ii) Advantages

(1) Allows for accurate capnography when attached as close as possible to the patient

(2) Lightweight

(3) Can be used to measure CO₂ and anesthetic gases

  iii) Disadvantages

(1) Condensation of humidified gases may cause frequent clogging of the tubing (water traps and filters must be used to prevent condensation from reaching the measuring chamber)

(2) Delay in measurement response time depending on the size and length of the tubing

    b) Mainstream capnometers

  i) Light absorption chamber directly attached to the airway

  ii) Advantages

(1) Direct measurement allows for fast response time

  iii) Disadvantages

(1) Bulky equipment placed directly in endotracheal tube

(2) Does not allow for measurement of anesthetic gases

1) Techniques of CO₂ measurement (4)

    a) Infrared spectrography

  i) Employed by most operating rooms

  ii) In the measuring chamber, gases are exposed to an infrared light beam

  iii) Each molecule (CO₂, anesthetic gases) has a specific absorbance spectrum of the infrared beam that can be quantified

    b) Mass spectrography

  i) Measurement of CO₂ and other gas concentrations on the basis of differing molecular weights

    c) Raman spectrography

  i) Use of spectral analysis of scattered light energy that results from an argon laser light source exposed to a gas mixture

    d) Colorimetric CO₂ analysis (5)

  i) pH sensitive test paper

  ii) When in the presence of CO₂, color change occurs

  iii) Often used to confirm CO₂ and endotracheal intubation outside of the operating room

4) Normal capnogram (6) (Fig. 10-1)

    a) Phase I

  i) Initiation of expiration

  ii) CO₂-free gas from anatomic dead space

    b) Phase II

  i) Expiration of mixture of dead space and alveolar gas

    c) Phase III

  i) Alveolar plateau

  ii) CO₂-rich gas from alveoli

    d) Phase IV or 0 (7)

  i) Inspiration

5) Clinical uses of capnography (4)

    a) Confirmation of endotracheal intubation

    b) Monitoring of adequacy of ventilation in controlled or spontaneously ventilating patients

    c) Noninvasive estimate of PaCO₂

  i) Assumes the normal 2 to 5 mm Hg difference between expired (PETCO₂) and arterial (PaCO₂) that exists in the awake state is present

  ii) The gradient between PETCO2 and PaCO2 may be increased with age, pulmonary disease, pulmonary embolus, low cardiac output, and hypovolemia

    d) Detection of patient disease

  i) Causes of increased CO₂ production

(1) Fever

(2) Sepsis

(3) Malignant hyperthermia

(4) Hyperthyroidism

(5) Shivering

  ii) Causes of decreased PETCO₂

(1) Decreased cardiac output

(2) Hypovolemiaism

(3) Pulmonary embolism

(4) Hypothermia (6)

(5) Hyperventilation (6)

  iii) Airway obstruction may be detected due to abnormalities in the capnography tracing

    e) Detection of problems with the anesthetic breathing system

  i) Rebreathing

  ii) Incompetent valves

  iii) Circuit disconnect

  iv) Circuit leak

6) Interpretation of abnormal capnograms (3,4,6) (Fig. 10-2)

    a) Rebreathing of CO₂

  i) Elevation in baseline CO₂ and Phase I

  ii) Can eliminate by increasing fresh gas flow or changing CO₂ absorber

    b) Obstruction to expiratory gas flow

  i) Prolonged Phase II and steeper Phase III slope

  ii) Occurs with bronchospasm, COPD, kinked endotracheal tube

    c) “Curare Cleft”

  i) Dip in Phase III

  ii) Indicates return of spontaneous respiratory efforts

    d) Cardiogenic oscillations

  i) Oscillations of small gas movements during phase III and IV (or 0)

  ii) Produced by aortic and cardiac pulsations

    e) Increased CO₂

  i) Elevated plateau height

  ii) Indicates increased CO₂ production states (see above), other source of CO₂ (as in laparoscopic surgery), or inadequate minute ventilation

    f) Decreased measured CO₂

  i) Decreased plateau height

  ii) May indicated decreased CO₂ production state (see above) or increased minute ventilation

    g) Incompetent inspiratory valve

  i) Prolonged Phase III with elevation of baseline CO₂ and plateau height

  ii) Results in rebreathing

  iii) May be difficult to detect without simultaneous analysis of flow waveforms (7)

    h) Esophageal intubation

  i) Initial presence of CO₂ followed by no CO₂ detection

References

1. American Society of Anesthesiology. Standards for Basic Anesthetic Monitoring. [Internet]. Park Ridge, IL: American Society of Anesthesiology, 2005 Oct 25, c2008. [cited 2010 Jul 7].

2. Greenberg SB, Murphy GS, Vender JS. Standard monitoring techniques. In: Barash PG, ed. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009:697–700.

3. Szocik JF, Barker SJ, Tremper KK. Fundamental principles of monitoring instrumentation. In: Miller RD, ed. Miller’s Anesthesia. 6th ed. Philadelphia, PA: Elsevier, 2005:1213.

4. Bhivani-Shankar K, Moseley H, Kumar AY, et al. Capnometry and anaesthesia. Can J Anaesth. 1992;39:617–632.

5. Moon RE, Camporesi EM. Respiratory monitoring. In: Miller RD, ed. Miller’s Anesthesia. 6th ed. Philadelphia, PA: Elsevier; 2005:1454.

6. Eisenkraft JB, Leibowitz AB. Hypoxia and equipment failure. In: Yao, FS, ed. Yao & Artusio’s Anesthesiology: Problem-Oriented Patient Management. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2008:1184–1187.

7. Bhavani-Shankar K, Philip JH. Defining segments and phases of a time capnogram. Anesth Analg 2000;91:973–977.

Invasive Arterial Blood Pressure Monitoring

Quinn Stevens, MD • Nathaen Weitzel, MD

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