Standard ASA Monitors



Continuous presence of a qualified anesthesia provider is a standard set by the ASA.


 

  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.


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An audible alarm should be set to notify the anesthesia provider of a hypoxic gas mixture.


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.


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Pulse oximetry Chapter 9 page 61


Capnography Chapter 10 page 65


 

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.


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EKG leads are color-coded to assist placement. RA and LA leads are easily remembered: imagine the LA becomes suntanned (black) driving a car with the window open, while the RA remains untanned (white) inside the car.


 

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).



Figure 8-1 Normal Five Lead ECG Lead Placement


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    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 8-2 Normal Configuration of a Three Lead EKG


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Figure 8-3 Modification of Three Lead ECG

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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.


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For most patients, printing the ECG tracing at the beginning of the case allows for later comparison should an intraoperative event occur.


 

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


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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


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Patients in the beach chair position are especially prone to cerebral ischemia even with “normal” brachial BPs.


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.


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Invasive arterial blood pressure monitoring, Chapter 11, page 70


 


Chapter Summary for Standard Monitors


 

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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.



9

Pulse Oximetry


 

Rachel Boggus, MD


 


Pulse oximetry is one of the most commonly employed monitoring modalities in anesthesia. It is a non-invasive way to monitor the oxygenation of a patient’s hemoglobin. A sensor with both red and infrared wavelengths is placed on the patient. Absorption of these wavelengths by the blood is measured and oxygen saturation (SpO2) can be calculated.


 

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
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(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


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SpO2 measurements are not accurate below 70%.


 

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)



Figure 9-1 The Oxygen-Hemoglobin Dissociation Curve


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The oxygen dissociation curve showing the relationship between SpO2 and PaO2. P50 is the PaO2 at which hemoglobin is 50% saturated with oxygen. The normal value is 27 mmHg. Adapted from Martin L. All you really need to know to interpret arterial blood gases. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 1999.


  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

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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



Table 9-1
Low Amplitude States
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SpO2 of 90% correlates to a PaO2 of 60 mm Hg.


 

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)



Table 9-2
Causes of Methemoglobinemia
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  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.



Chapter Summary for Pulse Oximetry


 

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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.



10

Capnography


 

Jessica Spellman, MD


 


Monitoring of CO2 through capnometry and capnography can be a valuable tool in detecting acute alterations in ventilation, metabolism, and circulation. Continuous monitoring of CO2 is an American Society of Anesthesiology (ASA) standard for basic monitoring in all patients receiving general anesthesia (unless invalidated by the nature of the patient, procedure, or equipment) and for confirming proper endotracheal tube and LMA placement (1).


 

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


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Capnography is 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


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Colorimetric CO2 analysis is often used to confirm CO2 and endotracheal intubation outside of the operating room.


 

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



Figure 10-1 Phases of a Normal Capnogram


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Reproduced from 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; 2008:1185, with permission.


    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


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Capnography can be used to monitor the adequacy of ventilation in controlled or spontaneously ventilating patients.


 

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Causes of increased CO2 production include: fever, sepsis, malignant hyperthermia, shivering.


Causes of decreased CO2 production include: decreased cardiac output, hypovolemia, pulmonary embolism, hypothermia, hyperventilation.


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



Figure 10-2 Abnormal Capnograms


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Reproduced from Eisenkraft JB, Leibowitz AB. Hypoxia and equipment failure. In: Yao FS, ed. Yao & Artusios Anesthesiology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2008:1186, with permission.


    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



Chapter Summary for Capnography


 

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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.



11

Invasive Arterial Blood Pressure Monitoring


 

Quinn Stevens, MD • Nathaen Weitzel, MD


 


Invasive arterial blood pressure monitoring allows for continuous, beat to beat monitoring of arterial blood pressure (BP) displayed as a waveform and provides access for arterial blood sampling.


 

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Dec 2, 2016 | Posted by in ANESTHESIA | Comments Off on Standard ASA Monitors

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