Anesthestic Monitoring (Anesthesia Text)

ASA Standards

Standard I: qualified anesthesia personnel

Standard II (“VOTC”): oxygenation, ventilation, circulation, temperature

  • Ventilation: ETCO2, inspired anesthetic gases
  • Oxygenation: SpO2 and inspired O2 (with an alarm)
  • Temperature
  • Circulation: HR, BP q5m, EKG


Blood Pressure

Indirect Techniques

When using the Korotkoff technique, the AHA recommends phase 5 (absence of any sound) as the diastolic pressure. The oscillometric method is only capable of measuring systolic pressure (cessation of oscillations) and MAP (point of maximal oscillations), the diastolic pressure must then be calculated. The dinamp device automates the oscillometric method, which overestimates diastolic blood pressure by ~ 10 mm Hg. The Finometer uses photoplethysmographic methods – a finger cuff is inflated to the point of maximal changes in light intensity, which occurs when transmural pressure is zero (i.e., arterial compliance is highest), and is highly accurate [Pickering TG.]

Direct BP Monitoring

Indications: 1) can’t get a cuff pressure 2) frequent blood analysis needed 3) tight blood pressure control is necessary 4) wide swings are expected 5) cardiopulmonary bypass

Multiple studies show that the risk of distal ischemia is < 0.1% after radial artery catheterization [Slogoff et. al.]. Acceptable sites include the radial artery, brachial artery, axillary artery, dorsalis pedis, and femoral artery

Intraoperative EKG[edit]

T wave is first sign of ischemia, followed by ST changes. Lead V5 alone will detect 75% of ischemic episodes in men 40 – 60 years of age [Whitcher et. al.], adding lead V4 increases this to 90%, and the combination of leads II, V4, and V5 add up to a 96% detection rate.


Echocardiographic response to passive leg raising may be a better indicator of response to volume therapy – Prospective study of 24 patients showed that a passive leg raising induced increase in stroke volume of 12.5% or more predicted an increase in stroke volume of 15% or more after 500 cc volume expansion with a sensitivity of 77% and a specificity of 100%, and that neither left ventricular end-diastolic area nor E/Ea predicted volume responsiveness [Lamia et. al.].

M-Mode: 1-D view, best used for determining velocity

B-Mode: 2-D view, giving a cross-sectional view of the heart

Pulsed-Wave: measures velocity using one crystal (for emission and reception), which has the advantage of localizing the tissue with which the velocity is measured, and the disadvantage of only being able to measure slow velocities (maximal Doppler shift measurable = 1/2 pulse repetition frequency, aka the Nyquist limit). Pulsed-wave Doppler also allows for color imaging (red = towards the transducer, blue = away)
Continuous-Wave: two crystals, allowing one to obtain information at higher velocities

Central Venous Pressure

Components of CVP Wave[edit]

“a wave” = atrial pressure. Vanishes in atrial fibrillation

“c wave” = closure of the tricuspid valve

“x descent” = ventricular systole

“v wave” = atrial filling / tricuspid closing

“y wave” = tricuspid opening

Validity of CVP and Volume Response

A systematic review of the literature (MEDLINE, Embase, Cochrane, and citation review), 24 studies met inclusion criteria, which included 803 patients, demonstrating a very poor relationship between CVP and blood volume as well as the inability of CVP/DeltaCVP to predict the hemodynamic response to a fluid challenge [Marik et al.]. Echocardiographic response to passive leg raising may be a better indicator of response to volume therapy – Prospective study of 24 patients showed that a passive leg raising induced increase in stroke volume of 12.5% or more predicted an increase in stroke volume of 15% or more after 500 cc volume expansion with a sensitivity of 77% and a specificity of 100%, and that neither left ventricular end-diastolic area nor E/Ea predicted volume responsiveness [Lamia et. al.].

Pulmonary Artery Catheter


Complication rate is only 0.5%, but can include dysrhythmias, catheter knotting, valve injury, and pulmonary artery rupture. Normal PAOP is 8-12 mm Hg, but increasing PAOP to 14-18 mm Hg can often increase cardiac output. Values > 18 mm Hg can cause dyspnea, at 20 mm Hg fluid begins to move into the alveoli, and > 30 mm Hg can cause frank pulmonary edema. Normal PvO2 is 40 mm Hg (saturation 75%).

Randomized, Controlled Trials of PA Catheter


Randomized (ESCAPE) trial of 433 congestive heart failure patients at 26 institutions, assigned to PAC vs. no PAC, goal of resolving clinical congestion, use of ionotropes was discouraged. Main outcome was 6 mo. mortality. Use of the PAC did not significantly affect the primary end point but in-hospital adverse events were more common among patients in the PAC group (21.9% vs 11.5%, p = .04) [Binanay et al.].

PAC-Man Trial

Randomized (PAC-Man) trial of 1041 patients from 65 ICUs in the United Kingdom, PAC vs. none. No difference in hospital mortality. 10% of PAC patients had a complication associated with their catheter, none of which were fatal [Harvey et al.].

French ICU Trial

676 patients in shock or ARDS, multicenter study in 36 French ICUs. No difference in mortality at 28 days or organ failure [Richard et al.].

Respiratory / Ventilation

Pulse Oximetry

Based on the Beers-Lambert Law (transmission = 10 ^ [-LA] = where L = length and A = absorbtivity). Pulse oximeters emit light at 660 nm (red) and 940 nm (infrared). Above SpO2 of 70% they are accurate to within ~ 3%. Measurement errors can be caused by methemoglobinemia (causes SpO2 to fixate at 85%), carboxyhemoglobin (values vary widely), methylene blue (SpO2 fixates at 65%), indigocyanine green, indigo carmine, patient movement, low blood flow, ambient light, shifts in the oxygen-hemoglobin curve, and nail polish

Cochrane Ratabase Review of Pulse Oximetry[edit]

A Cochrane database review examined the few randomised clinical trials of pulse oximetry have been performed during anaesthesia and in the recovery room which describe perioperative hypoxaemic events, postoperative cardiopulmonary complications and cognitive dysfunction. Two studies specifically addressed the outcomes in question [Bierman et al.] and [Moller et al.] both found no effect on the rate of postoperative complications using perioperative pulse oximetry.

Additional Studies

Two other studies [Moller et al.] and [Moller et al.] used hypoxemia to assess the value of perioperative monitoring (few clinical outcomes were given) and found that hypoxemia was reduced in the pulse oximetry group both in the operating theatre and in the recovery room. In the first study, postoperative cognitive function using the Wechsler memory scale and continuous reaction time was independent of perioperative monitoring with pulse oximetry, and in the second, postoperative complications occurred in 10% of the patients in the oximetry group and in 9.4% in the control group [Pedersen et al.].

CO2 Monitoring

Uses of CO2 monitoring

Uses of CO2 monitoring:

  1. Verifying ventilation: ALWAYS keep in mind that an esophageal intubation can produce CO2 for up to three tidal volumes
  2. Estimate of PaCO2: healthy people have 2-3% dead space and their ETCO2-PaCO2 gradient is 0.6 mm Hg according to Miller (likely a typo, should be 6 mm Hg), 5-10 mm Hg according to Barash
  3. Evaluation of dead space. Shunting has no effect on the ETCO2-PaCO2 gradient as shunted blood simply does not see the lungs. Dead space does decrease ETCO2 because the air trapped in “dead” regions does not exchange CO2, and mixes with perfused lungs during exhalation, lowering ETCO2 and increasing the gradient. Common causes of increased gradients include (causes of dead space) emboli, hypoperfusion, and COPD.

Anesthetic Gases

3.3 um monochromatic infrared light is passed through inhalational gases – because the absorption spectrum of inhalational agents is similar using this wavelength of monochromatic light, correct identification of the gas is essential. If a polychromatic infrared detector is used (7-13 um), the monitor can automatically detect the anesthetic agent used, as the absorption spectra at these wavelengths are relatively different.


On average, core temperature drops 1-1.5C in the first hour. Mild hypothermia can delay recovery from anesthesia.

Temperature and Cardiac Events

Traditionally it was thought that shivering associated with hypothermia could cause increased SBP, HR, and myocardial oxygen consumption, leading to ischemia. Initially, this was based only on retrospective studies suspicious for bias, however in 1997 Frank et. al., in a randomized, prospective study, demonstrated that high-risk patients assigned to only 1.3°C core hypothermia were three times as likely to experience adverse myocardial outcomes [Frank et al.; three hundred patients undergoing abdominal, thoracic, or vascular surgical procedures, randomized to 1.3°C of hypothermia vs. normal temperature, with the hypothermic group showing 3x as many cardiac morbid events]

Temperature and Infection

Temperature also affects infection rates – in 1996, Kurz et al. published the results of a randomized controlled trial examining the effects of hypothermia on the incidence of SSI. 200 patients undergoing colorectal surgery were randomized to either standard intraoperative thermal care (the hypothermia group) or additional warming (the normothermia group). A small, ~2°C difference in core temperature resulted in a 3-fold higher incidence of SSI in the hypothermia group (19% vs. 6%, p = 0.009) [Kurz et al.].

Temperature, Coagulation, and Wound Healing

Coagulation times and wound healing are impaired by low temperatures [Sessler D]. Proposed non-infectious consequences of hypothermia include increased duration of hospitalization, transfusion requirements, and likelihood of cardiac morbidity [Sessler D].

The best monitoring site is the PA catheter, after which is the bladder (rectum >> axillary but neither are as good as the bladder).


Sensory Evoked Potentials

Described in terms of site of origin (stimulus), latency, and amplitude.


Generally originate near the median/ulnar nerves or posterior tibials. Recording electrodes are on the scalp or spinal cord. Note that volatile anesthetics increase SSEP latency and decrease SSEP amplitude – nitrous oxide decreases SSEP amplitude but does not affect latency [Banoub et. al. Anesthesiology 99: 716, 2003]. The threshold for usefulness of SSEPs during volatile anesthesia is at ~ 0.5 – 0.75 MAC. Barbiturates, benzodiazepines, and opiates may interfere with SSEPs but to a much lesser extent than volatile anesthetics. SSEPs are controversial because their sensitivity is unestablished – it is clear that SSEPs showing prolonged increase in latency can be associated with severe neurologic injury, however the actual threshold (both in terms of duration and amount of latency) is not known [Kumar et. al. Anaesthesia 55: 225, 2001]

How Changes in Physiology Affect SSEPs

Temperature, SBP, PaO2, and PaCO2 all affect SEPs and must be controlled during surgery [Baoub et. al. Anesthesiology 99: 716, 2003]. Room temperature irrigation fluids can also affect SSEPs, thus body temperature fluids should be used for irrigation in neurosurgical cases


Similar to SSEPs, visual evoked potentials are highly sensitive to the use of anesthetic agents


More resistant to anesthetic influences than SSEPs and VEPs. According to Barash, a 1 ms increase in latency while the anesthetic regimen is held constant is considered significant.

Selection of Agent for Use With SEPs

All volatile agents depress evoked potentials. Nitrous oxide added to other volatile agents profoundly depresses the amplitude of both SSEPs and VEPs. Barash recommends using TIVA or at least above average IV opiates because they produce minimal changes in SEP waveforms. Dexmedetomidine can be added to the anesthetic regimen and will reduce MAC requirements while having essentially no effect on SSEP amplitude. That said, sevoflurane and desflurane have less effect on SEPs than earlier anesthetic agents, with Barash stating that desflurane

Motor Evoked Potentials

Not widely practiced – requires placement of a stimulating scalp electrode or magnetic coil (requires lower voltages) and an intramuscular recording electrode. MEPs are both difficult to obtain and have questionable accuracy. Still, they are sometimes used for intramedullary spinal tumors, scoliosis surgery, and intracranial tumors near the motor strip. Subject to the same effects on physiologic derangements as SEPs. Also profoundly affected by volatile anesthetic agents, less-so by nitrous oxide. Opiates have almost no effect. Nitrous oxide/opiate techniques have been successful with MEPs. Full paralysis makes the MEP essentially useless, however a continuous IV infusion titrated to 1-2 twitches will allow accurate MEP use

Bispectral Index

Intraoperative awareness is thought to occur in as many as 1:500 anesthetics, thus the BIS was developed as an additional tool – values from 45-60 are generally considered optimal. A Cochrane review concluded that BIS within 40 to 60 may improve anaesthetic delivery and postoperative recovery from relatively deep anaesthesia, and more important that BIS-guided anaesthesia significantly reduces the incidence of intraoperative recall in surgical patients with high risk of awareness (ex. TIVA) [Punjasawadwong et al.]. The largest study in Cochrane was Myles et. al. (2463 patients), the second-largest was only 268 patients

Myles: in favor of BIS

Randomized, double blind trial of 2463 “high risk” patients (caesarean section, high-risk cardiac surgery [EF <30%, cardiac index <2·1 L/min per m2, severe aortic stenosis, pulmonary hypertension, or undergoing off-pump coronary artery bypass graft surgery], acute trauma with hypovolaemia, rigid bronchoscopy, significant impairment of cardiovascular status and expected intraoperative hypotension requiring treatment, severe end-stage lung disease, past history of awareness, anticipated difficult intubation where an awake intubation technique was not planned, known or suspected heavy alcohol intake, chronic benzodiazepine or opioid use, or current protease inhibitor therapy, found two reports of awareness in the BIS-guided group (goal 40-60) and 11 reports in the routine care group (p=0.022), ie BIS-guided anaesthesia reduced the risk of awareness by 82% (95% CI 17-98%), although there was no overall difference in MAC [Myles et al.].

Avidan: against BIS

That said, BIS has received several criticisms. Most damaging are Avidan’s studies, the first of which included 2000 patients and showed no difference in awareness or use of volatile gas (note, however, that no patient had definite or probable awareness if BIS was kept under 50 and MAC > 1.0, suggesting that a combination of BIS < 50 and MAC > 1.0 may be a better threshold) [Avidan et al.]. Avidan’s study only included “high risk” patients (1 major or 2 minor criteria) – the major criteria were preoperative use of anticonvulsant agents, opiates, benzodiazepines, or cocaine, EF < 40%, history of awareness, history of difficult intubation or anticipated difficult intubation; ASA IV or V, aortic stenosis, end-stage lung disease, marginal exercise tolerance, pulmonary hypertension, planned open-heart surgery; and daily alcohol consumption, and the minor criteria were preoperative use of B-blockers, COPD, moderate exercise tolerance, smoking two or more packs of cigarettes per day, and BMI > 30. A follow-up, and even larger study by Avidan et al. showed nearly identical results – no difference in awareness between BIS-guided and ETAG guided anesthetic management in over 6000 high risk patients [Avidan et al.].

Additionally, it is not known whether a low BIS value in a head-injury patient can be relied upon.