What Intraoperative Monitoring Makes Sense? – )

Jay B. Brodsky

The routine practice of monitoring oxygenation, ventilation, circulation, and temperature during surgery is now the standard of care. However, with the possible exception of pulse oximetry and capnography, extensive physiologic monitoring has not been shown to reduce the incidence of adverse anesthestic-related events. Monitors are useful adjuncts, but they alone cannot replace careful observation by a vigilant anesthesiologist. (CHEST 1999; 115:101S-105S)

It was not until the first decade of this century that the surgeon Harvey Cushing suggested that BP be measured and recorded during surgery. He also recommended using a precordial stethoscope for continuous auscultation to monitor cardiac and respiratory function. Decades would pass before these radical ideas became widely accepted.

Until relatively recently, the anesthesiologist’s skill in recognizing and responding to clinical signs was of paramount importance. Intraoperative monitoring was limited to a BP cuff, an ECG, a stethoscope, and the eyes, ears, and touch of the anesthesiologist. The 1960s saw the introduction into clinical practice of invasive monitors. For the very first time, anesthesiologists were able to accurately measure and then manipulate physiologic variables in their patients. These monitors fostered the belief, still present and still largely unproved, that normalizing numbers is necessary and beneficial to our patients. We are now in an era characterized by widespread use of effective, relatively safe noninvasive monitors. Increasingly sophisticated instruments are used to measure both physiologic parameters in our patients and the performance of our equipment.

Invasive monitoring requires penetration of the skin or body by a needle, catheter, or surgical procedure, whereas noninvasive monitoring employs sensors that do not penetrate skin, mucosal membranes, or body orifices. In clinical practice, there is often a gradation of choices across the monitoring spectrum. For example, cardiac function monitors vary from noninvasive (ECG) to the invasive (central venous pressure line, pulmonary artery pressure line) with gradations of “invasiveness” in between (transesophageal echocardiography).

Technological enthusiasm, entrepreneurial involvement, and a sincere desire to know as much as possible about our patients has led to the view by many that “more is better.” They argue that additional information acquired from a extensive array of new monitors must surely benefit the patient. Until recently, there has been little evidence to support this hypothesis. Relatively small studies now suggest that a decrease in anesthetic risk can be associated with the use of certain new monitors (pulse oximetry, capnography).[1] Unfortunately, there is still no definite relationship between the number of variables monitored and the probability of satisfactory outcome.

In 1986, Eichhorn and his colleagues[2] at Harvard University described a number of adverse events related to anesthesia, and suggested that increased intraoperative monitoring might have improved patient outcome. The American Society of Anesthesiologists[3] (ASA) published guidelines for intraoperative monitoring that same year. As technology has evolved and practices have changed, revisions have become necessary. The current standards were last amended in 1996 (Table 1).

Table 1–ASA Standards for Basic Intraoperative Monitoring(*)


Standard 1: Qualified anesthesia personnel shall be present in

the room throughout the conduct of all general anesthetics,

regional and monitored anesthesia care

Standard 2: Oxygenation, ventilation, circulation, and

temperature shall be continually evaluated


Oxygen analyzer for inspired gases

Observation of the patient

Pulse oximetry



Observation of the patient

Observation of reservoir bag

End-tidal carbon dioxide analysis


Continuous ECG display

Heart rate and BP recorded every 5 min

Evaluation of circulation: auscultation of heart sounds,

palpation of pulse, pulse plethysmography, pulse oximetry,

intra-arterial pressure tracing


Core temperature and/or skin temperature

(*) The term continuously means prolonged without interruption while continually means repeated regularly and frequently.

Qualified anesthesia personnel must be present during all general, regional, and monitored anesthesia care to continuously monitor oxygenation, ventilation, circulation, and temperature.

When an anesthesia machine is used, the concentration of oxygen in the patient’s breathing system must be measured by an oxygen analyzer. Adequate illumination and exposure of the patient are also needed to assess skin color; in addition, during treatment with all anesthetics, a quantitative method for assessing oxygenation (pulse oximetry) must be employed.

Ventilation must be evaluated continually with observation of excursion of the chest and the reservoir breathing bag, and by auscultation of breath sounds.

Quantitative monitoring of carbon dioxide content and/or volume of expired gas is strongly encouraged. However, if an endotracheal tube or laryngeal mask airway is used, then correct positioning in the airway must be verified by continuous quantitative end-tidal carbon dioxide analysis using capnography, capnometry, or mass spectroscopy. When ventilation is controlled by a mechanical ventilator, continuous monitoring with a device capable of detecting disconnection of the breathing system must also be used.

Every patient having anesthesia must have a continuously displayed ECG and have arterial BP and heart rate determined and evaluated at least every 5 min. In addition, circulatory function must be evaluated continually by at least one of the following: palpation of a pulse, auscultation of heart sounds, monitoring of a tracing of intra-arterial pressure, ultrasound peripheral pulse monitoring, or pulse plethysmography. A means must also be available to measure the temperature.

These are minimum standards for every patient. Preexisting disease and complicating conditions differ for each patient, anesthetic agents and techniques are not the same, and surgeons and their ability to safely perform the same procedures vary widely. Therefore, selection of additional monitors may be needed for individual patients and all patients will not and should not be monitored the same way. Specialized expensive monitors such as transesophageal echocardiography or pulmonary artery pressure are required where there is preexisting medical disease, when special techniques such as controlled hypotension, hypothermia, or one-lung ventilation are employed, and when major physiologic insults such as massive blood loss are anticipated.

Initially it was fear of litigation that probably was the greatest force behind the rapid acceptance of routine monitoring in our operating rooms. Without any firm evidence of benefit, several state governments adopted health code regulations requiring specific monitors be used during the administration of general anesthesia. Extensive monitoring in turn has allowed the specialty of anesthesiology to have its malpractice liability reduced.[4] But reduction in insurance premiums is not by itself an accurate measure of patient outcomes.

The question that remains unanswered is “how much anesthesia morbidity and mortality can be prevented by use of state of the art monitoring?”

In 1954, Beecher and Todd[5] studied deaths associated with anesthesia. They reported an overall death rate of about 7 per 10,000 anesthetics. By the early 1960s, anesthesiologists had gained the ability to monitor and then support the respiratory and circulatory systems. However, Keats[6] noted that the published incidence of deaths with anesthesia as the primary cause 20 years after the report of Beecher and Todd[5] still ranged between 1.6 to 12 deaths/ 10,000 patients. By the 1980s, practice standards and new monitoring technology had been introduced and widely accepted, but overall anesthesia-related mortality does not seem to have changed dramatically.[7] The same complications that occurred before monitoring standards still continue to be problems despite the routine use of monitors (Table 2).

Table 2–Typical Anesthetic Mishaps


Airway/circuit integrity

Disconnect and/or misconnect

Intubation error (esophageal, endobronchial)

Airway obstruction

Gastric fluid aspiration


Operator error

Ventilator malfunction

Laryngospasm, straining

Anesthetic agents

Hypoxic mixture

Anesthetic overdose/underdose

Syringe swap/inappropriate drug

Allergic phenomenon/drug interaction

Residual drug effects (postoperative)

Intraoperative events





Acid/base imbalance

Malignant hyperthermia

Today, rather than lack of knowledge, it is failure to apply existing knowledge that is the most common denominator for adverse anesthetic events. Most preventable anesthetic incidents involve human error.[8,9] After human error, equipment problems are the most common cause of critical incidents in anesthesia. A critical incident is a human error or equipment failure that could have led (if not discovered or corrected) or did lead to an undesirable outcome. An undesirable outcome can range from prolonged hospital stay to death.[10]

One approach to the study of the role of monitors in preventing critical anesthesia incidents is to examine closed claims. In a closed claim study, relevant hospital and medical records, statements from the health-care providers, expert and peer review criticisms, and the cost of the settlement or jury award of an adverse anesthetic outcome are retrospectively examined by an expert anesthesiologist.[11]

A conclusion from the ASA Closed Claims Study for data from 1,175 analyzed claims was that the combination of pulse oximetry and capnography “could be expected” to help prevent anesthetic-related morbidity and mortality.[12] The conclusions from these retrospective analyses of adverse events that consider the concept of “could have been prevented” are speculative and based on assumptions. These conclusions assume that the monitors would be applied and used correctly, would function continuously, and the data obtained from the monitors would be interpreted and acted on appropriately.

Monitors were often used in the cases studied, yet adverse events still occurred. Eichhorn et al,[2] among the earliest and strongest proponents for monitoring standards, noted that at least 50% of the intraoperative accidents in their report were not preventable, even with implementation of monitoring.

The more monitors routinely used, the more dollars expended. Financial impact for each new monitor must be measured against benefit. The probability of benefit is situation dependent. The actual financial costs to the individual patient and to society of increased monitoring are unknown. We have no clear-cut means of distinguishing between what is cost-effective and what is not. All monitoring devices from the simplest to the most complex have both medical and financial costs.

We do have some data on the potential benefits vs cost for pulse oximetry. Both prospective[13,14] and retrospective[15] studies have reported increased detection of hypoxemia, hypoventilation, endobronchial intubation, and myocardial ischemia in patients monitored with a pulse oximeter. Since problems with airway ventilation leading to hypoxemia and hypoventilation are estimated to be responsible for about 30% of the preventable deaths and cases of brain damage in Australia, and since one death solely attributable to anesthesia occurs for every 26,000 cases in that country, the Australian Incident Monitoring Study estimated that routine use of oximetry can prevent one anesthetic death for each 78,000 operations. The cost of monitoring with an oximeter would add an additional $2 to each patient’s bill. Therefore, the authors concluded that one anesthetic death could be prevented for [is less than] $200,000.[16] This is an extremely small additional price to pay, especially if one considers the high cost of long-term care for a brain-damaged patient or the cost of a malpractice settlement resulting from a preventable anesthetic death.

Acquisition costs for monitors have decreased considerably over the past decade. For example, in 1988, the cost of a pulse oximeter ($5,000), automatic BP cuff ($2,300), and ECG ($4,500) totalled [is greater than] $12,000.[17] In 1998, a monitor combining all of those functions can be purchased for $3,500.

Besides the financial costs for each new monitor added, the hidden costs associated with their use must also be considered. These include the following: (1) distraction–the anesthesiologist’s and surgeon’s attention may be distracted by “false” positive data and ringing alarms; (2) false sense of security and relaxation of vigilance due to “false negative” data; (3) detraining effect–dependence on sophisticated monitors leads to loss of the recognition of physical signs and symptoms, with a shift of the anesthesiologist’s focus from the patient to the monitor; (4) medical legal implications–should the procedure be canceled if a special monitor is not available?; (5) increased time–time is required to train individuals to use and interpret the data generated by a new monitor, to set up and calibrate the monitor, and to affix the monitor to the patient; (6) risk of injury–monitors, sensors, and probes can burn, constrict, compress, crush, shock, and cut the patient. Tissue damage can result from a defective machine (electrical shock from an improperly grounded ECG) or from a normally functioning unit (skin burn from a transcutaneous electrode). Injuries can also be a consequence of insertion of an invasive monitor (for example, nosebleed from a nasopharyngeal temperature probe, arterial thrombosis from an intra-arterial catheter, cardiac perforation from a pulmonary artery catheter).

What intraoperative monitoring makes sense? The specific roles of several intraoperative monitors during general anesthesia were studied by analyzing 2,000 incidents reported to the Australian Incident Monitoring Study.[1,15] In [is greater than] 50% of incidents, at least one monitor detected the problem before clinical signs were apparent. It should be noted that in almost half the incidents, monitors initially failed to detect the problem.

The combination of pulse oximetry (27%) and capnography (24%) detected more than half of the incidents that were detected by monitors. The ECG (19%), various BP monitors (12%), low pressure circuit alarm (8%), and the circuit oxygen analyzer (4%) were also very helpful. The remaining monitors each detected [is less than] 1% of the incidents (Table 3).

Table 3–Routinely Used Intraoperative Monitors


Oxygen supply failure alarm: audible alarm when line pressure falls

below a minimum

Oxygen analyzer: audible alarm when fraction of inspired oxygen

falls below a minimum

High pressure alarm: audible alarm when circuit pressure above a


Low pressure alarm: audible alarm if preset pressure below a


Pressure gauge: alarms if patient not ventilated or has been subject

to high airway pressures for up to 5 min

Spirometer: measures minute ventilation

Gas analyzer: detects presence and percentages of gases present

Capnograph: audible alarm for apnea, hypocarbia, and/or


Pulse oximeter: audible alarm for oxygen saturation at preset levels

Pulse meter: audible beep for heart rate and rhythm (pulse

oximeter, or plethymosgraphy, or ECG)

ECG: audible beep; alarms when heart rate above or below preset


Intra-arterial BP

Noninvasive BP: audible alarms when BP above or below preset


Manual BP


Peripheral nerve stimulator


Using a theoretical analysis of the monitors used in this study, the authors suggested a priority for monitor acquisition for those with limited resources. The order was stethoscope, followed by sphygmomanometer, oxygen analyzer (if nitrous oxide is used), pulse oximeter, capnograph, high pressure alarm (if patients are mechanically ventilated), and a low pressure alarm (or spirometer with an alarm). They recommended that an ECG, a defibrillator, a spirometer, and a thermometer should also be available.[15]


To my knowledge, statistically valid data do not yet exist, perhaps as a consequence of the extremely large number of patients required to identify a change in the incidence of a specific adverse outcome.[18] However, few would question that the addition of physiologic monitoring (especially oximetry and capnography) has contributed to a significant decline in the number of catastrophic anesthetic events.[19] But increasingly sophisticated monitors do not ensure increased patient safety.[20] Monitors by themselves are not therapeutic–they require understanding and interpretation, after which a rational action must be undertaken. The difference between potential and actual benefit can be determined only by the user.

Although increased monitoring may reduce risk, other changes such as better preoperative preparation and maintenance during procedures, better anesthetic agents, and higher standards in anesthesia training programs may each play a significant role in reducing morbidity and mortality. However, the same tragedies that occurred before modern monitors (esophageal intubation, anesthetic overdose, etc) continue to occur. In many instances, the event can be recognized and treated without sophisticated monitors. Human vigilance cannot be delegated to an assortment of sensors and alarm systems.

All patients should not be monitored the same way. Individual needs must be identified. The ASA states that monitoring standards are intended to encourage quality of care, but observation of the guidelines by themselves is not a guarantee for a specific patient outcome.[3] Monitors are certainly a useful adjunct, but they cannot be a substitute for careful observation and common sense.


[1] Symposium–the Australian Incident Monitoring Study. Anaesth Intensive Care 1993; 21:501-695

[2] Eichhorn JH, Cooper JB, Maier WR, et al. Standards for patient monitoring during anesthesia at Harvard Medical School. JAMA 1986; 256:1017-1020

[3] American Society of Anesthesiologists. Standards for basic intraoperative monitoring. American Society of Anesthesiologists Newsletter 1986

[4] Zeitlan GL, Cass WA, Gessner JS. Insurance incentives and the use of monitoring devices. Anesthesiology 1988; 69:441

[5] Beecher HK, Todd DP. A study of deaths associated with anesthesia and surgery. Ann Surg 1954; 140:2-34

[6] Keats AS. What do we know about anesthetic mortality? Anesthesiology 1979; 50:387-392

[7] Cohen MM, Duncan PG, Pope WDB, et al. A survey of 112,000 anaesthetics in one teaching hospital (1975-1983). Can Anaesth Soe J 1986; 33:22-31

[8] Cooper JB, Newbower RS, Long CD, et al. Preventable anesthesia mishaps: a study of human factors. Anesthesiology 1978; 49:399-406

[9] Williamson JA, Webb RK, Runciman WB, et al. Human failure: an analysis of 2,000 incident reports. Anaesth Intensive Care 1993; 21:678-683

[10] Cooper JB, Newbower RS, Kitz RJ. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection. Anesthesiology 1984; 60:34-42

[11] Caplan RA, Posner KL, Ward RJ, et al. Adverse respiratory events in anesthesia: a closed claims analysis. Anesthesiology 1990; 72:828-833

[12] Tinker JH, Dull DL, Caplan RA, et al. Role of monitoring devices in prevention of anesthetic mishaps: a closed claims analysis. Anesthesiology 1989; 71:541-546

[13] Moller JT, Pedersen T, Rasmussen LS, et al. Randomized evaluation of pulse oximetry in 20,802 patients: I. Design, demography, pulse oximetry failure rate, and overall complication rate. Anesthesiology 1993; 78:436-444

[14] Moller JT, Johannesen NW, Espersen K, et al. Randomized evaluation of pulse oximetry in 20,802 patients: II. Perioperative events and postoperative complications. Anesthesiology 1993; 78:445-453

[15] Webb RK, Van Der Walt JH, Runciman WB, et al. Which monitor? An analysis of 2,000 incident reports. Anaesth Intensive Care 1993; 21:529-542

[16] Runciman WB. Qualitative versus quantitative research–balancing cost, yield and feasibility. Anaesth Intensive Care 1993; 21:502-505

[17] Whitcher C, Ream AK, Parsons D, et al. Anesthetic mishaps and the cost of monitoring: a proposed standard for monitoring equipment. J Clin Monit 1988; 4:5-15

[18] Eichhorn JH. Prevention of intraoperative anesthesia accidents and related severe injury through safety monitoring. Anesthesiology 1989; 70:572-577

[19] Pierce EC Jr. Monitoring instruments have significantly reduced anesthetic mishaps. J Clin Monit 1988; 4:111-114

[20] Moyers J. Monitoring instruments are no substitute for careful clinical observation. J Clin Monit 1988; 4:107-111

(*) From the Department of Anesthesiology, Stanford University School of Medicine, Stanford, CA.

Correspondence to: Jay Brodsky, MD, Department of Anesthesiology, H 3580, Stanford University Medical Center, Stanford, CA, 94305; e-mail: Jbrodsky@leland.stanford.edu

COPYRIGHT 1999 American College of Chest Physicians

COPYRIGHT 2000 Gale Group

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