Summary
Patient monitoring is fundamental to the job of the anesthesiologist. Anesthetic drugs and surgical procedures can produce rapid changes in patient physiology and these changes may last throughout the perioperative period. It is essential that anesthesiologists have the capability to monitor these changes in real time to optimize patient safety and care throughout the perioperative period. The information acquired via various monitoring devices can be used to maintain quality patient care, but it does not guarantee any particular outcome. Multiple medical societies, including the preeminent anesthesiology society the American Society of Anesthesiologists (ASA), have agreed upon a standard set of monitoring devices referred to as the ASA standard monitors. Other monitoring devices and invasive monitoring methods may be used, in addition to the standard ASA monitors, depending on specific patient and intraoperative surgical concerns. It is important that anesthesiologists are aware of the variety of devices at their disposal, so that they may appropriately utilize those monitors to optimize and ensure quality patient care.
Introduction
Patient monitoring is fundamental to the job of the anesthesiologist. Anesthetic drugs and surgical procedures can produce rapid changes in patient physiology and these changes may last throughout the perioperative period. It is essential that anesthesiologists have the capability to monitor these changes in real time to optimize patient safety and care throughout the perioperative period. The information acquired via various monitoring devices can be used to maintain quality patient care, but it does not guarantee any particular outcome. Multiple medical societies, including the preeminent anesthesiology society, the American Society of Anesthesiologists (ASA), have agreed upon a standard set of monitoring devices referred to as the ASA standard monitors. Other monitoring devices and invasive monitoring methods may be used, in addition to the standard ASA monitors, depending on specific patient and intraoperative surgical concerns. It is important that anesthesiologists are aware of the variety of devices at their disposal, so that they may appropriately utilize those monitors to optimize and ensure quality patient care.
Standard Intraoperative Patient Monitoring
A basic standard of monitoring has been recommended by various societies, including the ASA, for any procedures performed with an anesthetic administered by an anesthesiologist. The common theme among these standard requirements is their ability to monitor a patient’s oxygenation, ventilation, circulation, and temperature. These four components provide a thorough, albeit not comprehensive, physiologic snapshot of a patient. During most procedures, these four components can be monitored via the standard ASA monitors. These include pulse oximetry to ensure adequate oxygenation, arterial blood pressure monitoring usually with a noninvasive blood pressure (NIBP) cuff and electrocardiography (ECG) to ensure adequate circulation, capnography to ensure adequate ventilation, and a thermometer to measure temperature. While these monitors are incredibly useful, they do not replace a vigilant clinician closely observing the patient throughout the procedure.
Pulse Oximetry
Pulse oximetry provides a significant amount of information with regard to oxygenation and circulation. The oximeter utilizes the differing light absorptions of oxyhemoglobin and deoxyhemoglobin to determine the overall oxygen saturation of the patient’s hemoglobin. Oxygenated hemoglobin absorbs more infrared light at approximately 940 nm, and deoxyhemoglobin absorbs red light at 660 nm. The oximeter is typically placed on a single digit or earlobe, the nasal septum, or another part of the patient’s body in which the oximeter can pulse light through the tissue and subsequently register it on the other side. As oxygenated blood pulses through the tissue between the oximeter, a ratio is generated via these differing absorptions to determine an approximation of the partial pressure of oxygen in the patient’s blood and the resulting oxygen saturation (see Table 4.1). The oxygen saturation is measured during arterial pulsations of blood, which also allows for a relatively accurate measurement of the pulse rate and rhythm.
Table 4.1 Partial pressure of arterial oxygen (PaO2) with blood oxygen saturation (SaO2) estimations
| PaO2 (mmHg) | SaO2 (%) |
|---|---|
| 100 | 100 |
| 95 | 97 |
| 85 | 96 |
| 75 | 95 |
| 60 | 90 |
| 50 | 85 |
| 40 | 75 |
| 30 | 60 |
Most pulse oximeters generate a plethysmographic waveform and emit a pitched tone with each pulsation that correlates with various levels of oxygenation. The waveform can demonstrate whether or not the probe is positioned appropriately, and a change in waveform may indicate decreased perfusion. A change in the pitch with a proper waveform may represent decreased oxygenation. This audiovisual tool helps anesthesiologists to both see and hear the patient’s rate, rhythm, and oxygenation while performing other tasks to address multiple patient care issues.
Of note, pulse oximetry may not always be accurate. Significant patient movement can disrupt wavelength monitoring and make it difficult for the oximeter to determine pulsations. Additionally, if the patient has altered forms of hemoglobin (that is, methemoglobin and carboxyhemoglobin), the oximeter will not provide a reliable representation of oxygen saturation. Carboxyhemoglobin will result in a falsely high reading, as it absorbs infrared light similarly to oxyhemoglobin. Methemoglobin will absorb light closer to 640 nm and consequently may decrease the average oxygenated hemoglobin reading registered by the oximeter. Thus, while pulse oximetry has enormous utility, its limitations and possibility for error should be kept in mind, so as to avoid misinterpretations of the patient’s physiology.
Blood Pressure Monitoring
Blood pressure monitoring is a direct measurement of circulation, and a blood pressure measurement within normal range typically represents adequate perfusion. For the vast majority of cases, NIBP monitoring every 3–5 minutes is adequate to ensure hemodynamic stability. There are multiple techniques for NIBP monitoring. The most common method is via oscillometry, which measures oscillations that occur with arterial pulsations after compression with a pressure cuff. Maximum oscillations occur at the point of the mean arterial pressure (MAP), and systolic and diastolic blood pressures are subsequently calculated based on the measured MAP and the measured oscillations. Oscillometry can be inaccurate if the patient does not have a sinus rhythm, if an inappropriate cuff size is used, or in cases of extreme hypertension or hypotension. Other forms of NIBP monitoring can be utilized and should be readily available if the automated oscillometer malfunctions or is unavailable. These include pulse palpation of the extremities with or without cuff occlusion to estimate systolic pressure and auscultation via a stethoscope while using a sphygmomanometer or a Doppler probe which measures sound waves from the traveling pressure wave in the artery.
During procedures that require beat-to-beat blood pressure monitoring, such as procedures that could result in significant blood loss or rapid blood pressure changes or procedures that require multiple instances of intraoperative blood sampling, intraarterial blood pressure monitoring can be utilized. The risks of placing an intraarterial catheter include bleeding, nerve damage, thrombosis, arterial dissection, aneurysm, hematoma, infection, and vascular compromise at the puncture site. The procedure is performed in a sterile fashion by use of a small-gauge catheter. Common sites of arterial cannulation are the radial, axillary, femoral, and dorsalis pedis arteries, secondary to their ease of access and ease of securing the catheter externally without dampening of the arterial pressure waveform. Other sites for cannulation include the brachial or ulnar arteries. The brachial artery is often avoided secondary to its proximity to the median nerve, as well as the lack of collateral blood flow should thrombosis or other injury to the artery occur. The ulnar artery has also historically been avoided, as it is considered the primary arterial supply to the hand and is often deeper and more tortuous than the radial artery, making it more difficult to access.
When intraarterial blood pressure monitoring is used, pressure transducers utilize mechanical energy from pressure changes within the artery to generate electrical energy and a digital signal that is then transferred to the computer monitor observed by the anesthesiologist. When the patient is in the supine position, the transducer should be placed at the level of the heart for an accurate MAP reading that determines cardiac perfusion and cerebral perfusion at the level of the circle of Willis. In cases where the patient’s position is altered, the transducer should be maintained in a position that allows for accurate measurement of the vital organs. Most commonly, when a patient is placed in an upright or sitting position, the level of the transducer should be elevated to coincide with the level of the circle of Willis to ensure adequate cerebral perfusion.
The appearance of the arterial waveform may vary with the different vessel sites, but the waveform has some standard components. As shown in Figure 4.1, the initial spike is the systolic upstroke and is a surrogate for left ventricular contractility. This spike is followed by a small dip and then a dicrotic notch, representing the end of systole and closure of the aortic valve. Lastly, a second drop in the waveform back to baseline is representative of the diastolic pressure and peripheral vascular resistance. Significant variability in the waveform with respirations or intraabdominal pressure changes can be indicative of decreased intravascular volume.
Figure 4.1 Standard intra-arterial blood pressure waveform and its correlation with electrocardiography. (1) Systolic upstroke followed by (2) the systolic peak pressure; (3) the systolic decline leading to the dicrotic notch (4); (5) represents the diastolic runoff ending at (6), the end-diastolic pressure.
Capnography, Gas Analysis, and End-Tidal Carbon Dioxide
While delivering any anesthetic, the ability to perform real-time inhalational gas analysis can be an important tool to monitor a patient’s ventilation, level of sedation, adequacy of analgesia, and airway patency while simultaneously indicating possible pathologies. Exhaled gas analysis is typically performed via infrared spectroscopy and is available with most anesthesia machines. Exhaled gas from a sidestream port passes through infrared light and the different components of that gas reflect or absorb infrared at different rates. This allows the sensor to determine what concentrations of gases are present. The composition of the expired air is then displayed as various percentages, which can inform the anesthesiologist how much nitrous oxide, volatile anesthetic gas, and carbon dioxide the patient is exhaling.
An oxygen analyzer is available with every anesthesia machine and is paramount to ensure that a patient does not receive a hypoxic gas mixture. Modern-day oxygen analysis occurs either via a galvanic fuel cell or, most commonly, via a paramagnetic oxygen analyzer, which takes advantage of the electron imbalance in the oxygen molecule to create a magnetic field. While most anesthesia machines will perform an oxygen calibration with an automated check, it is important to ensure that the machine is appropriately calibrated to the atmospheric oxygen level (normal 21%) prior to any procedure to ensure adequate measurements during the delivery of anesthetic.
Carbon dioxide analysis in the form of end-tidal carbon dioxide (ETCO2) can provide valuable information regarding the patient’s metabolic status, respiratory pattern, and ventilation. The gas analyzer can generate a waveform based on the quantity of carbon dioxide exhaled, known as a capnograph. Additionally, the ETCO2 can be used to monitor a patient’s respiratory pattern and assess clinical changes.
In a spontaneously ventilating patient, an increase in the number of respirations, as measured by ETCO2, can represent inadequate sedation or increased analgesic requirement. Similarly, a decrease in ETCO2 can represent oversedation or decreased ventilatory drive. In an intubated patient who is mechanically ventilated, the ETCO2 pattern can provide information regarding the adequacy of ventilation based on the waveform of the capnograph. For example, a “shark fin” waveform (see Figure 4.2) is indicative of an obstructive respiratory disorder, including acute bronchospastic disease or chronic obstructive pulmonary disease. Abrupt decreases in the capnograph waveform may be indicative of: a circuit disconnect or circuit leak; decreased perfusion in the setting of new-onset pulmonary embolism, heart failure, anaphylaxis, or cardiac arrest; or severe airway obstruction (that is, mucus plugging or severe multifocal bronchospasm). Changes in capnography, coupled with clinical observations, can guide and improve management.
