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Although some might consider electronic devices the only cardiovascular monitors, the fundamental basis for circulatory monitoring remains in the eyes, hands, and ears of the anesthesiologist. In many ways, the anesthesiologist's senses capture more information than even the most sophisticated electronic monitors. Combined with knowledge, experience, and sound clinical judgment, the physician's senses offer an integrated, panoramic view of the patient's condition, made even more valuable by an understanding of the clinical context present. Whereas instruments accurately and monotonously collect volumes of quantitative data, the clinician integrates, evaluates, and interprets the data and, in so doing, provides the most important aspect of patient monitoring.[1] Just as inspection, palpation, and auscultation are the cornerstones for standard physical examination of the cardiovascular system, these procedures are fundamental elements of perioperative cardiovascular monitoring. However, they must be adapted and focused on the unique requirements of surgical and critically ill patients, and their limitations must be recognized.
Many standard physical means of assessing the circulation are used throughout an operation. For many healthy patients undergoing minor procedures, these physical signs may provide a considerable fraction of the total cardiovascular monitoring. When the operation becomes more complex or the patient comes to surgery with more advanced or unstable cardiovascular disease, the extent of supplemental electronic monitoring grows accordingly. However, careful physical assessment still provides the clinician with an all-important backup system to confirm or refute information derived from other monitoring devices. The most obvious, perhaps trivial example is a patient whose electrocardiogram (ECG) shows asystole. Detection of a normal pulse by direct palpation focuses the anesthesiologist on correcting the monitoring artifact rather than initiating cardiopulmonary resuscitation. The clinical examination provides redundant monitoring for patient safety; the heart rate is monitored continuously by the ECG and confirmed when required by intermittent palpation of the pulse.
Early advances in cardiovascular assessment beyond the methods of basic physical examination were not readily accepted into medical practice. Around 1900, the introduction of sphygmomanometry to measure blood pressure was criticized because it would weaken clinical acuity by blunting the senses and acute perceptions of the clinician. Even stethoscopy had its critics, simply on the basis that it placed the physician at greater distance from the patient.[2] Today, these concerns appear foolish considering our patient care responsibilities in unusual environments such as magnetic resonance scanners, where remote monitoring is required. Nonetheless, there can be little doubt that increased reliance on electronic monitoring devices has diminished our fundamental clinical skills of physical assessment.
Palpation of the pulse and its rate and character should not be forgotten in the perioperative setting. Unique constraints in the operating room may dictate which pulses are accessible for monitoring. An anesthetist standing at the patient's head during induction of anesthesia can easily palpate the carotid artery, the superficial temporal artery anterior to the ear, or the facial artery at the mandible. When only an arm is accessible, the axillary, brachial, radial, and ulnar pulses may be examined. Even procedures necessitating that the anesthetist be positioned near the patient's feet permit palpation of femoral, popliteal, dorsalis pedis, and posterior tibial pulses. Many of these vessels also serve as suitable sites for sampling arterial blood or direct arterial cannulation for pressure monitoring. Perhaps most important, one should consider the unique opportunities for pulse monitoring during surgery rather than just the constraints of this environment. During cardiac surgery, the beating heart may be observed directly, and palpation of the ascending aorta by the surgeon provides a useful estimate of aortic blood pressure. In fact, the surgeon's evaluation of any arterial pulse within the surgical field should be considered whenever severe hemodynamic instability develops.
Clinical evaluation of venous pressure during anesthesia is technically difficult because of unusual patient positions, positive-pressure mechanical ventilation, and lack of access to observe the neck veins. However, early recognition of venous obstruction in an extremity or the head and neck may have important clinical consequences. Not only might this observation herald hemodynamic problems for the patient, but communicating these observations to the operating surgeon should also allow timely correction of associated technical problems.
Given that the cardiovascular system is responsible for the transport of substrates and by-products to and from all organ systems, monitoring end-organ function reflects the adequacy of performance of the cardiovascular system. Inspection of mucous membranes, skin color, and skin turgor can reveal pertinent clues about hydration, oxygenation, and perfusion. Additional simple clinical techniques include empirical estimation of fluid deficits and measurement of intraoperative blood loss. Decreased urine output may indicate hypovolemia or reduced cardiac output, and altered mental status may be a sign of inadequate cerebral perfusion. Unfortunately, preexisting end-organ dysfunction may confound interpretation of these simple physical signs and measurements. Furthermore, drugs may alter organ function directly. For example, general anesthesia makes it impossible to monitor mental status or sensory or motor function by physical examination, and diuretic therapy obviates monitoring urine output as an indicator of overall systemic perfusion. As a result, simple clinical assessment of end-organ function is of limited value in many anesthetized or critically ill patients, so additional cardiovascular monitoring techniques are required.
Although Laennec is credited with introducing the stethoscope into general medical practice in 1818, nearly a century elapsed before Harvey Cushing proposed in 1908 that the stethoscope be used as a routine continuous cardiopulmonary monitoring device during surgery.[3] Today, intraoperative monitoring with either a precordial or an esophageal
Stethoscopy provides a simple and reliable means of listening to heart and breath sounds continuously throughout an operation. The most common equipment used for precordial stethoscopy consists of a heavy metal bell or accumulator attached to a length of rubber or plastic extension tubing and a custom-molded monaural plastic earpiece. Electronically amplified stethoscopes have been designed in an attempt to improve the quality and clarity of heart and breath sounds, and wireless systems using radio-transmitted signals allow continuous monitoring without the anesthetist being tethered to the patient by the stethoscope extension tubing.[6] [7] [8] However, stethoscopes with these electronic modifications have not supplanted the standard inexpensive mechanical device in everyday practice.
Though minimally invasive and practical only for a patient who is undergoing general endotracheal anesthesia, the esophageal stethoscope provides monitoring benefits not available with its precordial cousin. Clear breath sounds and distinct heart sounds are audible in most patients with the stethoscope tip positioned 28 to 30 cm from the incisors.[9] Esophageal temperature can be measured with a thermistor incorporated in the tip of the stethoscope. Specially configured stethoscopes permit transesophageal ECG recording, which may be useful in diagnosing atrial arrhythmias, right ventricular ischemia, or posterior left ventricular ischemia.[10] [11] The esophageal stethoscope may also be a valuable therapeutic tool for treating intraoperative sinus bradycardia or junctional rhythm. For this special application, transesophageal atrial pacing can be accomplished with a stethoscope equipped with bipolar pacing electrodes on its outer surface.[12] [13] Although use of the esophageal stethoscope is generally without significant risk, it has been associated with occasional complications, including hypoxemia from unintended tracheobronchial placement or compression of the membranous posterior portion of the trachea in small infants, loss down the esophagus, detachment of the acoustic cuff, and distortion of surgical anatomy in the neck. [14] [15] [16] Placement of the esophageal stethoscope may also cause pharyngeal or esophageal trauma and interfere with nasogastric tube positioning or transesophageal echocardiography.
Despite the apparent value of the precordial or esophageal stethoscope for basic monitoring of patient safety, its widespread application in clinical practice has diminished in recent years.[15] [17] This decline may be driven by the routine use of pulse oximetry, capnography, and other electronic safety monitors that have become ubiquitous and even required by law in some locations. As a practical matter, clinicians may not recognize the loss of heart or breath sounds as readily as might be expected because of reliance on more modern electronic devices or the distractions and noise pollution inherent in the operating room. These practice patterns appear to be borne out by the 1993 Australian Incident Monitoring Study, which noted that a stethoscope was used in only 5% of the 1256 critical incidents reported. Furthermore, it was the first monitor to detect the morbid incident—cardiac arrest—in only one instance.[17]
The current role of intraoperative stethoscopy may be summarized as follows:
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