Medical brain notes 7

  Introduction Imagine stepping into an operating room. You see a patient draped for the operation, the surgical team, the anesthesiologist, an anesthesia machine, a ventilator, one or more infusion pumps, bags with intravenous fluids, and a monitor with a screen full of curves and numbers. But the picture is not static. The people move, the bellows of the ventilator go up and down, the drip chambers of the infusion sets show drops of fluids, and on the monitor the ECG, blood pressure, SpO 2 , and capnographic patterns run across the screen. You behold this scene that presents an enormous amount of continuously changing data. You also hear the surgeon asking for an instrument, the scrub nurse saying something to the circula-tor, the anesthesiologist conveying to the surgeon information from the patient’s medical record, the ventilator puffing, and a monitor beeping. Depending on your experience, you will know how to interpret what your senses absorb. You can imagine the scene with ...

  Focused monitoring Our goals in monitoring the patient under anesthesia are driven by two consid-erations: ·             Are we ventilating the patient’s lungs optimally and giving just the right amount of drugs and fluids? In other words, we monitor so that we can titrate our ministrations to a conceptual optimum. ·             Do the data we gather from the patient and the equipment indicate potential danger or trends that require our intervention? In other words, for safety’s sake, we monitor variables that can indicate threatening problems, be they the consequence of anesthetic or surgical actions or based on the patient’s disease. Many signals we monitor subserve both titration and safety. For example, during anesthesia, we observe the patient’s response to electrical stimulation of a motor nerve (the “twitch monitor”) in order to  titrate  the administrati...

  Instruments that supplement clinical monitoring As we begin to focus on instruments to aid us in our monitoring task, we also need to ask for justification for their use. Does this monitor offer benefits that justify the cost (amortization of the instrument and cost of consumable supplies and, don’t forget, time needed for application) and the potential hazards inherent with the use of the monitor? Several instruments have been identified as essen-tial minimal monitors  always  to be used. With others, the clinician must decide whether a cost–benefit assessment justifies its use. Many monitors will be used routinely, others only with special indications. We must also point out that, over time, clinical practice changes with changing assessment of the value of this or that monitor. The American Society of Anesthesiologists has published Minimal Monitoring Standards for patients undergoing general anesthesia. 1  In brief, these standards call for the monitoring of th...

  Pulse oximetry An old saying goes:  The lack of oxygen not only stops the machinery, it wrecks it . Hypoxia of the brain first causes confusion, then coma, and eventually irreversible brain damage. Other organs follow that pattern, even though most can survive hypoxia longer than the brain. Thus, knowing whether arterial blood carries oxy-gen to the organs assumes great importance. Because oxyhemoglobin is red and reduced hemoglobin bluish, this color difference can be exploited to assess the oxygenation of blood. Clinically, we recognize cyanosis, but we cannot well grade the degree of bluishness. Enter pulse oximetry. The concept is what you might call “elegant.” A probe sends light impulses into a finger (or earlobe or nose or toe) and then collects the light that has passed through the tissue. The light comprises two wavelengths: one (infra-red) more likely to be absorbed by oxyhemoglobin, the other (red) by reduced hemoglobin. By rapidly (too rapid for the eye to recogn...

  The electrocardiogram Intraoperative electrocardiography does not draw on the full power of this sophisticated monitor. Instead of 12 leads, we usually settle for just three or five leads. A little ditty helps with remembering where to put the leads: White on right, red to ribs, and what is left over to the left shoulder. With five leads, we add a brown lead for the V 5 position (over the fifth rib in the anterior axillary line) and a green lead that goes to the right side and serves as a ground. The ECG leads can either be positive or negative, and the lead selector switch changes the polarity of the leads. Think of the negative lead as the exploring sensor. ·             Lead I looks across the chest: White on right shoulder is negative Black on left shoulder is positive Red on ribs is ground. ·             Lead II looks along the axis of the heart: White on right shoul...

  Capnography The delivery of carbon dioxide to the lungs depends first on the metabolic production of carbon dioxide. Capnography, therefore, says something about metabolism which may be depressed by cold or fired up during hyperthermia. Capnography depends on blood flow to the lungs. It therefore says something about circulation, specifically that regarding pulmonary blood flow. The delivery of carbon dioxide in the expired gas requires ventilation of alveoli and trans-port of alveolar gas to the outside. Capnography therefore says something about ventilation. Because the ambient air is free of carbon dioxide (well, not com-pletely free with only about 0.03% in air), the appearance of carbon dioxide in the inspired gas must mean that carbon dioxide is being added to the gas or that the patient is re-inhaling the carbon dioxide he just exhaled, for exam-ple from a breathing circuit with a defective valve that causes the dead space in the circuit to increase. Thus, capnography, the...

  Neuromuscular function Because we use neuromuscular blocking agents (muscle relaxants, for short) so frequently, we need to monitor the degree of relaxation. Clinical judgment goes a long way, but instruments can gauge the degree of relaxation and provide numer-ical assessment. For this purpose, we use a nerve stimulator that delivers short pulses of a direct current. We use two stick-on electrodes placed fairly close together (Fig.  7.3 ) over the course of a nerve (usually the ulnar nerve close to the wrist), and select one of several patterns of stimuli. Ideally, the current is well below the level to stimulate the muscle directly, as a healthy muscle will respond to strong, direct stimulation even in the presence of neuromuscular blocking agents. Thus, we are looking for maximal stimulation of the nerve only. Submax-imal stimulation of the nerve can induce variability of response and thus make it  impossible to tell whether an observed depression must be attributed ...

  Doppler and ultrasound The Doppler principle has been applied to monitoring in anesthesia. We can place a Doppler pencil probe over a vessel to identify blood flow or, with a broader emitter/receiver head, place it over the chest to detect the blood flow in the right  atrium.   When air appears in the blood flowing into the heart, it changes the reflective characteristics of the blood, easily detected by the Doppler signal, which is transformed into a swooshing noise. While ultrasound has been used for many years to spy on babies still in the womb and to view the functioning heart through the chest wall, more recently the equipment has been miniaturized into a finger-sized probe that views the heart from behind, through the wall of the esophagus (Fig.  7.5 ). This advance gives us a hands-free, relatively stable (and relatively non-invasive) view of the heart that does not impinge on the operative field. The technology continues to advance but currently allows view...

  The electroencephalogram and evoked responses In anesthesia, we expend much more effort in monitoring the cardiovascular and respiratory system than the nervous system, even though anesthesia is all about putting nervous function out of commission long enough to abolish awareness or at least the perception of pain. The reason for our bias against monitoring nervous activities is that we can afford to overdose the nervous functions and put them completely to rest as long as we continue to satisfy the basal needs for substrate and oxygen to brain and nerves. Hence, we worry more about the circulation than about the brain. However, when the systemic circulation is doing well but blood supply to all or parts of the brain or spinal cord is threatened, we need to monitor their function. Two methods are available: the electroencephalogram and evoked responses. The EEG as recorded by experts requires a montage with many electrodes. In anesthesia, that is not practical and in the operatin...

  Arterial catheter The ease with which a small catheter can be inserted into an artery, usually the radial, has caused many patients to be monitored with arterial catheters (often  called “lines” which is not an ideal term, as a line has no lumen, something arterial and venous catheters distinctly possess).   Before inserting a catheter into a radial (rarely the ulnar) artery, many clinicians like to check the patency of the volar arterial arch that connects radial and ulnar arteries. In the so-called Allen’s test, the hand is blanched, both arteries occluded by external pressure, then one occluded artery is freed. If now the entire hand, rather than the vascular bed of just one artery, turns pink, we accept the idea that the volar arch is patent and should one artery become obstructed by a clot or through damage to the intima, the other artery will prevent necrosis of fingers. For this and all other invasive pressure measurements, we use saline or heparin-filled non-com...