Anesthesiology clinics
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Anesthesiology clinics · Dec 2006
Intraoperative monitoring with transesophageal echocardiography: indications, risks, and training.
There are benefits and risks to the use of TEE. The benefits are derived from the physiologic information that TEE provides, which may not be as readily obtained by any other technique. The risks of TEE are those related to mechanical trauma from the probe, as well as those of an incorrect TEE interpretation by the echocardiographer. ⋯ The authors have discussed some of the issues involved with each class-I indication. Performance of the TEE is not an end in itself and should not distract the anesthesiologist from the primary goal of patient care. With proper training and experience, the anesthesiologist may learn how to use TEE to improve patient care.
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Traditionally, hypothermia has been thought of and used perioperatively as a presumptive strategy to reduce cerebral and myocardial tissue sensitivity to ischemia. Evidence, however, is mounting that maintenance of perioperative normothermia is associated with improved outcomes in patients undergoing all types of surgery, even cardiac surgery. Ambient environmental temperature is sensed by free nerve endings in the dermal and epidermal layers of the skin, which are the axonal extensions of thermosensitive neurons found in the dorsal root ganglia. ⋯ Heating intravenous fluids does not warm patients, but does prevent fluid-induced hypothermia in patients given large volumes of fluid. This article examined the evolutionary adaptations people possess to combat inadvertent hypothermia and hyperthermia. Because thermal disturbances are associated with severe consequences, the standard of care is to monitor temperature during general anesthesia and to maintain normothermia unless otherwise specifically indicated.
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Depth-of-anesthesia monitoring with EEG or EEG combined with mLAER is becoming widely used in anesthesia practice. Evidence shows that this monitoring improves outcome by reducing the incidence of intra-operative awareness while reducing the average amount of anesthesia that is administered, resulting in faster wake-up and recovery, and perhaps reduced nausea and vomiting. As with any monitoring device, there are limitations in the use of the monitors and the anesthesiologist must be able to interpret the data accordingly. ⋯ Several monitoring devices are commercially available. The BIS monitor is the most thoroughly studied and most widely used, but the amount of information about other monitors is growing. In the future, depth-of-anesthesia monitoring will probably help in further refining and better understanding the process of administering anesthesia.
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As outlined in Table 1, the nonthermodilution techniques available to measure cardiac output are noninvasive and clinically applicable to a variable degree. The truly noninvasive monitors are bioimpedance and CO2 re-breathing. The latter, however, requires the patient to be intubated, and the former continues to be evaluated with regard to correlation with the thermodilution standard. ⋯ Pulse contour analysis requires an arterial catheter, and two of the three available monitors require external calibration, while the third has not been validated adequately. The reader can see that all four approaches continue to be refined, with new analysis algorithms and monitors continuing to appear on the market. In the absence of true tissue oxygenation monitors, it seems likely that some or all of these alternatives to thermodilution will play a greater role in the care of patients where measurement of cardiac output is desired.
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Pressure monitoring systems influence the contour of the displayed wave-forms and, on occasion, can introduce significant artifact in the pressure traces. It is important to understand the technical details of invasive pressure monitoring to interpret better the information presented. Careful observation of the arterial pressure waveform can provide information about ventricular function, the arterial system, and ventricular preload. ⋯ CVP monitoring is also used to assess intravascular volume, but this measurement is significantly influenced by ventricular compliance and intrathoracic pressure. Under most clinical circumstances, a trend in CVP values or its change with therapeutic maneuvers is more reliable than a single measurement. Like arterial pressure waveforms, CVP waveform morphology can provide important information about clinical pathophysiology.