Michael C. Damask

Rapid increase in desflurane concentration is associated with greater transient cardiovascular stimulation than with rapid increase in isoflurane concentration in humans.

BACKGROUND Increases in desflurane and isoflurane concentrations can transiently increase arterial blood pressure or heart rate or both during induction of anesthesia. The current study tested the hypothesis that a rapid increase of desflurane concentration in humans increases sympathetic activity and hormonal variables and heart rate and arterial blood pressure more than does an equivalent increase in isoflurane concentration. METHODS Twelve healthy male volunteers were assigned randomly to receive desflurane and on a separate occasion isoflurane. After induction of anesthesia with propofol 2 mg/kg, anesthesia was maintained at 0.55 MAC (desflurane, 4.0%; isoflurane 0.71% end-tidal) for 32 min. Mechanical ventilation maintained normocapnia throughout anesthesia. Mean arterial blood pressure and heart rate were recorded continuously, and arterial blood was sampled for plasma catecholamine and vasopressin (AVP) concentrations, and plasma renin activity. Anesthetic concentration was increased rapidly to 1.66 MAC (desflurane, 12.0%; isoflurane 2.12% end-tidal), and maintained at this concentration for 32 min, and then rapidly decreased to and maintained at 0.55 MAC for an additional 32 min. RESULTS Neither anesthetic produced sympathetic or cardiovascular stimulation during their initial rapid wash-in to 0.55 MAC. The rapid increase to 1.66 MAC increased mean arterial blood pressure, heart rate, and plasma epinephrine and norepinephrine concentrations, and plasma renin activity with both desflurane and isoflurane, the former usually producing a response of greater magnitude than the latter. Plasma AVP concentration increased with desflurane only. Increased mean arterial blood pressure returned to control in 4 min. Heart rate decreased 50% of the difference between its peak and the value at 32 min at 1.66 MAC in 2 min with desflurane and in 4 min with isoflurane but did not return to the value at 0.55 MAC with either anesthetic. With desflurane, plasma epinephrine and AVP concentrations decreased quickly from their peak values, remaining elevated for 8 min. Decrease of concentrations of desflurane and isoflurane from 1.66 MAC to 0.55 MAC rapidly decreased heart rate and increased mean arterial blood pressure with both anesthetics. Thirty-two minutes after return to 0.55 MAC, with both anesthetics, only heart rate remained increased relative to the values at 32 min of the initial period of 0.55 MAC anesthesia. CONCLUSIONS In healthy male volunteers, rapid increases of desflurane or isoflurane from 0.55 to 1.66 MAC increase sympathetic and renin-angiotensin system activity, and cause transient increases in arterial blood pressure and heart rate. Desflurane causes significantly greater increases than isoflurane, and also causes a transient increase in plasma AVP concentration. The temporal relationships suggest that the increased sympathetic activity increases mean arterial blood pressure and heart rate, with mean arterial blood pressure also increased by increased plasma AVP concentration, whereas the delayed, increased plasma renin activity is likely a response to the ensuing hypotension, or earlier inhibition by AVP, or both.

A systematic method for validation of gas exchange measurements.

The measurement of gas exchange is useful, but thus far, has not been practical during the mechanical ventilation of critically ill patients. To validate two new commercial instruments, (Siemens-Elema Servo Ventilator 900B, Beckman Metabolic Cart), the authors constructed a lung model into which they delivered CO2 and N2 at precise rates to simulate Co2 production (Vco2) and O2 consumption (Vos). The model consists of 13.5-1 gas jar with an attached one liter anesthesia bag. The lung model was ventilated at present tidal volumes and frequencies. The authors also compared the measured respiratory quotient (RQ) with the known RQ of burning methanol (RQ = 0.67) in the jar. When the model was ventilated with levels of tidal volume and gas exchange applicable to adults, both instruments measured V02 within 5 to 13% of predicted values. Varying the FI02 did not significantly affect this accuracy. At tidal volumes below 350 ml, the difference increased between predicted VCO2 and measured VCO2. The difference between measured vs. the actual RQ of methanol was 5 and 1.5% in the Siemens-Elema and Beckman Systems, respectively.

Artifacts in measurement of resting energy expenditure

Measurements of gas exchange have been demonstrated to be clinically useful in the care of critically ill and malnourished patients. Using principles of indirect calorimetry, resting energy expenditure (REE) can be calculated from gas exchange data and used as the basis for designing a nutritional support regimen as well as for following the patients metabolic state. This study demonstrates that a relatively minor procedure, such as percutaneous muscle biopsy, can induce temporary but major increases in gas exchange and lead to an overestimation of REE. Four studies were performed on 3 healthy adult subjects admitted to the Surgical Metabolism Unit for nutritional study. A percutaneous muscle biopsy was performed with the subject inside a canopy with continuous recording of oxygen consumption (VO2) and carbon dioxide production (VCO2). After the muscle biopsy, VCO2 and VO2 increased 93 and 103% (at their peak value), respectively. The mean duration that these changes persisted at least 15% above control was 10.6 +/- 7.8 (SD) and 11.4 +/- 5.9 min of VCO2 and VO2, respectively. Thus, considerable artifacts in the estimation of REE can occur due to painful stimuli.

Response to Tubular Airway Resistance in Normal Subjects and Postoperative Patients

Critically ill patients must often breathe spontaneously through an endotracheal tube that acts as a fixed inspiratory and expiratory tubular airway resistor. Although this practice is common, its effect on the pattern of breathing is not known. The mean breathing patterns of seven normal, healthy male subjects and eight male patients who had undergone upper abdominal surgery 2–4 days previously were studied breathing through a mouthpiece fitted in random order with a 5, 6, 7, 8, or 15 mm diameter (17 mm long) resistor. These diameters were selected because they simulate the pressure-flow relationships of adult endotracheal tubes. With the 15 mm aperture, the patients had a greater breathing frequency (f) than did the normal subjects (21 ± 5 [SD] vs. 14 ± 4 breaths/min, P < 0.01) as well as a smaller mean tidal volume (VT). In both groups, minute ventilation (VE) and f progressively decreased as resistance was increased by decreasing the aperture size from 15 to 6 mm. In the normal subjects, but not the patients, &OV0312;T also progressively decreased. When the diameter was decreased from 6 mm to 5 mm, there were increases in VT and decreases in f that were more marked in the normal subjects. In both groups, the changes in VE were accompanied by decreases in mean and peak inspiratory and expiratory flow rates. Throughout the study, oxygen consumption (&OV0312;O2) and carbon dioxide production (&OV0312;CO2) did not change. This, coupled with the decreases in &OV0312;E resulted in decreases in the ventilatory equivalents to CO2 and O2 (&OV0312;E/VCO2, VE/VO2). This study demonstrates that increasing tubular airway resistance significantly alters respiratory pattern without affecting gas exchange. When interpreting the breathing pattern of patients whose trachea is intubated, the clinician should consider the effects of the fixed tubular resistance. These results also reinforce previous admonitions that in patients with respiratory dysfunction, it is important to use as large an endotracheal tube as possible and allow spontaneous respirations only once substantial clinical improvement has occurred.

General versus epidural anesthesia for femoral-popliteal bypass surgery☆

This study examines whether epidural anesthesia is more effective than general anesthesia using an inhalation agent in controlling cardiovascular responses during femoral-popliteal bypass surgery. Nineteen patients were randomized into two groups: general anesthesia (n = 10) and epidural anesthesia (n = 9). The patients who underwent general anesthesia received sodium pentothal and succinylcholine for induction of anesthesia and 60% N2O, 40% O2, and 1% to 1.5% isoflurane for maintenance. Fifteen minutes before extubation, the patients received morphine sulfate 0.05 mg/kg intravenously (IV). The group that underwent epidural anesthesia received anesthesia to T-10 (through a catheter placed in the L4-5 interspace using 3% 2-chloroprocaine). Thirty minutes after the last dose, 0.05 mg/kg IV was administered. Hemodynamic variables were recorded at selected intervals during the operation and for 60 minutes in the recovery room. In the general anesthesia group, mean arterial pressure (MAP) and rate pressure product (RPP) significantly decreased (p less than 0.05) during the operation as compared with preoperative values. Following intubation and skin incision, 5 minutes after extubation, and after 60 minutes in the recovery room, MAP, heart rate (HR), and RPP were significantly greater (p less than 0.05) as compared with intraoperative periods. In the epidural anesthesia group, there were clinically important decreases in MAP and RPP after reaching T-10 and skin incision. The general anesthesia patients showed higher MAP, HR, and RPP 5 minutes after extubation and after 60 minutes in the recovery room. Epidural anesthesia patients showed stable hemodynamic patterns throughout the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Is sufentanil removed by blood conservation devices

To conserve blood during open heart surgery, cell savers and hemoconcentrators are used. Cell savers retrieve and filter shed blood from the operative field and then wash and separate reconcentrated erythrocytes from a supernatant by centrifugation. Hemoconcentrators are extracorporeal devices that extract an ultrafiltrate from the circulating perfusate during cardiopulmonary bypass. Both cell saver supernatant and hemoconcentrator ultrafiltrate are discarded. Twenty patients were anesthetized with a single dose of sufentanil, 30 microg/kg, and the cell saver supernatant and hemoconcentrator ultrafiltrate were analyzed for sufentanil. The supernatant contained only 0.1% of the total administered dose. Hemoconcentrators from two different manufacturers were tested, and 0.1% of the administered sufentanil was detected in one ultrafiltrate and none was found in the other. Thirty minutes after induction of anesthesia, the plasma sufentanil concentration was 8.5 ng/mL (1.3% of the given dose); 1 hour later, it was 4.9 ng/mL (0.8%). During cardiopulmonary bypass, the plasma level decreased to 2.5 ng/mL (0.6%); after bypass, it fell to 1.5 ng/mL (0.3%). It is concluded that intravenous (IV) sufentanil rapidly leaves the plasma compartment, and little remains available to be extracted by the devices used to process and conserve blood.

Metabolic Measurements During Mechanical Ventilation

Michael C. Damask Benedict-Roth spirometer with a CO2 absorber included in the Department of Anesthesiology anesthesia circuit. Historically, the most frequently used Columbia University College of Physicians and Surgeons indirect methods for metabolic measurement during mechanical ventilation are the Douglas bag technique [7], the open INVASIVE METHODS circuit flow-through technique [8], and closed circuit spiroM ETABOLIC rate can be clinically measured using the Fick metry [9]. M principle to calculate oxygen consumption (VO2) and In the most common method for measuring metabolic rate carbon dioxide production (VCO2) [1]. Oxygen consumed (or during mechanical ventilation, expired gas is collected in a carbon dioxide produced) is calculated as the product of the Douglas bag or large spirometer. Concentrations of 02 and difference in gas concentration between mixed venous and CO2 are then measured using either gas meters, chemical gas arterial blood (cm3 gas/cm3 blood), and the cardiac output analyzers, mass spectrometry, Haldane techniques, or the (cm3 blood/min). This method requires a pulmonary artery Scholander technique. Once the expired minute volume and catheter to sample mixed venous blood. Cardiac output is the inspired 02 and CO2 concentrations are known, V02 and measured by either thermodilution or indicator-dilution VCO2 are calculated. It is essential to obtain very accurate method. The problems associated with invasive methods are measurements of both gas concentrations and gas volume. numerous, which makes their clinical application difficult. However, Douglas bags are notoriously cumbersome and The insertion of a pulmonary catheter requires skill and proper subject to leakage, and can collect gas for only a limited time monitoring facilities, and carries definite risks of complication period (2-6 min). [2]. To measure blood oxygen and carbon dioxide concentrations accurately, the Van Slyke method should be employed OPEN CIRCUIT FLOW-THROUGH TECHNIQUE [3]. The common practice of calculating blood gas content An open circuit flow-through method has been used for from partial pressure and hemoglobin concentration meacontinuous metabolic measurements mainly on spontanesured with clinical analyzers may be a source of error. ously breathing patients, but also for patients on mechanical Incomplete mixing of the indicator, dye, or cold solution in the ventilators [8, 10]. With the open circuit method, the patient cardiopulmonary circulation is the most common problem Of breathes through a valve that separates inspired air from direct cardiac output measurements, and therefore of indirect expired air, which is collected and analyzed. Lister et al. [1 1 ] metabolic rate measurements. Changes in blood flow during used this method to measure oxygen uptake in spontaneously measurement is another source of error [4]. Also, with the breathing infants and children. In this system (Fig. 1), the direct method, careful attention must be paid to gas temperahead of the patient is placed in a chamber through which a ture and pressure. stream of room air flows. This arrangement allows for the continuous entry of room air into the system, while expired NON-INVASIVE METHODS gas diluted with room air is removed and collected. The Direct calorimetry uses physical methods for measurement mixture is sampled, and the 02 concentration is measured by of free-body heat production, whereas indirect calorimetry a paramagnetic oxygen analyzer. The V02 is calculated by measures energy released as heat from chemical reactions in multiplying the gas flow rate through the system by the the body [5]. Metabolic measurement by indirect means is difference in oxygen concentrations between room air and preferable because indirect methods are non-invasive and can exhaled air mixed with room air. The total gas flow through be adapted to flow-through techniques, making it possible to the system is measured before sampling mixed expired gas. monitor V02 and VCO2 continuously. However, the major Recently, a more sophisticated open circuit flow-through problem with these techniques is that they require many technique has been reported, which measures the metabolic components, thus making them too cumbersome for use in an rate in premature infants by using more accurate CO2 and 02 operating room or intensive care unit. Also, the time and cost analyzers [121. With this technique, two dependent variables of assembling these systems make them impractical in must be considered: the flow of gas through the system, and routine clinical use. For the most part, these systems have the oxygen concentration in the expired gas/room air mixture. been used as research tools on patients spontaneously As the flow rate increases, the difference between the breathing room air, but not on those who require mechanical oxygen concentration of room air and that of mixed expired ventilation. gas decreases. Since, in order to prevent the loss of expired Shackman et al. [6] performed one of the first studies air from the system, flow rates are usually five or ten times measuring V02 of mechanically ventilated patients under the minute volume, the sampled mixed expired 02 and CO2 anesthesia. The device used for these measurements was a concentrations approximate room air. Therefore, the instruments used must be capable of accurately measuring small differences in 02 concentration [11]. The accuracy of the Michael C. Damask, MD is Assistant Professor open circuit method is decreased because of the high and of Anesthesiology at Columbia University Coloften variable inspired oxygen concentration that is typically lege of Physicians and Surgeons, and Attendrequired by seriously ill patients [131. Under anesthesia, the Ing Anesthesiologist at Columbia-Presbyterian measurement of inspired and expired volumes with the open Medical Center, Anesthesia Service, New circuit technique becomes inaccurate if nitrous oxide (N20) is York, NY 10032. used [141.

Physiologic Requirements During Rewarming: Suppression of the Shivering Response

Intraoperative hypothermia has become a common occurrence. Postoperative rewarming often is accompanied by shivering and results in increased metabolic and circulatory demands. We examined the metabolic, hemodynamic, and biochemical variables in 2 groups of hypothermic (greater than 35.8 degrees C) patients requiring mechanical ventilation after a major operation. One was observed during routine medical management whereas the other group received 40 mg of metocurine iodide and then observed during routine medical management. All patients were allowed to rewarm passively. O2 consumption (VO2, ml/min, STPD), CO2 production (VCO2, ml/min, STPD) and respiratory quotient (RQ) measurements were made every 15 min using a Beckman Metabolic Measurement Cart. Esophageal temperature, arterial blood pressure, heart rate (HR), rate pressure product, CVP, arterial blood gases, serum lactate concentration, and duration of shivering also were recorded. Suppression of the shivering by metocurine increased rewarming time significantly and decreased VCO2, VO2, HR, rate pressure product, mean arterial pressure (MAP), and the O2 cost of rewarming. Thus, the elimination of shivering during postoperative rewarming is associated with a decrease in caloric, metabolic demands and myocardial work (as assessed by the rate pressure product) while rewarming time is prolonged. In the postoperative, hypothermic, critically ill patient, suppression of the shivering response in selected patients may be indicated.