Coasting: worth the effort?

A new anesthesia machine incorporates a "coasting mode", but the extent to which a coasting technique can maintain anesthesia at the end of a procedure under optimal conditions (closed circuit anesthesia) remains unknown. Sixty-nine patients undergoing peripheral or abdominal surgery were assigned to 1 of 9 groups, depending on when desflurane coasting (in O2/air) was started (after 4, 9, 16, 25, 36, 49, 64, 81, or 100 min). The end-expired desflurane concentration was maintained at 4.5% in O2/air prior to coasting with a conventional anesthesia machine. After initiating coasting (using a closed-circuit technique), we examined when the end-expired desflurane concentration reached 70, 60, 50, and 40% of its value during maintenance (= 30, 40, 50 and 60% decrement times, respectively). Decrement times increased with increasing duration of anesthesia, and varied widely. After 64 min of maintenance anesthesia, the end-expired desflurane concentration remained at or above 70, 60, 50, and 40% of its maintenance value during 10.3 +/- 2.3, 16.0 +/- 3.5, 25.0 +/- 5.9, and 45.4 +/- 19.3 min, respectively (average +/- standard deviation). Coasting can briefly maintain anesthesia towards the end of a procedure. While savings with an automated coasting mode are likely to be modest per patient, they may become substantial when multiplied by the number of procedures per day per operating room with no increase in the clinical workload of the anesthesia provider.

Influence of steep Trendelenburg position and CO(2) pneumoperitoneum on cardiovascular, cerebrovascular, and respiratory homeostasis during robotic prostatectomy.

BACKGROUND: The steep (40 degrees ) Trendelenburg position optimizes surgical exposure during robotic prostatectomy. The goal of the current study was to investigate the combined effect of this position and CO(2) pneumoperitoneum on cardiovascular, cerebrovascular, and respiratory homeostasis during these procedures. METHODS: Physiological data were recorded during the whole surgical procedure in 31 consecutive patients who underwent robotic endoscopic radical prostatectomy under general anaesthesia. Heart rate, mean arterial pressure, central venous pressure, Sp(o(2)), Pe'(co(2)), P(Plat), tidal volume, compliance, and minute ventilation were monitored and recorded. Arterial samples were obtained to determine the arterial-to-end-tidal CO(2) tension gradient. Continuous regional cerebral tissue oxygen saturation (Sct(o(2))) was determined by near-infrared spectroscopy. RESULTS: Although patients were in the Trendelenburg position, all variables investigated remained within a clinically acceptable range. Cerebral perfusion pressure (CPP) decreased from 77 mm Hg at baseline to 71 mm Hg (P=0.07), and Sct(o(2)) increased from 70% to 73% (P<0.001). Pe'(co(2)) increased from 4.12 to 4.79 kPa (P<0.001) and the arterial-to-Pe'(co(2)) tension difference increased from 1.06 kPa in the normal position to a maximum of 1.41 kPa (P<0.001) after 2 h in the Trendelenburg position. CONCLUSIONS: The combination of the prolonged steep Trendelenburg position and CO(2) pneumoperitoneum was well tolerated. Haemodynamic and pulmonary variables remained within safe limits. Regional cerebral oxygenation was well preserved and CPP remained within the limits between which cerebral blood flow is usually considered to be maintained by cerebral autoregulation.

Desflurane consumption with the Zeus during automated closed circuit versus low flow anesthesia.

INTRODUCTION: During automated closed-circuit anesthesia (CCA), the Zeus (Dräger, Lübeck, Germany) uses a high initial fresh gas flow (FGF) to rapidly attain the desired agent and carrier gas concentrations, resulting in a desflurane consumption well above patient uptake. Because both FGF and carrier gas composition can affect consumption, we determined the Zeus' agent consumption with automated CCA and with automated low flow anesthesia (LFA) (= maintenance FGF of 0.7 L min(-1)) with 3 different carrier gases. METHODS: After IRB approval, 65 ASA PS I or II patients undergoing general surgery received desflurane in either O2, O2/air, or O2/N2O, with the Zeus to maintain the end-expired concentration (FA) at 6, 6, and 4% and the F1O2 at 1.0, 0.6, and 0.4, respectively. In addition, patients were assigned to either automated CCA (O2 n = 11; O2/air n = 11; O2/N2O n = 11) or automated LFA (selected FGF 0.7 L min(-1)) (O2 n = 12; O2/air n = 11; O2/N2O n = 9). Demographics and desflurane consumption at 2, 4, 6, 8, 10, 20, 30, 40 and 50 min were compared. RESULTS: With the same carrier gas, desflurane consumption was lower with the CCA mode than with LFA mode after 4 min in the O2 groups, 6 min in the O2/air groups, and 30 min in the O2/N2O groups. Within each mode, desflurane consumption in the O2 and O2/air groups was identical at all times. Despite the use of a lower FA in the N2O groups, initial desflurane consumption was higher than in the O2 and O2/air groups, but it was lower later (> or = 15 min) only with LFA. DISCUSSION: After 50 min, desflurane consumption with automated CCA is lower than with automated LFA. However, initial agent consumption is complex, and N2O in particular may increase initial desflurane consumption (though ultimately resulting in lower desflurane usage because of its MAC sparing effect) because initial FGF is increased to rapidly reach the target concentrations. Differences in desflurane consumption only become apparent after FGF has stabilized to the target FGF.

Pulmonary gas exchange is well preserved during robot assisted surgery in steep Trendelenburg position.

INTRODUCTION: During robot assisted hysterectomies and prostatectomies, surgical exposure demands the application of a CO2 pneumoperitoneum with a very steep Trendelenburg position (40 degrees). The extent to which oxygenation and ventilation might be compromised intra-operatively remains poorly documented. METHODS: Dead-space ventilation and venous admixture were determined in 18 patients undergoing robot assisted hysterectomy (n = 6) or prostatectomy (n = 12). Anesthesia was maintained with desflurane in O2 or O2/air, with the inspired O2 fraction left at the discretion of the attending anesthesiologist. Controlled mechanical ventilation was used, but 15 min after assuming the Trendelenburg position and up until resuming the supine position pressure controlled ventilation was used. Dead-space ventilation and venous admixture were determined using Bohr's formula and Nunn's iso-shunt diagram, respectively, at the following 7 stages of the procedure: 15 min after induction; 5 min after applying the CO2 pneumoperitoneum (intra-abdominal pressure 12 mm Hg) but while still supine; 5, 60, and 120 min after assuming the Trendelenburg positioning; and 5 and 15 min after reassuming the supine position. RESULTS: Venous admixture did not change. Dead-space ventilation increased after Trendelenburg positioning, and returned to baseline values after resuming the supine position. However, individual patterns varied widely. DISCUSSION: The lung has a remarkable yet incompletely understood capacity to withstand the effects of a CO2 pneumoperitoneum and steep Trendelenburg position during general anesthesia. While individual responses vary and should be monitored, effects on dead-space ventilation and venous admixture are small and should not be an obstacle to provide optimal surgical exposure during robot assisted prostatectomy or hysterectomy.

Special aspects of pharmacokinetics of inhalation anesthesia.

Recent interest in the use of low-flow or closed circuit anesthesia has rekindled interest in the pharmacokinetics of inhaled anesthetics. The kinetic properties of inhaled anesthetics are most often modeled by physiologic models because of the abundant information that is available on tissue solubilities and organ perfusion. These models are intuitively attractive because they can be easily understood in terms of the underlying anatomy and physiology. The use of classical compartment modeling, on the other hand, allows modeling of data that are routinely available to the anesthesiologist, and eliminates the need to account for every possible confounding factor at each step of the partial pressure cascade of potent inhaled agents. Concepts used to describe IV kinetics can readily be applied to inhaled agents (e.g., context-sensitive half-time and effect site concentrations). The interpretation of the F(A)/F(I) vs time curve is expanded by reintroducing the concept of the general anesthetic equation-the focus is shifted from "how F(A) approaches F(I)" to "what combination of delivered concentration and fresh gas flow (FGF) can be used to attain the desired F(A)." When the desired F(A) is maintained with a FGF that is lower than minute ventilation, rebreathing causes a discrepancy between the concentration delivered by the anesthesia machine (=selected by the anesthesiologist on the vaporizer, F(D)) and that inspired by the patient. This F(D)-F(I) discrepancy may be perceived as "lack of control" and has been the rationale to use a high FGF to ensure the delivered matched the inspired concentration. Also, with low FGF there is larger variability in F(D) because of interpatient variability in uptake. The F(D)-F(I) discrepancy increases with lower FGF because of more rebreathing, and as a consequence the uptake pattern seems to be more reflected in the F(D) required to keep F(A) constant. The clinical implication for the anesthesiologist is that with high FGF few F(D) adjustments have to be made, while with a low FGF F(D) has to be adjusted according to a pattern that follows the decreasing uptake pattern in the body. The ability to model and predict the uptake pattern of the individual patient and the resulting kinetics in a circle system could therefore help guide the anesthesiologist in the use of low-flow anesthesia with conventional anesthesia machines. Several authors have developed model-based low FGF administration schedules, but biologic variability limits the performance of any model, and therefore end-expired gas analysis is obligatory. Because some fine-tuning based on end-expired gas analysis will always be needed, some clinicians may not be inclined to use very low FGF in a busy operating room, considering the perceived increase in complexity. This practice may be facilitated by the development of anesthesia machines that use closed circuit anesthesia (CCA) with end-expired feedback control--they "black box" these issues (see Chapter 21). In this chapter, we first explore how and why the kinetic properties of intravenous and inhaled anesthetics have been modeled differently. Next, we will review the method most commonly used to describe the kinetics of inhaled agents, the F(A)/F(I) vs time curve that describes how the alveolar (F(A)) approaches the inspired (F(I)) fraction (in the gas phase, either "fraction," "concentration," or "partial pressure" can be used). Finally, we will reintroduce the concept of the general anesthetic equation to explain why the use of low-flow or closed circuit anesthesia has rekindled interest in the modeling of pharmacokinetics of inhaled anesthetics. Clinical applications of some of these models are reviewed. A basic understanding of the circle system is required, and will be provided in the introduction.

Large volume N2O uptake alone does not explain the second gas effect of N2O on sevoflurane during constant inspired ventilation.

BACKGROUND: The second gas effect (SGE) is considered to be significant only during periods of large volume N(2)O uptake (VN(2)O); however, the SGE of small VN(2)O has not been studied. We hypothesized that the SGE of N(2)O on sevoflurane would become less pronounced when sevoflurane administration is started 60 min after the start of N(2)O administration when VN(2)O has decreased to approximately 125 ml min(-1), and that the kinetics of sevoflurane under these circumstances would become indistinguishable from those when sevoflurane is administered in O(2). METHODS: Seventy-two physical status ASA I-II patients were randomly assigned to one of six groups (n=12 each). In the first four groups, sevoflurane (1.8% vaporizer setting) administration was started 0, 2, 5 and 60 min after starting 2 litre min(-1) O(2) and 4 litre min(-1) N(2)O, respectively. In the last two groups, sevoflurane (1.8 or 3.6% vaporizer setting) was administered in 6 litre min(-1) O(2). The ratios of the alveolar fraction of sevoflurane (Fa) over the inspired fraction (Fi), or Fa/Fi, were compared between the groups. RESULTS: Sevoflurane Fa/Fi was larger in the N(2)O groups than in the O(2) groups, and it was identical in all four N(2)O groups. CONCLUSIONS: We confirmed the existence of a SGE of N(2)O. Surprisingly, when using an Fa of 65% N(2)O, the magnitude of the SGE was the same with large or small VN(2)O. The classical model and the graphical representation of the SGE alone should not be used to explain the magnitude of the SGE. We speculate that changes in ventilation/perfusion inhomogeneity in the lungs during general anaesthesia result in a SGE at levels of VN(2)O previously considered by most to be too small to exert a SGE.