| JOURNAL HOME | CME HOME | THIS MONTH | PAST ISSUES | ETOC | COLLECTIONS |
| AUTHORS | REVIEWERS | EDITORIAL BOARD | FEEDBACK | RSS | HELP |
| ||||||||||||||
| ||||||||||||||
|
|
|
|
||||||||||||
|
||||||||||||||
*Department of Anesthesia and Perioperative Care, University of California, San Francisco, California; and Department of Anesthesia, Stanford University, Stanford, California
Address correspondence and reprint requests to Edmond I Eger II, MD, Department of Anesthesia, S-455, University of California, San Francisco, CA 94143-0464. Address e-mail to egere{at}anesthesia.ucsf.edu.
 |
Abstract |
|---|
 Top Abstract IntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
|
Introduction |
|---|
|
TopAbstractIntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
This essay uses drawings to enhance understanding. The drawings indicate the relative capacities of compartments, the occupancy by anesthetics, and the movements of anesthetic. The total capacities are proportional to the size of circles or ovals representing tissues, with the lung—specifically, the functional residual capacity (FRC)—acting as the standard (i.e., all capacities are referenced to the lung capacity—the FRC—to hold anesthetic). The arrows indicate the movement of anesthetic: the broader and/or longer the arrow, the greater the movement. The figures illustrate concepts but are not necessarily precise (quantitative) reflections of the kinetics. We begin with a discussion of the induction of anesthesia in the patient with normal morphology.
|
The Four Factors Governing Uptake of Potent Inhaled Anesthetics |
|---|
|
TopAbstractIntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
|
|
However, uptake (the second factor) opposes the effect of ventilation. The relationship at the concentrations of potent inhaled anesthetics often used is a simple one: if uptake removes half of the anesthetic inspired, then the alveolar concentration equals half the concentration being inspired. If uptake removes two thirds of the anesthetic, then the alveolar concentration equals one third the concentration being inspired.
Three elements determine uptake. The first of these, the one that distinguishes the inhaled anesthetics, is solubility in blood, defined as the blood/gas partition coefficient, or 
. The blood/gas partition coefficient equals the concentration of anesthetic in blood divided by the concentration in gas when the two phases are in equilibrium (have the same partial pressure of anesthetic). Desflurane has the smallest blood/gas partition coefficient of presently available potent inhaled anesthetics (Table 1), and, accordingly, for a given alveolar concentration, the uptake of sevoflurane or isoflurane exceeds that of desflurane. These differences influence the rate of increase in the alveolar concentration, an increase that often is taken as a surrogate for the rate of the induction of anesthesia; the rate for desflurane exceeds that of sevoflurane, and that of sevoflurane exceeds that of isoflurane.
|
Cardiac output and the alveolar-to-venous (A-v) blood partial pressure difference constitute the remaining two elements that govern uptake. An increase in cardiac output (Q) or the A-v blood partial pressure difference will increase uptake. Uptake (U) may be calculated as follows:
|
|
For an anesthetic such as desflurane, the initial uptake of anesthetic is less than the amount delivered. At the start of anesthesia, A-v is maximal because no anesthetic is in venous blood. Thus, initially, for a given alveolar concentration (i.e., for A), uptake is proportional to blood solubility x cardiac output. For desflurane, for a healthy adult, uptake would be proportional to 0.45 x 5.4 L/min, or 2.43 L/min (proportional to but not equal to except at 100% anesthetic; at, say, 10% desflurane, uptake would equal 0.1 x 0.45 x 5.4 L/min, or 0.243 L/min). This is less than the alveolar ventilation (4 L/min); thus, the arrow out of the lungs (the loss of anesthetic) will be approximately half as thick as the arrow delivering desflurane into the lungs (Fig. 3). With sevoflurane, the arrow out of the lungs (0.65 x 5.4 L/min, or 3.51 L/min) would be approximately as thick as the arrow into the lungs, and for isoflurane, the arrow (1.4 x 5.4 L/min, or 7.56 L/min) would be nearly twice as thick as the arrow into the lungs.
|
|
The arrow representing cardiac output in Figure 3 actually represents several blood flows (several arrows), one for each tissue of the body (Fig. 4A). The tissues may be differentiated into three tissue groups depending on their blood flow and their capacity to hold anesthetic (Tables 2 and 3) (3–8). The vessel-rich group (VRG) consists of brain, heart, liver/intestine, and kidney. The capacity of the VRG (or any tissue group) equals the volume of the group (in this case, 6 L; Table 3) times its affinity for the anesthetic, as defined by the tissue/gas partition coefficient (0.58), or 3.48 L. Thus, for desflurane, the VRG is less than twice as large as the 2-L FRC (Fig. 4A).
|
|
The VRG receives the largest fraction of the cardiac output and thus, initially, the largest fraction of the anesthetic taken up. Accordingly, in Figure 4A, the largest arrow goes from the lung to the VRG (including the brain, which, although part of the VRG, is displayed separately for reasons related to intertissue diffusion, discussed later). Anesthetic also goes to the muscle group (MG) (and also skin, because skin has the same perfusion and solubility characteristics as muscle) and to the fat group (FG). The arrow going to the muscle is smaller than that going to the VRG because muscle receives less of the cardiac output, and the arrow going to the fat is smaller than that going to muscle because still less of the cardiac output goes to fat in a healthy, lean adult.
Compare the sizes (capacities) of the tissue groups with each other and with the FRC (as noted previously, the FRC is the reference for all capacities). Although the VRG receives a large fraction of the cardiac output, it constitutes only 9% of the body mass, and the circle indicating its capacity to hold anesthetic is small (actually two circles, because the brain is part of, but is separated from, the remaining tissues that constitute the VRG).
The size of the circle for the MG exceeds that of the circle representing the VRG because muscle (and skin) make up 50% of the body mass in a lean adult, or approximately 33 L. The muscle/gas partition coefficient of 0.78 (Table 1) produces an MG capacity of 26 L. Thus, the area of the circle for the MG is 13 times larger than the area of the oval representing the lung. Although fat constitutes only 20%–25% of the body mass in a healthy adult, the circle representing the FG is much larger than that for muscle because the capacity of fat (because of the enormous solubility of anesthetic in fat) is much larger than that of muscle. (A fourth tissue group, the vessel-poor group, made up of ligaments, tendons, cartilage, bone, and other avascular tissues, does not contribute to uptake because of its minimal or absent perfusion and thus is not considered in this discussion.)
Tissue/Gas Versus Tissue/Blood Partition Coefficients.
The reader may wonder at the use of both Table 1 (tissue/gas partition coefficients) and Table 2 (tissue/blood partition coefficients). The two tables are simply connected: a tissue/blood partition coefficient for a given anesthetic is calculated by dividing a tissue/gas partition coefficient by the blood/gas partition coefficient for that anesthetic.
The two partition coefficients tell somewhat different things. A tissue/gas partition coefficient indicates the relative capacities of all tissues to hold the anesthetic in question. The product of the tissue volume times the tissue/gas partition coefficient equals the capacity of the tissue to hold anesthetic relative to the lung (the reference capacity defined by the FRC). Tissue/gas partition coefficients also indicate the relative capacity of a given tissue to hold one anesthetic as opposed to another. Thus, the capacity of muscle to hold sevoflurane is 2.2 times (i.e., 1.7/0.78) the capacity of muscle to hold desflurane; the capacity of fat to hold sevoflurane is 2.8 times the capacity of fat to hold desflurane.
A tissue/blood partition coefficient allows an estimate of the rate at which an anesthetic partial pressure may be developed or decreased in a given tissue. This may make use of the time constant. The time constant equals the capacity of a tissue to hold anesthetic divided by the flow of blood to that tissue. For example, consider the VRG. For this essay, we attribute a blood flow of 30.1 mL/min to each 100 mL of tissue. (Note that this is an average and would be more for some tissues [e.g., gray matter] and less for others [e.g., white matter].) For desflurane, each 100 mL of VRG has a capacity of 100 mL x 1.30, the desflurane VRG/blood partition coefficient (Table 3). This gives a time constant of 130 mL/30.1 mL/min, or 4.3 min. Now, the time constant always is the time to reach 63% of equilibrium, and this invariable connection makes the time constant useful. Thus, the partial pressure of desflurane in the brain may reach 63% of the arterial partial pressure in 4.3 min.
Half-time (50% of equilibrium) values also may be calculated with this technique: the half-time equals the time constant times 0.7. Thus, the half-time for the VRG for desflurane equals 3.0 min and for sevoflurane equals 4.0 min. For fat, the half-times are 860 min for desflurane and 1540 min for sevoflurane.
How are these thoughts illustrated in the Figure 4B drawing for 5 min (and for later drawings)? At 5 min, the VRG has reached two thirds of the way to equilibration. Accordingly, the arrow leaving the brain is two thirds as long as the arrow to it (i.e., two thirds as much anesthetic leaves as enters). Also, the amount of desflurane lodged in the brain (the area of the speckled circle in the brain) is two thirds the size of the circle representing the capacity of the brain). In contrast to the VRG, equilibration of the MG has just begun. With a time constant of 38.1 min, equilibration is but 15% complete, and the area of the speckled circle representing desflurane in the MG is 15% of the area of the circle representing the capacity of the MG—and the arrow returning to the lungs from the MG is 15% of the arrow going to the MG. For the FG at 5 min, equilibration equals 0.4%, and the area of the speckled circle is very small relative to the large circle representing fat capacity.
A small amount of anesthetic (overall, approximately 0.02% of what is taken up) is lost from the liver by metabolism of desflurane (metabolism is the third factor that governs the uptake and distribution of potent inhaled anesthetics). A larger amount of isoflurane (0.2%) and sevoflurane (5%) is lost in this manner.
By 5 min, uptake by the VRG has decreased substantially. This, and a slight decrease in uptake by the MG, decreases the inspired concentration (FI) needed to sustain the alveolar concentration (FA) at MAC. Thus, at 5 min, the arrow representing FI is only 29% longer than the arrow representing FA.
Intertissue Diffusion.
Something else appears at 5 min. Some anesthetic delivered to parts of the VRG transfers to adjacent tissues, particularly fat, by intertissue diffusion (the fourth factor that governs the uptake and distribution of potent inhaled anesthetics). Perl et al. (9) suggested the importance of intertissue diffusion to anesthetic uptake and distribution 40 yr ago. Thus, anesthetic moves from intestine to mesenteric and omental fat; from kidney to perirenal fat; and from heart to pericardial fat. The area of the thin checkered layer at the edge of the FG represents the amount of anesthetic transferred to the fat participating in the anesthetic transfer from the VRG to fat. Anesthetic may also be transferred from skin to subcutaneous fat or to fat that is interwoven (intercalated) into muscle (consider prime, choice, commercial, and cooker cuts of beef). However, at this point in the anesthetic process, the partial pressure of anesthetic in skin and muscle is much less than that available in the VRG, and the amount transferred is smaller than the amount transferred from the VRG. Another intertissue transfer occurs from gray matter to white matter (10). These transfers by intertissue diffusion mean that a small part of the VRG (part of the intestine, liver, kidney, and heart) and part of the muscle/skin are slower to reach equilibrium because anesthetic is continuously lost to adjacent fat. Thus, intertissue diffusion decreases the arrow returning from the VRG (excluding the brain) to the lung. As we shall see shortly, intertissue diffusion loss from the MG will also limit the size of the arrow returning from muscle.
Both direct and indirect evidence support the notion that anesthetic transfers from one tissue to another or part of one tissue to another part of that tissue (e.g., gray matter to white matter) (10). A perirenal rim of anesthetic in perirenal fat directly demonstrates the movement of anesthetic from kidney to fat (8). The capacity of nitrous oxide to diffuse through plastics (11–14) and natural membranes (15–18) provides further evidence. However, the most convincing evidence for the importance of intertissue diffusion to anesthetic uptake comes from studies of inhaled anesthetic elimination. These indicate that intertissue diffusion accounts for approximately 30% of the anesthetic taken up (3–7). These studies estimate that intertissue diffusion occurs with time constants of approximately 200–400 min for potent inhaled anesthetics: the time constant increases in proportion to the fat/blood partition coefficient (Table 3).
Fifty Minutes into Anesthesia.
By 50 min, the brain has completely equilibrated with the desflurane partial pressure brought to it from the lungs (Fig. 5A; the area of the speckled circle representing desflurane completely overlies the circle representing the capacity of the brain to hold desflurane, and the arrow returning from the brain to the lungs is as long as the arrow to the brain). Were it not for losses by intertissue diffusion, the desflurane partial pressure in the MG would reach 74% of the partial pressure delivered to it from the lungs (the area of the speckled circle would be 74% of the area of the larger circle representing the MG). The arrow returning from the MG to the lungs is approximately two thirds of the arrow to the MG. By 50 min, additional anesthetic is transferred from the VRG and muscle/skin to fat by intertissue diffusion. Some of the anesthetic transferred by intertissue diffusion returns to the VRG (at 50 min, the fat affected by intertissue diffusion has reached 20% equilibration, in contrast to bulk fat—the FG—which has reached only 4% equilibration; thus, uptake by bulk FG continues essentially unchanged). Because of transfer of anesthetic by intertissue diffusion, the arrow returning from the VRG continues to be smaller than the arrow going to the VRG. The decreased uptake by muscle and by intertissue diffusion is reflected in a smaller FI needed to sustain FA; the FI arrow now is 12% longer than the FA arrow.
|
How Does Sevoflurane Differ from Desflurane?
Both desflurane and sevoflurane are classified as poorly soluble potent inhaled anesthetics but, on average, sevoflurane is twice as soluble in blood and tissues as is desflurane (Tables 1–3). The drawings in Figure 5 reflect these differences. The capacities of the VRG, MG, and FG to hold sevoflurane exceed those capacities for desflurane, and the areas of the circles representing these tissue groups are larger. Although the amount of sevoflurane taken up into these tissues and transferred by intertissue diffusion is also larger, the rate of equilibration is slower (equilibration is less complete). Loss of sevoflurane by metabolism is 100 times greater, but even this larger loss does not materially affect uptake. Whereas the arrow indicating the FI for desflurane is 12% greater than the arrow repre-senting FA, for sevoflurane the arrow is 21% greater. This is an example of something called "overpressure," the use of a larger concentration to sustain a target alveolar concentration. For more soluble anesthetics, the overpressure needed (the extent to which FI must exceed FA) can be substantial.
Two-Hundred Minutes into Anesthesia.
By 200 min, except for the portion involved in intertissue diffusion, the MG has equilibrated with the desflurane brought to it, and the blood from muscle returns to the lungs with nearly as much desflurane as when it left the lungs (Fig. 6A). Thus, the area of the speckled circle indicating desflurane in the MG nearly completely occupies the capacity of the MG, and the arrow from the MG to the lung approaches the one from the lung. Anesthetic continues to be transferred from muscle/skin to fat by intertissue diffusion, and this keeps the arrow from MG to lung smaller than the arrow to MG. At this time, approximately 58% of the anesthetic transferred by intertissue diffusion returns to the VRG (the intertissue arrow from the FG is now half the size of the arrow to the FG). Because of the transfer of anesthetic by intertissue diffusion, the arrow returning from the VRG continues to be smaller than the arrow going to the VRG. The bulk FG (i.e., that not involved in intertissue diffusion) continues to remove nearly all anesthetic brought to it. The desflurane present in bulk fat is substantial, but by 200 min, only 15% equilibration has been achieved. The marked decrease in uptake by muscle and by intertissue diffusion is reflected in a decreased uptake at the lung and a decrease in the size of FI relative to FA; the FI arrow now is only 5% greater than the arrow indicating FA.
|
How Does Isoflurane Differ from Desflurane?
Isoflurane is approximately four times more soluble than desflurane in blood and tissues, and thus the representations of tissues for isoflurane (Fig. 6B) are four times (sometimes more) larger than those for desflurane (Fig. 6A). As with desflurane, by 200 min, the VRG and MG have equilibrated with the isoflurane (speckled circles) brought from the lungs—except for the portions connected to fat by intertissue diffusion. Relatively more isoflurane is transferred by intertissue diffusion and by transfer to bulk fat, but the extent of equilibration is less than with desflurane (40% vs 58% for intertissue diffusion and 9% vs 15% for bulk fat). Uptake to fat by these two routes produces a need for a greater FI for isoflurane. The arrow indicating FI is 20% greater than the FA arrow for isoflurane and only 5% greater for desflurane.
Rebreathing and the Vaporizer Setting Needed to Sustain a Constant Alveolar Anesthetic Concentration
Thus far, we have considered the inspired concentration of anesthetic (FI) needed to sustain a constant alveolar concentration (FA). FI will equal the concentration delivered from the vaporizer (FD) if the inflow rate equals or exceeds minute ventilation, but if minute ventilation exceeds inflow rate (i.e., if rebreathing occurs), then FD must be more than FI to compensate for the effect of rebreathed gas depleted of some anesthetic (i.e., containing gas with a concentration equal to FA; Fig. 7). The inflow rate usually is set at something less than minute ventilation, often between 1 and 3 L/min. A slower inflow rate increases the economy of anesthesia, retains heat, and increases the humidity of respired gases.
|
Two factors affect the FD required to sustain FA at a constant level. First, if FA differs but slightly from FI (i.e., if uptake is small), then the rebreathing of some gas containing FA will minimally affect the FD required to sustain FA at a constant level. Conversely, an increase in the difference between FI and FA increases the FD required. Second, an increase in rebreathing (i.e., as occurs with a decrease in inflow rate) increases the amounts of rebreathed gas containing FA and therefore will increase the FD required to sustain constant FA. The next drawings superimpose the effect of rebreathing on previous drawings.
Five minutes into anesthesia, FD for desflurane must exceed FI by 137% if the inflow rate is set at 0.5 L/min (Fig. 8A). However, if the inflow rate is 2 L/min, then FD needs to be only 30% more than FI. The figure shows the extra (added) percentage needed in FD to sustain the FI. Note that the complexity of the system depicted in Figure 7 is reduced to just a circle in Figure 8.
|
At 50 min of anesthesia with desflurane, equilibration of the VRG and partial equilibration of the MG markedly decrease uptake and thus decrease the effect of rebreathing (Fig. 8B). Now, even at an inflow rate of 0.5 L/min, FD must exceed FI by only 44%, and at 1 L/min the difference is 21%. The substitution of sevoflurane for desflurane increases these differences because of the greater solubility and uptake associated with sevoflurane (Fig. 8C). At a 1 L/min inflow (the slowest inflow rate suggested by the sevoflurane package label), FD must exceed FI by 48%, rather than the 21% needed with desflurane.
At 200 min of desflurane anesthesia, equilibration of the VRG and the MG and the partial equilibration of bulk fat and fat served by intertissue diffusion further decrease uptake and, thus, decrease the effect of rebreathing (Fig. 9A). Now, even at an inflow rate of 0.5 L/min, FD must exceed FI by only 7%. In contrast, at 200 min of isoflurane anesthesia, continued uptake by bulk fat and intertissue diffusion and the greater solubility of isoflurane increase the needed FD (Fig. 9B). At an inflow rate of 0.5 L/min, FD must exceed FI by 94%; at 1 L/min, by 46%; and at 2 L/min, by 22%.
|
These effects are summarized for the three potent inhaled anesthetics for the relationship for FD/FA and FI/FA in Figure 10. The effects of solubility (isoflurane > sevoflurane > desflurane) and inflow rate are apparent.
|
Factors Influencing Uptake and the Development and Elimination of an Anesthetizing Concentration
Several factors influence uptake and the development and elimination of an anesthetizing concentration of an inhaled anesthetic. The importance of solubility is obvious from the preceding discussion. Solubility is the crucial factor that distinguishes one anesthetic from another. Increasing duration deposits increasing amounts of anesthetic in depots such as muscle and fat and thereby tends to delay recovery (19,20). An increase in ventilation hastens the increase in the alveolar concentration of a potent inhaled anesthetic, with a greater effect with more soluble anesthetics (21). By increasing uptake (at least initially), an increase in cardiac output hinders the rate of increase of the alveolar concentration of anesthetic and has a greater effect with more soluble anesthetics (21). Ventilation/perfusion inequalities complexly influence inhaled anesthetic kinetics (22). Aging decreases the rate of increase of the alveolar concentration of a potent inhaled anesthetic (23,24), and solubility changes with increasing age (25). Body habitus may also influence pharmacokinetics. We next will examine this point in detail because its effect has not been described fully and because habitus is changing in ways that importantly affect anesthesia.
Obesity, an Epidemic That Has Implications for Anesthesia
Excess fat (a body mass index [BMI] >25 kg/m2) in North America and other parts of the world has reached epidemic proportions (26–28). From 1991 to 1998, the percentage of United States citizens with a BMI >30 kg/m2—i.e., obesity—increased by 50% (29).
Obesity at all levels (morbid to overweight by 40 pounds) presents a growing challenge to the anesthesiologist, who must deal with a decreased FRC and a decreased compliance (30), an increased incidence of intra- and postoperative atelectasis (31), difficulty in tracheal intubation (32), an increase in airway resistance that may resemble asthma (30), an increased capacity to metabolize anesthetics such as halothane (33) or enflurane (34) (but not, apparently, sevoflurane) (35), a greater surgical demand for relaxation, and more (36). Anesthesia may exaggerate the decrease in FRC far more in obese patients than in normal-weight patients (37).
Of immediate relevance to this report is the need to restore the obese or overweight patient to his or her preanesthetic state as rapidly as possible after surgery. The obese or overweight patient presents kinetic issues that may delay recovery and, thereby, add to the risk of anesthesia. A few kinetic studies have been performed in patients. Obesity increases uptake in two ways. The greater fat burden increases the blood flow directed to bulk fat, and uptake by the FG must increase. Obesity also may increase fat surfaces accessible by intertissue diffusion (e.g., intraabdominal fat and fat intercalated in muscle), and this, too, must increase uptake.
Compare the conditions displayed in Figure 6A to the condition found with obesity (Fig. 6C). The larger FG in Figure 6C increases the deposition of desflurane into bulk fat and into fat reached by intertissue diffusion. The larger uptake by fat is reflected in a larger arrow for FI.
In summary, the increased size of the FG differentiates the obese patient from the normal-weight patient in several ways. First, the larger volume of fat requires an increased blood delivery of anesthetic to bulk fat, a delivery that continues throughout the course of anesthesia. Obesity increases cardiac output (38). Despite a greater flow of blood to fat, bulk fat never equilibrates or begins to approach equilibration with the anesthetic brought to it, even with prolonged anesthesia. Second, the larger volume of fat presents a larger surface for anesthetic transfer by intertissue diffusion, particularly transfer from intestine to omental and mesenteric fat, but also from muscle to intercalated fat and from dermis to subcutaneous fat. Thus, the morbidly obese patient may have three times as much fat as the normal-weight patient and a roughly proportional increase in anesthetic acquired by intertissue diffusion. Third, the press of abdominal contents on the diaphragm can decrease the lung size (FRC). Fourth, the increased fat, the increased need to perfuse fat, and an increased work of breathing modestly increase metabolism, cardiac output, and breathing.
Obesity does not change the VRG or the MG: these tissues do not change in size or perfusion consequent to obesity (except that hepatic size and blood flow may increase). However, the VRG and MG are important because they serve as conduits for the anesthetic stored by intertissue diffusion. Another minor change, an increase, may occur with hepatic metabolism of anesthetic.
Recovery from Anesthesia
Two elements determine recovery. First is the concentration of anesthetic in the effect compartment (i.e., where the anesthetic causes anesthesia) that permits awareness, or MACawake. MACawake differs among inhaled anesthetics and between inhaled and IV anesthetics (Fig. 11) (39–41). Similar values are found for MACawake for desflurane, isoflurane, and sevoflurane, and thus this factor does not distinguish among the anesthetics in time to recovery.
|
The second element that determines recovery is the clearance of anesthetic from the effect site. Several factors influence clearance. Anesthetic in tissue depots and the solubility of the anesthetic in blood (the blood/gas partition coefficient) will determine the rate of decrease of anesthetic in the arterial circulation during recovery from anesthesia because solubility determines the clearance of anesthetic at the lungs. If the solubility () of the anesthetic is very small, most of the anesthetic will be cleared by ventilation and will thus not recirculate and delay recovery. The equation given after this paragraph shows that as approaches 0, clearance approaches 100% (42). That is, a low solubility allows clearance of most of the anesthetic by the lungs, leaving little to recirculate and delay recovery. At zero solubility, it does not matter how much anesthetic is stored in tissue depots; no anesthetic can reach the arterial blood. The duration of anesthesia or body habitus cannot affect recovery. However, if the solubility is appreciable, some anesthetic will not be cleared (Fig. 12), will recirculate, and will delay recovery.
|
|
|
By allowing deposition of more anesthetic in tissue depots (compare Figs. 4–6), an increasing duration of anesthesia delays recovery. That is, an increasing duration can increase the delivery of anesthetic from the MG and FG, and (if not cleared at the lungs) this will delay recovery from anesthesia.
As in healthy patients, the morbidly obese patient may awaken sooner after desflurane than after isoflurane or propofol (43). Immediate awakening also may occur more rapidly after desflurane than after sevoflurane anesthesia (44). Similarly, sevoflurane appears to provide a slightly more rapid washin and washout of anesthetic in the morbidly obese patient than does isoflurane (45), and the use of sevoflurane may allow an earlier recovery from anesthesia (46,47) and an earlier discharge of the morbidly obese patient from the postanesthesia care unit than does isoflurane (46).
What strategies might be used to hasten recovery? As noted above, one is the selection of less-soluble anesthetics, a strategy that increases the clearance at the lungs. Also, if the lower solubility extends to a lower tissue/blood partition coefficient in cerebral tissues, the lower solubility will shorten the time constant for the brain. This, too, adds to the speed of recovery.
On the induction of anesthesia, "overpressure" can compensate for the hindering effect of greater solubility. Thus, we may provide an inspired concentration of 4% or 5% halothane to more rapidly produce an alveolar concentration of 1% to 2%. However, we have no such ability during recovery; we cannot supply a negative anesthetic partial pressure for the patient to breathe. However, we can take advantage of the ventilatory component (VA) in the above equation. As VA increases, the percentage clearance increases (at very large values for VA, the percentage clearance approaches 100%.
However, increasing VA introduces two limiting factors. First, when hyperventilation ceases, the decrease in Paco2 produced by hyperventilation results in apnea or depressed ventilation (48), and this hinders further anesthetic elimination. Second, the decrease in Paco2 produced by hyperventilation decreases cerebral blood flow (49) and, thus, lengthens the time constant for elimination of anesthetic from the central nervous system. This, too, slows recovery. We can compensate for these two limiting factors by adding carbon dioxide to the inspired gases during hyperventilation at concentrations sufficient (e.g., 5%) to prevent a decrease in Paco2 (50). Such a strategy produces a more rapid recovery (51).
|
Summary |
|---|
|
TopAbstractIntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
|
Appendix 1 |
|---|
|
TopAbstractIntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
|
|
is the blood/gas partition coefficient, e is the natural logarithm, time is in minutes from the start of anesthesia, and TC is the time constant for the tissue in question (Table 3). |
|
|
|
|
Footnotes |
|---|
Dr. Eger is a paid consultant to Baxter Healthcare Corp.
Accepted for publication September 16, 2004.
|
References |
|---|
|
TopAbstractIntroductionThe Four Factors Governing...SummaryAppendix 1References |
|---|
