Royse, Colin F. MBBS, MD, FANZCA; Royse, Alistair G. MBBS, MD, FRACS
The development of new local anesthetics has occurred as a consequence of deaths after accidental IV injection of bupivacaine identified >20 yr ago (1). The S(−) enantiomer of bupivacaine has less myocardial depressant potential than the R(+) enantiomer (2), which led to development of 2 commercially available drugs aimed at increasing the margin of safety with accidental overdose, ropivacaine and levobupivacaine. Ropivacaine is the S(−) isomer of the propyl analog of mepivacaine and bupivacaine, whereas levobupivacaine is the S(−) enantiomer of bupivacaine. Although these two compounds are similar, they may be sufficiently different to present different myocardial depressant profiles.
Cardiac toxicity is not a single entity, but can be expressed in terms of reduced contractility, impaired diastolic function, vasodilation or vasoconstriction, or effects on conduction and arrhythmogenicity. Chang et al. (3) showed that all 3 drugs were equivalent in fatal cardiac toxicity with direct intracoronary injection of drug.
Despite our desire for safer drugs, there is still considerable controversy as to whether the increased cost of the newer drugs is worth it (4). The problem is that local anesthetic catastrophe is very uncommon, with an estimated incidence of 1:10000 epidural or regional nerve blocks (5). Studies on local anesthetic toxicity have tended to examine the dose required to elicit symptoms or to produce cardiovascular toxicity. Toxicity is inferred from the dose required to produce these end-points. These represent “blunt” research tools, because they do not identify cardiac dysfunction at subcatastrophic doses. It is often assumed that hypotension after regional analgesia, especially with epidural analgesia, is caused by sympathectomy, without consideration of the possible direct effects of the absorbed drug.
Our aim was to produce dose-response curves of local anesthetic drugs using indices of systolic and diastolic left ventricular (LV) function, derived from pressure volume (PV) loops using conductance volumetry, to identify whether there are differences in contractility or vascular effects among bupivacaine, levobupivacaine, and ropivacaine at doses that may be encountered in normal clinical practice.
The project was approved by the University of Melbourne Animal Ethics Committee. The primary measurement technique is to measure PV loops with conductance volumetry during an incremental infusion of local anesthetic. Four groups of 7 rabbits were tested (nonrecovery experiment), 1 group each for bupivacaine, levobupivacaine, and ropivacaine, and a control group receiving general anesthesia and a 0.9% NaCl infusion only. The theory and methods of conductance measurements have been extensively described (6,7). We will describe the instrumentation and measurement techniques, and then summarize the intervention protocols.
New Zealand white rabbits (2.1–4.2 kg) were anesthetized with propofol (10 mg/kg bolus, then 20–50 mg · kg−1 · h−1), tracheally intubated, and mechanically ventilated. Core temperature was maintained at 39°C using a Harvard warming blanket (Edenbridge, Kent, UK). The heart was exposed via median sternotomy, and a 3F combined micromanometer pressure and dual-field conductance catheter (SPR-855; Millar Instruments, Houston, TX) was inserted into the LV through an apical stab. The conductance catheter is a 10-electrode dual field system incorporating a micromanometer that has a high frequency response to 10 KHz. A silicone sling was placed around the inferior vena cava to perform acute preload reduction. Fluid-filled catheters were inserted into the right internal jugular vein and advanced into the right atrium, and into the right carotid artery. The animals were killed at the end of the study with IV pentobarbitone 2 mg/kg.
The electrical impedance, measured using 3 segments within the LV, was recorded with a CFL-512 Cardiac Function Laboratory, and analyzed off-line with propriety software (CD Leycom, Zoetermeer, The Netherlands). High-fidelity pressure data were simultaneously acquired and displayed with impedance measurements as real-time PV loops. Five milliliters of blood was withdrawn for measurement of specific blood resistance (ρ). Volume calibration was performed by injection of 0.07 mL/kg 10% saline into the right atrium (8,9) with measurement of the parallel conductance (Vc). Data for load-independent measurements were obtained during the first 10–15 beats after caval occlusion (3–5 s).
Acquired primary data included the mean arterial blood pressure (MAP), right atrial pressure (RAP), heart rate (HR), continuous LV pressure, and LV volume.
The following measurements were performed on the primary data offline:
1. Stroke volume (SV) = end-diastolic volume − end-systolic volume (mL)
2. Cardiac index (CI) = (SV × HR)/body weight (L · min−1 · kg−1)
3. Systolic vascular resistance index (SVRI) = [(MAP − RAP)/CI] × 80 (dyne · cm · s−5 · kg−1)
4. Ejection fraction (EF) = (end-diastolic volume − end-systolic volume)/end-diastolic volume (%)
5. Isovolumetric relaxation pressure half-time (τ) = time from end-systole until pressure declines by half (ms)
6. LV end-diastolic pressure (LVEDP mm Hg)
7. From PV loops:
a. ESPVR = end-systolic PV relation (mm Hg/mL)
b. EDPVR = end-diastolic PV relation (mm Hg/mL)
c. V0 is the x axis intercept of the ESPVR
d. V100 is a measurement of contractility derived from the ESPVR and x intercept (6). It defines the volume intercept of the ESPVR curve at a pressure of 100 mm Hg. A larger value denotes worse, and a smaller value denotes better, contractility. It helps to overcome the problem of parallel shift in the ESPVR, such as occurs during myocardial ischemia or potentially from a drug effect. A parallel shift in the slope is indicative of a change in contractility, but the value for ESPVR remains the same. We have chosen V100 as the primary end-point of contractility (Fig. 1).
We adopted the concept of “clinical equivalence” in determining the infusion protocol. That is to say, levobupivacaine and bupivacaine are infused at 0.125% concentration for epidural blockade, whereas ropivacaine is infused at 0.2%. In clinical practice, the infusion rate (mL/h) is similar among the three drugs. Previous studies have tended to compare the three drugs at equivalent actual doses (mg/kg) (10,11). These studies have been criticized because the dose of ropivacaine administered in clinical practice would be less than bupivacaine or levobupivacaine if the same infusion rate is used. Accordingly, the infusion protocol outlined in Table 1 is based on a ratio of 0.125:0.2 for levobupivacaine and bupivacaine versus ropivacaine. Commercially available local anesthetic solutions were diluted in 0.9% NaCl solution so that a constant infusion of 60 mL/h was infused for all groups. The control solution was 0.9% NaCl solution alone, infused at the same rate.
Each local anesthetic was alternated with successive rabbits (e.g., rabbit 1 bupivacaine, rabbit 2 levobupivacaine, rabbit 3 ropivacaine, rabbit 4 control) so as to reduce the effect of seasonal variability or breeding mates on the results. The measurements described above were performed at baseline and at the end of each 5-min infusion cycle.
Our model aims to simulate absorption of a drug into the venous system, rather than accidental IV injection. Essentially each additional dose is administered via slow infusion over a 5-min period. Although we cannot directly compare doses in rabbits to doses in humans, if one scales up our doses to a 70-kg human, the dose delivered from a typical epidural infusion would be equivalent to that reached at the 5- to 10-min stages, whereas the dose delivered during a brachial plexus block, or from extensive local anesthetic infiltration, would lead to an accumulated dose over 40 min that could be equivalent to the end stages of the protocol. Similarly, if the epidural infusion rate was inadvertently changed to the “IV fluid infusion rate” of perhaps 125 mL/h, after 1 h of infusion, it could also lead to systemic doses reached at 35–40 min of the protocol.
Data are presented graphically as dose-response curves for each of the variables (error bars indicate within-subject sem). The effect of each drug was measured as the change from baseline values for each measurement period. Statistical analysis was performed with repeated-measures analysis of variance to extract the Group × Dose interaction term, allowing for multisample asphericity by applying the Greenhouse-Geisser correction. The interaction term tests for differences in profile over Dose between the Drug and Control groups (12). Data from our preliminary studies in the rabbit show that a sample size of 7 in each group would provide adequate power to detect a 20% difference in the end-systolic PV curve (2P = 0.05, power 0.8). This sample size is consistent with other cardiovascular studies in small animals using conductance volumetry (13–16). Analysis was performed using SPSS version 12 (SPSS Inc., Chicago, IL). Two-sided P ≤ 0.05 was regarded as statistically significant.
There were no significant differences among groups for any of the baseline variables measured, all P > 0.05 (Table 2).
The baseline values and maximal change from baseline are shown in Table 2. The characteristics of the control group were: increases in HR, CI, EF, LV end-diastolic volume (LVEDV) and LVEDP; reduction in SVRI and EDPVR; contractility showed improvement over time; and there was essentially no change in MAP, RAP, or τ.
Representative PV loops with control and the 40-min measurements overlaid are shown for each of the drugs and control in Figure 2. The change from baseline values for V100 is shown in Figure 3. A smaller value of V100 indicates improved contractility. Bupivacaine (P = 0.019) and levobupivacaine (P = 0.013) caused a reduction in contractility when compared with control, whereas contractility in the ropivacaine group was not significantly different to control (P = 0.064). For bupivacaine and levobupivacaine, the significant deviation in contractility from control occurred between an accumulated dose of 1.32 and 2.66 mg/kg, whereas ropivacaine did not show difference from control at any stage of the protocol.
There were no significant differences in EDPVR for all 3 drugs against control (bupivacaine, P = 0.123; levobupivacaine, P = 0.122; and ropivacaine, P = 0.228) (Fig. 4). There were no significant changes from control in τ for any of the drugs (bupivacaine, P = 0.057; levobupivacaine, P = 0.178; and ropivacaine, P = 0.186).
There were no significant differences between each drug against control for LVEDP (bupivacaine, P = 0.547; levobupivacaine, P = 0.465; ropivacaine, P = 0.493); or for LVEDV (bupivacaine, P = 0.776; levobupivacaine, P = 0.311; and ropivacaine, P = 0.363).
The change in values from baseline for CI, SVRI, EF, and MAP is shown in Figure 5. CI was reduced by bupivacaine (P = 0.003), but was not significantly altered with levobupivacaine (P = 0.084), or ropivacaine (P = 0.110). SVRI was increased by bupivacaine (P = 0.015) and ropivacaine (P = 0.017), but there was no significant change compared with control for levobupivacaine (P = 0.167). Levobupivacaine, however, demonstrated a biphasic vascular response where maximal vasodilatation occurred at a delivered dose of 0.26 mg/kg (20 min) thereafter increasing vascular resistance, exceeding control by the end of the protocol. Neither bupivacaine nor ropivacaine demonstrated a biphasic response, both increasing vascular resistance with increasing dose. EF was reduced by all 3 drugs compared with control (bupivacaine, P = 0.009; levobupivacaine, P = 0.029; and ropivacaine, P = 0.006). There were no significant differences in the MAP from baseline values between each drug against control (bupivacaine, P = 0.366; levobupivacaine, P = 0.536; ropivacaine, P = 0.261). There were no significant differences in HR for each drug compared against control (bupivacaine, P = 0.159; levobupivacaine, P = 0.259; ropivacaine P = 0.166), although HR decreased for all 3 drugs and increased in the control group (Table 2). There were no significant differences in the RAP from baseline values between each drug against control (bupivacaine, P = 0.674; levobupivacaine, P = 0.493; ropivacaine, P = 0.396).
Changes in vascular tone (vasoconstriction or vasodilation) are plotted against changes in contractility (V100) in Figure 6. Bupivacaine worsens contractility but also causes vasoconstriction. Levobupivacaine impairs contractility to the same degree as bupivacaine, but shows a biphasic change in vascular tone. Ropivacaine does not alter contractility compared with control, but is a mild vasoconstrictor.
There were no significant differences for pH among groups, P = 0.482 (control, 7.52 ± 0.02; ropivacaine, 7.51 ± 0.04; bupivacaine, 7.53 ± 0.04; levobupivacaine, 7.47±.02); or for Paco2, P = 0.483 (control, 29.2 ± 3.0; ropivacaine 28.9 ± 4.4; bupivacaine, 28.7 ± 4.0; levobupivacaine 35.7 ± 4.0 mm Hg); or for Pao2, P = 0.974 (control, 230.9 ± 47.8; ropivacaine, 209.8 ± 58.7; bupivacaine 208.7 ± 46.3; levobupivacaine, 203.6 ± 43.1 mm Hg).
Our study shows that levobupivacaine and bupivacaine significantly impaired myocardial contractility at doses that could be delivered in normal clinical practice, whereas ropivacaine did not. Ropivacaine does induce mild vasoconstriction, which was responsible for a reduction in the EF, but without compromise to cardiac output. Levobupivacaine reduced contractility to the same degree as bupivacaine and induced a biphasic vascular response, with maximal vasodilation at a small dose, thereafter increasing vascular resistance. Bupivacaine reduced contractility and increased vascular resistance, causing the most severe reduction in CI and EF. Diastolic function was not significantly changed by any of the drugs.
This is the first study of the cardiovascular effects of these local anesthetics in animals using integrated PV loops to obtain load-independent measurements of myocardial function. PV loops acquired during a rapid reduction in preload produce measurements of myocardial contractility and diastolic function that are relatively insensitive to ventricular loading conditions (7). This is of particular importance when evaluating drugs that affect ventricular function as well as altering vascular resistance. Reduction in ejection phase indices of LV function, such as cardiac output, dP/dtmax or EF, can be caused by either reduction in contractility or increases in vascular resistance, or by a combination of both mechanisms (17). Because differences in vascular effects have been identified in this study, the use of load-dependent measurements may lead to incorrect interpretation of contractility variables. Groban et al. (18) (using anesthetized dogs) and Scott et al. (19) (using volunteers) conducted small-dose range studies, but used estimates of EF with echocardiography as measurement of contractility. Caution should be exercised in ascribing changes in contractility based on changes in EF, particularly if the drugs have direct vascular effects, or if afterload is increased secondary to anxiety, such as might occur in volunteers when they begin to experience symptoms of central nervous system toxicity. Our data, however, are consistent with these studies in that ropivacaine seems to have less myocardial depression than bupivacaine or levobupivacaine.
Our animal model represents a simulation of what might occur in clinical practice, but caution must be exercised in extrapolating these data to the human situation. Although rabbits are frequently used for cardiovascular pharmacology experiments, they represent a small animal model (much faster HR than humans), are anesthetized, and the experiments are performed with the chest open and pericardium incised. The myocardial function in the control group improved over time, along with progressive vasodilation. The animals were subjected to “cardiac surgery” in order to instrument them for PV loops, and required differing amounts of anesthesia for different levels of stimulation during the experiment, in a similar manner to what might occur to humans undergoing surgery of this type. After sternotomy, the animals required a slower infusion rate of propofol to maintain adequate anesthesia, which may account for the progressive increase in contractility in the control group. The animals also received intravascular fluid administration, which might account for the increased cardiac output over time. Therefore, we have directly compared each drug against the control group, and have not made the assumption that this type of surgical model will not change over the time course of the experiment. Local anesthetics were infused IV, whereas in clinical application, the drug would enter the bloodstream after absorption from the site of administration. We have no direct evidence that the plasma levels reached in the rabbit would exactly match those in a human. The same measurement protocol, however, allows us to compare differences among drugs. This animal model is as close to an in vivo human experiment as can be achieved, and the use of load-independent measurements of contractility and diastolic function improved our ability to differentiate effects on the myocardial versus vascular system, which cannot easily be done in a human experiment. Additionally, we chose to infuse drugs at “clinically equivalent” dose rates, using proprietary drugs, rather than matching drug dosages in moles/gram, or adjusting differing compositions of levobupivacaine versus bupivacaine. This approach overcomes the criticism of previous studies wherein all three drugs were infused at the same dose rate, which assumes that the drugs are equipotent, and administered at the same dose in clinical practice (10,20,21). Our approach, however, is potentially disadvantageous to ropivacaine. This model, therefore, simulates everyday clinical practice rather than a rare catastrophic accident. We are not suggesting that ropivacaine does not impair contractility, and indeed there is a clear trend that ropivacaine impaired myocardial contractility, but that it did not achieve statistical significance. A larger study may have shown a small but significant difference. Trend data for each drug plotted against baseline measurements are shown in Figure 7.
Despite reductions in contractility, the MAP was preserved for all three drugs. This suggests that reflex mechanisms are active to maintain MAP despite general anesthesia with propofol, and concomitant local anesthetic infusion. The degree of vasoconstriction in part may relate to reflex mechanisms in response to a decrease in contractility and CI. This might explain why there is a biphasic response seen with levobupivacaine—vasoconstriction at a larger dose when impairment of contractility is maximal. The vasoconstriction seen with ropivacaine is minimal, and because there is no significant change in contractility, may be caused by the direct effects of the drug. We cannot fully differentiate direct effects of the drugs versus reflex mechanisms with this model, but it is a reasonable indicator of what may happen to the human in clinical practice. This is of some concern, because MAP measurement is the most frequently used indicator of hemodynamic stability in patients with epidural or regional anesthesia. Deterioration in global systolic performance will be detected at a relatively late stage, when myocardial dysfunction may be severe. Hypotension may occur earlier if there is concomitant vasodilation resulting from a sympathectomy. In this regard, levobupivacaine may be disadvantageous, because the vasodilation occurred at a small dose range, which in our simulation might occur at dose rates encountered during an epidural infusion.
We conclude that significant decline in contractility from control occurs with bupivacaine and levobupivacaine, but not with ropivacaine, at doses achievable in routine clinical practice.
We are grateful to Ms. Karen Groves (BBus) and Ms. Linda Cornthwaite-Duncan (Department of Pharmacology, University of Melbourne, Victoria, Australia) for their assistance in the conduct and analysis of this work. We thank Dr. John Ludbrook, MD, DSc, AStat (Biomedical Statistical Consulting Pty. Ltd., Melbourne, Victoria, Australia) for performing the statistical analysis and for manuscript review. We are grateful to AstraZeneca Pty. Ltd., and the CASS Foundation, for educational grants in support of this project.
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