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The direct effects of ketamine-xylazine (KET-XYL) on vascular function have not been investigated in rabbits. The short-term cardiovascular effects of intravenous (IV) KET-XYL bolus injection, therefore, should be investigated using vascular ultrasonography.
In this prospective experimental study, KET-XYL anesthesia was induced IV in 9 female New Zealand White rabbits before 3 defined test bolus injections of KET-XYL were given IV. Before and for 10 min after each KET-XYL injection vascular and hemodynamic variables were recorded at the left common carotid artery (ACC) after the 1st injection, and at the abdominal aorta (AA) after the 2nd injection. Echocardiography was performed after the 3rd injection to investigate changes in cardiac parameters.
Ketamine-xylazine IV caused a significant increase in vessel diameter at the ACC and AA. Average volumetric flow significantly decreased at the ACC and pulsatility index significantly decreased at the AA. Fractional shortening (FS) and heart rate significantly decreased, while mean arterial blood pressure initially increased.
Bolus injections of KET-XYL IV produced a transient vasodilatation at the ACC and AA. Despite central vasodilatation, bradycardia, and decrease of FS and average volumetric flow (VFave), mean arterial blood pressure did not significantly decrease indicating well-preserved cardiovascular compensatory mechanism after the ratio and doses of KET-XYL IV bolus injections used in this study.
Les effets directs du mélange kétamine-xylazine (KET-XYL) sur la fonction vasculaire n’ont pas été étudiés chez les lapins. De ce fait, les effets cardiovasculaires à court terme d’une injection intraveineuse (IV) d’un bolus de KET-XYL devraient être étudiés à l’aide d’ultra-sonographie vasculaire.
Dans cette étude prospective expérimentale, une anesthésie au KET-XYL a été induite IV chez 9 lapines de race Nouvelle-Zélande avant que 3 injections rapides définies de KET-XYL ne soient données IV. Avant l’injection et pour 10 minutes après chaque injection de KET-XYL les variables vasculaires et hémodynamiques étaient enregistrées au niveau de l’artère carotide commune gauche (ACC) après la 1ère injection, et au niveau de l’aorte abdominale (AA) après la 2e injection. Une échographie a été effectuée après la 3e injection pour étudier les changements des paramètres cardiaques.
Une injection IV de KET-XYL a causé une augmentation significative du diamètre des vaisseaux aux sites AAC et AA. Le flot volumétrique moyen a diminué de manière significative au site AAC et l’index de battement des vaisseaux a diminué de manière significative au site AA. On nota une réduction significative de la fraction de raccourcissement (FS) et du rythme cardiaque, alors que la pression artérielle moyenne augmenta initialement.
Les injections rapides IV de KT-XYL ont causé une vasodilatation transitoire aux sites ACC et AA. En dépit d’une vasodilatation centrale, d’une bradycardie et d’une diminution de FS et du flot volumétrique moyen, la pression artérielle moyenne n’a pas diminué de manière significative indiquant ainsi un mécanisme de compensation cardiovasculaire bien préservé après les ratios et injections rapides IV de KET-XYL utilisés dans la présente étude.
(Traduit par Docteur Serge Messier)
Ketamine is classified as an N-methyl-D-asparate (NMDA) receptor antagonist producing a dissociative anesthetic state. Due to a combination of inhibitory effects on the parasympathetic system and stimulatory sympathomimetic effects on the heart, ketamine has been shown to increase heart rate and arterial pressure. The increase in arterial pressure was shown to be associated with an increase in cardiac output, but no change in stroke volume (1). Accordingly, Riou et al (2) demonstrated that ketamine has a dual opposing effect on the myocardium: a positive inotropic effect associated with increased Ca2+ influx, and a negative intropotic effect possibly because of an impaired function of the sarcoplasmatic reticulum. Regarding the ketamine related action on vascular resistance, several studies have demonstrated that ketamine directly relaxes the vascular smooth muscle layer while causing a sympathetically mediated vasocon-striction (3). The net effect is that systemic vascular resistance is not significantly altered by ketamine (4,5).
Xylazine is a α2-adrenoceptor-agonist which acts on the pre- and postsynaptic nerve terminals. Several studies have shown that xylazine alone affects the cardiovascular system causing a statistically significant decrease in heart rate, cardiac output, aortic flow, an initial increase in blood pressure and peripheral resistance followed by a decrease of both (6,7).
The combination of ketamine and xylazine (KET-XYL) has been used for many species over the years and remains a popular combination for intramuscular (IM) and intravenous (IV) anesthesia in rabbits (8). In previous studies, different doses of ketamine and xylazine resulted in large variations in cardiac function (9,10); it has also been shown that the KET-XYL ratio, in particular, has a significant impact on cardiac contractility (11). According to Sandford and Colby (12) KET-XYL in a combination of 35 mg/kg (KET) and 5 mg/kg (XYL) induced a drop of blood pressure, heart rate, and respiratory rate in rabbits. Furthermore, fractional shortening (FS) and cardiac output (CO) as measurement for systolic function, significantly decrease under KET-XYL anesthesia, whereas the suppressive effects of the combination on both heart rate (HR) and FS were mainly attributed to xylazine (11).
The production of profound cardiovascular effects has been a consistent finding in earlier studies. However, there is only little known about the direct cardiovascular effects of intravenous KET-XYL bolus injections. Ultrasound imaging technologies, including Doppler-based modes, can be used for the visualization of direct vascular effects of anesthetic drugs (7,13).
The present study, therefore, was intended to clarify, in vivo, the short-term cardiovascular effects of IV ketamine-xylazine bolus injections in rabbits by using vascular ultrasound technology.
Nine healthy female New Zealand White rabbits with a body weight (BW) of 2.83 ± 0.34 kg [mean ± standard deviation (s)] and between 10- and 12-weeks-old were used in the study. They were obtained from a colony free of respiratory pathogens (Asamhof, Bad Kissingen, Germany). The animals were housed separately in cages on dust-free wooden shavings. Mean room temperature was maintained at 19°C ± 2°C, relative humidity was maintained between 50% and 60%, and a light cycle (12 h of light and 12 h of darkness) was induced. Rabbits were fed a commercial pelleted diet (Altromin; Lage, Germany); autoclaved hay and water were available ad libitum. All animals were allowed to acclimate to their environment for at least 7 d before the onset of the experiment. Experiments were approved by the local animal care committee and were in accordance with the German Animal Welfare Act.
On the day of the experiment, each rabbit was weighed and clinically examined for behavior, respiration, and cardiovascular variables. Experiments were conducted between 9 am and 12 am. A local anesthetic (EMLA cream; AstraZeneca GmbH, Germany) was put on the skin of the left ear of each rabbit, and a catheter [Vasofix; 20 SWG (1.1 × 33 mm) B. Braun Melsungen AG, Germany] was inserted in the median auricular artery for arterial blood pressure measurements and another catheter [Vasofix; 22 SWG (0.9 × 25 mm), B. Braun Melsungen AG, Germany] was inserted in the lateral auricular vein. Anesthesia was induced by IV administration of ketamine (Narketan 10; Vetoquinol/Chassot GmbH, Ravensburg, Germany), 6.0 mg/kg and xylazine (Xylapan; Vetoquinol/Chassot GmbH), 0.6 mg/kg. After the swallowing reflex was lost, the trachea was intubated (inner diameter of endotracheal tube, 2.5 to 3.0 mm) without direct laryngeal observation. Each rabbit was shaved in preparation for ultrasonographic measurements and placed on a heating-pad to maintain body temperature at 37°C to 38°C.
Ultrasonographic measurements of the left common carotid artery (ACC) and 2-dimensional (2-D) guided M-mode echocardiography were conducted with the rabbits positioned in dorsal recumbency. For ultrasonography of the abdominal aorta (AA), the rabbits were positioned in right lateral recumbency. In 7 animals, plane of anesthesia lightened after the test bolus injection. In these animals additional doses of ketamine 4.0 mg/kg and xylazine 0.4 mg/kg were given IV (additional KET-XYL bolus injections) during the probe positioning before injection of the 2nd and 3rd test bolus injection.
Rabbits were ventilated (Anesthesia Workstation; Hallowell EMC, Voelker GmbH, Kaltenkirchen, Germany) with 100% oxygen at a rate of 29 to 32 breaths/min and peak ventilation pressure of 8 to 10 cm H2O. Monitoring of mean arterial pressure (MAP), heart rate (HR), end-tidal CO2 (PE′CO2), and arterial oxygen saturation (SpO2) was conducted by use of a patient monitor (Datex Ohmeda S/5, Type F-CM1.00, Helsinki, Finland, pressure transducers: Hellige Type 4-327-I).
Three test bolus injections using ketamine 4.0 mg/kg and xylazine 0.4 mg/kg were given IV. The ultrasonographic examination was divided into 3 stages. Vascular and hemodynamic alterations induced by the first test bolus injection were recorded by ultrasonography at the ACC. After the second ketamine-xylazine test bolus ultrasonographic indicators were measured at the AA. Finally, after the third test injection echocardiographic parameters of the heart were recorded to investigate alterations of cardiac parameters.
Each stage of the experiment began with measurement of baseline values. Baseline measurements at the various locations were determined only after a stable anesthetic plane was evident, which was defined as a mean HR, MAP, SpO2, Pe′CO2, and ultrasonographic variables with no obvious fluctuation (± 5% of initial values) for 5 min. Baseline ultrasonographic data were measured at the carotid artery before the first test bolus injection, at the abdominal aorta before the second test bolus injection, and at the heart before the third test bolus injection.
After the injections, vascular and echocardiographic images were recorded at 30 s, 1 min, and then at 1-minute intervals for up to 10 min. The injection volume was adjusted with saline solution (0.9% NaCl; Delta Select GmbH, Pfullingen, Germany) to achieve 1.0 mL/bolus, and each injection was administered during a period of 20 s. Time 0 was defined as the end of each ketamine-xylazine injection. During ultrasonographic examinations of the ACC and AA, vessel images and velocity spectra were recorded for subsequent determination of vessel diameter (D), peak systolic velocity (psBFV), minimum diastolic velocity (mdBFV), end-diastolic velocity (edBFV), and time-average blood flow velocity (Vave).
The resistance index (RI) of the vessels, which was derived from psBFV and edBFV, is an important index that reflects the vascular resistance distal to the point of Doppler imaging. The RI was calculated using the following equation (14):
The pulsatility index (PI) was also used to characterize peripheral vascular resistance in accordance with the following equation (15):
where: mdBFV is minimum diastolic blood flow velocity, and Vave is time-average blood flow velocity.
Average volumetric flow (VFave) in the AA and ACC was calculated (16) as:
where: r is the vessel radius.
where: LVEDD is the left ventricular end-diastolic diameter, and LVESD is the left ventricular end-systolic diameter.
Hemodynamic data, including HR, MAP, SpO2, Pe′CO2 and the plethysmographic amplitude (PA) were recorded concurrently during each section of the ultrasonographic examination.
A 10-MHz linear transducer (FLA 10 MHz 1A; GE Vingmed, Horten, Norway) was used to conduct vascular imaging. For echo-cardiography, a 10-MHz sector transducer (FPA, 10 MHz 2A, GE Vingmed) was used. The transducers were used in conjunction with an ultrasonographic system (A/S System FIVE/REM, GE, Vingmed).
Vascular variables of the ACC and AA were measured in accordance with a previously described method (13). Doppler evaluations were conducted in pulse-wave mode. Recorded velocity spectra were assessed for quality on the basis of clarity of the visual and audible signal and then stored for subsequent measurement of psBFV, minimum diastolic blood flow velocity, edBFV, and time-average blood flow velocity (Figure 1). From these variables, the RI, PI, and average volumetric flow were derived. Furthermore, 2-D images of the vessel wall were assessed and stored for subsequent measurement of the luminal diameter between the leading edge of the innermost echogenic layer by cursor adjustment (20,21).
A right parasternal view was used for the echocardiographic assessment. Two-dimensional M-mode short-axis views at the level of the chorda tendinae were recorded for measurement of ventricular dimensions (left ventricular end-systolic and end-diastolic diameters). Fractional shortening of the left ventricle was derived from these variables.
Mean ± s values were reported for all data. Statistical comparisons were for an exploratory data analysis; thus, no correction of α error rate was considered. Value of α = 0.05 level was used to outline significant differences for each statistical comparison.
To evaluate overall patterns for variables of interest, linear mixed models (LMM) with monotonous (linear) or transient (quadratic) time effects were calculated. The LMM approach properly reflects the structure of repeated data and accounts for correlation between measurements within the same subjects. Autoregressive correlation structures (first order) as well as random effects for each rabbit were considered in the regression analysis. Effects of time were specified first by graphic assessment and verified by stepwise model derivation. When a significant effect of time was detected during LMM analysis, a post hoc Student t-test for paired samples was used to assess differences between each time point during the examination period of 10 min and the baseline value. Furthermore, the Friedman test followed by Student t-tests for paired samples were used to separately compare related samples for equivalent time points after the 1st, 2nd, and 3rd bolus for the variables HR, MAP, SpO2 and Pe′CO2. All statistical analyses were conducted with commercially available software (SPSS, Version 15.0; SPSS, Chicago, Illinois, USA).
Data for vascular ultrasonographic measurements of the left common carotid artery after injection of the first ketamine-xylazine test bolus were determined (Table I).
Vessel diameter significantly increased immediately after injection and remained significantly increased for 6 min after the first test bolus injection (significantly transient). Accordingly, psBFV significantly decreased immediately after injection (for 3 min after test bolus injection). Furthermore, VFave significantly decreased directly after injection. Based on the results for the mixed-model regression, edBFV, RI, and PI of the left common carotid artery were not significantly changed after injection of the first ketamine-xylazine test bolus injection.
Data for vascular ultrasonographic measurements of the abdominal aorta after the second test bolus injection of ketamine-xylazine were determined (Table II).
Luminal diameter of the abdominal aorta was significantly increased after injection of the ketamine-xylazine bolus. The psBFV and PI were significantly decreased, with a maximum at 30 s and 1 min after ketamine-xylazine injection, respectively. Based on the results of the mixed-model regression, edBFV, RI, and VFave of the abdominal aorta were not significantly changed after injection of the ketamine-xylazine bolus.
Selected echocardiographic variables measured after injection of the 3rd bolus of ketamine-xylazine were determined (Table III).
Fractional shortening significantly decreased, and LVESD significantly increased for 7 min with a maximum directly after injection. However, based on the results of the mixed-model regression, no significant change of LVEDD was detected.
A significant decrease in HR occurred after each bolus injection of ketamine-xylazine. Furthermore, after the 1st injection, there was immediately after injection, a significant decrease in end-tidal CO2 which then showed a significant increase 5 min after injection. The MAP showed a significant increase for 7 min after the 1st bolus injection. Based on the mixed model regression, SpO2 - did not significantly change after bolus injection.
If the values of the 1st bolus injection are compared with the corresponding values of the 2nd and 3rd bolus injections, HR, in particular, decreased (not significantly) after the 2nd and 3rd ketamine-xylazine injections.
Recorded plethysmographic amplitude revealed a reversible flattening, especially immediately after the first bolus injection (data not shown).
Induction of anesthesia by ketamine (6.0 mg/kg) and xylazine (0.6 mg/kg) was fast and without excitations. Duration of the anesthetic effect was quite variable with each rabbit; therefore, 7 of 9 animals had to be given additional ketamine-xylazine injections after measurements at the ACC and the AA, respectively, to maintain anesthesia. Recovery of the animals was very fast, sometimes accompanied by excitations. One animal was excluded from analysis at the heart because a high additional KET-XYL bolus injection was necessary during ultrasound measurements to deepen anesthesia which was considered to adulterate test bolus observations.
The purpose of this study was to investigate some short-term cardiovascular effects of intravenous ketamine-xylazine bolus injections using ultrasound measurements in anesthetized rabbits. Dopplersonography is widely accepted as an accurate, non-invasive method for evaluating blood flow in a variety of vessels, such as, the extracranial carotid system and the abdominal aorta (22). Percutaneous sonographical examination allows an immediate visualization of vascular effects and is comparable with perivascular ultrasonic flow probes as a non-invasive method (23). In this study, vascular dimensions were measured using ultrasound at the common carotid artery and the abdominal aorta. These are central elastic-type arteries and are heavily involved with cardiac contractility in the transmission of the pulse wave (24). The combination of Doppler-flow technology using high-resolution vessel images, with echocardiography and hemodynamic monitoring can provide extensive information about the cardiovascular effects of drugs. Furthermore, changes in systemic vascular resistance within the distribution area of measured vessels can be determined indirectly by calculation of the resistance and pulsatility index (14,15).
The combination of ketamine and xylazine has been used for many species over the years and remains a popular combination for intramuscular and intravenous anesthesia in rabbits (8).
In the present study, a vasodilatation of both central vessels, ACC (6.7%) and AA (6.3%), which correlated with a decreased peak systolic blood flow velocity (ACC: 37.9%, AA: 34.1%), was observed after intravenous bolus injection of ketamine 4.0 mg/kg and xylazine 0.4 mg/kg. In goats, Lee et al (7) observed a similar effect using xylazine alone (0.2 mg/kg IV) which increased the luminal diameter of the carotid artery by 20% to 30%, but did not show a significant impact on blood flow velocity. This effect was supposed to be caused by inhibition of peripheral α1-adrenoceptors resulting in a relaxation of the smooth muscle layer of central vessels (7). According to Ruffolo et al (25) α2-adrenoceptors are also associated within the intima and may play a role in the release of an endogenous relaxing factor from the endothelium. Accordingly, ketamine also acts directly on the vascular smooth muscle layer to cause relaxation by decreasing Ca2+ influx and by inhibiting mobilization of intracellular Ca2+. Reicher et al (26) measured a marked cerebral vasodilatation in rabbits using ketamine alone. This effect was associated with ketamine-mediated inhibition of tension-dependent Ca2+– influx in myocytes of the vessel wall (27).
In the present study, edBFV, RI, PI and changes in plethysmographic amplitude (PA) were recorded during the measurements which could be useful indicators of peripheral resistance (14,15,28,29). Specific changes of the plethysmographic amplitude and specific features of the waveform can be used to identify normal and abnormal peripheral perfusion patterns (30). According to Erhardt et al (31) hypotension or peripheral vasoconstriction, or both, is associated with a flattening of the plethysmographic amplitude. Ketamine-related action on peripheral vascular resistance directly dilates the vascular smooth muscle layer while causing a sympathetically mediated vasoconstriction (3) resulting in a totally non-altered systemic vascular resistance (4,5). In contrast, regarding the action of xylazine, Haskins et al (6) showed that 1.0 mg/kg of this drug produces an initial significant increase of peripheral vascular resistance which was followed by a decrease of the parameter. Accordingly, a transient flattening of the plethysmographic amplitude, a decrease of edBFV at the ACC by 35.9% and a slightly increase of RI and PI was observed in the current study immediately after the first KET-XYL injection, in particular. However, the vasoconstrictive effect was very smooth as parameters were not significantly changed. In accordance with Haskins et al (6), prolongation of KET-XYL anesthesia is accompanied by a reduction of peripheral vascular resistance which was indicated in the present study by an increase of edBFV and a reduction of RI (not significantly changed) and PI (significant) measured at the AA after the second test bolus injection.
Furthermore, the average volumetric flow immediately decreased for 1 min at the ACC (significant) and for 30 s in the AA; indicating a short lasting but significant (first test bolus injection) decrease of cardiac output immediately after KET-XYL injection. The observed fall in cardiac output corresponded to a measured significant increase of LVESD and a significant decrease in fractional shortening until 7 min after the 3rd test bolus injection. These findings are in accordance with data reported by Stypmann et al (32) and Xu et al (11); xylzine (4 mg/kg) and ketamine (50 mg/kg) combination IM was associated in their studies with a significant decrease of fractional shortening and cardiac output, a reversal of the E/A-ratio and an increase in the left ventricular ejection time and isovolumetric contraction time and therewith in the myocardial performance index (Tei index), indicating a significant global impairment of systolic and diastolic function.
According to Kunst et al (33), the negative inotropic effect of ketamine is caused by a decrease in intracellular Ca2+ and net trans-sarcolemmal Ca2+ influx. However, the direct negative inotropic effect of ketamine on myocardium can be masked by central stimulation. This is probably an indirect effect, with a combination of inhibitory effects on the parasympathetic vagal innervation of the heart and a stimulatory sympathomimetic effect on the heart (34). Virtue et al (35) suggested that the stimulatory cardiovascular effects of ketamine could result from endogenous release of catecholamines. Dowdy and Kaya (36) proposed that the pressor effect of ketamine results from desensitization of the arterial baroreceptors. Furthermore, Miletich et al (37) reported that in the isolated rat heart, ketamine interferes with the re-uptake of norepinephrine and that some of the cardiovascular effects of ketamine could result from a similar action on the postganglionic adrenergic neuron. Regarding xylazine-related cardiac effects, Haskins et al (6) and Stypmann et al (32) mainly associated the decrease in cardiac output with the drug-related bradycardia. The decrease in heart rate, however, may be caused by a direct or indirect increase in vagal tone, enhanced baroreceptor reflex activity, or a decrease in sympathetic activity (6). Xylazine effects are further attributed to its α2-receptor stimulating effect which decreases the central release of the transmitter norepinephrine (6).
Accordingly, in the present study, HR decreased after each KET-XYL injection; however, despite this, MAP was significantly increased after the 1st test bolus injection and did not further change after the 2nd and 3rd test bolus injections. These findings are in accordance with data reported by Haskins et al (6): there, 1.0 mg/kg xylazine followed by ketamine (10 mg/kg) caused an initial significant decrease in heart rate, cardiac output, but additionally, increases in systemic blood pressure and central venous pressure by increasing peripheral vascular resistance. In the present study, peripheral vascular resistance in the distribution area of the ACC slightly increased. The short initial hypertension followed by a longer hypotensive period, characteristic of xylazine, can also be observed in dogs (6) and cats (38), due to activation of peripheral α1- and α2-adrenoceptors and central α2-adrenoceptors. Alpha1-adrenoceptors are generally localized postsynaptically at the smooth muscle layer of the arterioles and venules and at the myocardium. They produce a constriction of the vessels causing an increase of blood pressure and myocardial contractility (25). Alpha2-adrenoceptors are generally localized presynaptically and postsynaptically (in the CNS). The presynaptic peripheral adreno-ceptor activation inhibits the release of noradrenalin, acetylcholine, serotonin and dopamine, the postsynaptic central adrenoceptor activation reduces the sympathetic tone, followed by a decrease of blood pressure, and stimulates the vagal tone (39,40). However, in the present study, a hypotension did not occur after prolongation of anesthesia, indicating a well-preserved cardiovascular compensatory mechanism by using the proposed ratio and doses of KET-XYL.
The study’s principal limitation stems from the fact that ultrasound measurements were not taken simultaneously at the various locations; therefore, cumulative effects have to be assumed rather than comparing the data measured at the different locations. A further limitation is that it was necessary to administer additional doses of KET-XYL individually, as the plane of anesthesia lightened after the test bolus injections. Different cumulative effects have to be assumed, therefore, than comparing the data measured at abdominal aorta and the heart. To minimize direct effects of additional KET- XYL bolus injections, stable baseline values were always awaited by monitoring the animals with a 10-minute break before a new test bolus was administered.
However, hemodynamic variables were simultaneously recorded throughout the different parts of the study and comparing changes after the 2nd and 3rd in variables like HR, MAP, SpO2, and Pe′CO2 test bolus injections with those of the 1st test bolus injection, in particular, a decrease of HR occurred (not significant), confirming a slight accumulation of xylazine and a slight potentiation of the bradycardia during further bolus application.
In summary, the current study indicates that the IV bolus injection of ketamine 4.0 mg/kg and xylazine 0.4 mg/kg induces a significant, transient decrease in the diameters of the ACC and the AA. This was associated with a slight increase of peripheral vascular resistance, a significant decrease of volumetric flow at the ACC, and a decrease of pulsatility index at the AA after the 2nd test bolus injection. Furthermore, the 3rd test bolus injection was associated with a significant decrease of fractional shortening and LVESD. Regarding hemodynamic parameters, only HR significantly decreased after each test bolus injection with an increasing effect comparing the values of the first with those of the 2nd and the 3rd test bolus injections. Finally, MAP significantly increased after the 1st test bolus injection and did not change further after the 2nd and 3rd test bolus injections.
In conclusion, in the present study KET-XYL bolus injections did induce a central vasodilatation and a short-lasting and transient decrease of cardiac function. The impact on peripheral vascular resistance was biphasic and slight; only indicating a significant change (decrease of PI) after the 2nd test bolus injection. However, despite central vasodilatation, bradycardia, decrease of FS and VFave, mean arterial blood pressure did not significantly decrease after all test bolus injections, indicating a well-preserved cardiovascular compensatory mechanism after the used ratio and doses of KET-XYL IV bolus injections.
The authors are grateful for the contributions of Heike Wamser.