Influence of tidal volume and positive end-expiratory pressure on inspiratory gas distribution and gas exchange during mechanical ventilation in horses positioned in lateral recumbency.

OBJECTIVE: To study effects of intermittent positive-pressure ventilation (IPPV) with large tidal volumes and addition of positive end-expiratory pressure (PEEP) on maldistribution of ventilation in anesthetized horses positioned in lateral recumbency. ANIMALS: 6 healthy adult horses. PROCEDURE: Anesthesia was induced by i.v. infusion of thiopental sodium and guiafenesin and was maintained with supplemental doses of thiopental and i.v. infusion of chloral hydrate. Functional separation of the lungs was achieved, using a tube-in-tube intubation technique. Intermittent positive-pressure ventilation of both lungs with air was done by use of an anesthetic circle system and a ventilator. Data were collected during spontaneous respiration and during IPPV, using increasing tidal volumes with and without PEEP of 10 and 20 cm of H2O. RESULTS: Uneven distribution of inspired gas between the lungs that existed during spontaneous respiration was not altered by IPPV and large tidal volumes. Addition of PEEP caused a significant and reversible shift of inspired gas to the dependent lung and preferentially increased functional residual capacity of the nondependent lung. This was accompanied by significant increase in PaO2. With IPPV, the combined effects of PEEP and large tidal volume caused an increase of the fractional distribution of inspired gas to the dependent lung from 34% to 50%, accompanied by an increase in PaO2 and alveolar dead space of both lungs. CONCLUSIONS AND CLINICAL RELEVANCE: Use of PEEP during IPPV changes distribution of inspired gas. Increased in PaO2 can be attributed to improved ventilation-perfusion, especially in the dependent lung, in which previously collapsed lung units might have been reopened and participated again in gas exchange after redistribution of inspired gas. The most pronounced effects of IPPV and PEEP were associated with high airway pressures, which are likely to offset the beneficial effects of the increase of PaO2 on total oxygen availability to the tissues because of the expected negative effects on cardiac output.

Distribution of inspired gas to each lung in the anaesthetised horse and influence of body shape.

The distribution of inspired gas to each lung, time constants of the lungs and parameters of gas exchange were studied in 2 groups of horses (mean bwt 606 kg), anaesthetised using thiopentone and chloral hydrate and breathing room air. One group (n = 4) had a downward curved abdominal contour (round-bellied) and the other group (n = 4) had an upward curved abdominal contour (flat-bellied). An equal distribution of inspired gas between the lungs existed in both groups in dorsal recumbency. Flat-bellied horses maintained this equal distribution in lateral recumbency whereas in round-bellied horses an uneven distribution of tidal volume (VT) developed. The percentage of (VT) distributed to the dependent lung was 23% and 38% for left and right lateral recumbency respectively. The distribution of VT agreed with the ratio of time constants of the lungs in flat-bellied horses but differed markedly from this ratio in round-bellied horses suggesting that, in the latter, factors other than compliance and resistance play a role in distribution of ventilation. Round-bellied horses had a lower PaO2 and a larger (A-a)PaO2 than flat-bellied horses in all body positions. The results are compatible with the known hypothesis that pressure exerted by abdominal contents on the dependent lung and diaphragm is an important factor in ventilation/perfusion mismatch of the anaesthetised horse.

Differential artificial ventilation in anesthetized horses positioned in lateral recumbency.

Effects of differential ventilation on gas exchange were studied in 7 isoflurane-anesthetized, laterally recumbent horses, and were compared with effects of conventional ventilation, using similar minute volume. A tracheal tube-in-tube intubation technique allowed each lung to be connected separately to an anesthetic circle system with a ventilator. Two distribution patterns of tidal volume were investigated; half the tidal volume was distributed to each lung and two-thirds the tidal volume was distributed to the dependent lung. Effects of the combination of these patterns with positive end-expiratory pressure (PEEP) of 10 and 20 cm of H2O to the dependent lung were investigated. Differential ventilation maintained PaCO2, but significantly increased PaO2 from 180 to 270 mm of Hg (+44%) and decreased shunt perfusion from 22 to 19% (-15%), regardless of the distribution pattern used. Mean airway pressure was lower than the value detected during conventional ventilation. The combination of differential ventilation with selective PEEP was followed by a decrease in PaCO2 and further increase of PaO2 and decrease of shunt, which were similar for both distribution patterns. Effects of PEEP of 20 cm of H2O were more pronounced than those of PEEP of 10 cm of H2O. Owing to the combined effects of differential ventilation and selective PEEP, PaO2 increased to 399 mm of Hg and shunt decreased to 15%. This represents increase of 112% and decrease of 33% respectively, compared with values for conventional ventilation. Mean airway pressure increased maximally to 23 cm of H2O, which was 11 cm of H2O greater than the value for conventional ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of medetomidine on the pharmacokinetics of propofol in dogs.

Pharmacokinetic variables of propofol were investigated in 6 mixed-breed dogs, and the effect of medetomidine (10 micrograms/kg of body weight) on these kinetics was investigated using a two-way crossover design. On 2 occasions, dogs received either a bolus dose of propofol sufficient to allow endotracheal intubation, followed by an infusion of propofol (0.4 mg/kg/min) for 120 minutes, or medetomidine (10 micrograms/kg, IM), 15 minutes prior to induction of anesthesia as described, followed by infusion of propofol (0.2 mg/kg/min). Dogs given medetomidine received atipamezole (50 micrograms/kg, IM) at the end of the 120-minute propofol infusion. Blood propofol concentration was measured, using high-performance liquid chromatography with fluorescence detection. Mean elimination half-life, blood clearance, mean residence time, and mean volume of distribution at steady state, were 486.2 minutes, 34.4 ml/kg/min, 301.8 minutes, and 6.04 L/kg, respectively, in the absence of medetomidine, and 136.9 minutes, 36.2 ml/kg/min, 215.1 minutes, and 3.38 L/kg, respectively, in the presence of medetomidine. Mean time to walking without ataxia was 174 minutes in the nonpremedicated dogs (with a median blood propofol concentration of 2.2 micrograms/ml) and was 160 minutes in the premedicated dogs in which median blood propofol concentration was 1.03 microgram/ml.

A tracheal tube-in-tube technique for functional separation of the lungs in the horse.

The tube-in-tube technique for the functional separation of the lungs in intact anaesthetised adult horses is based on fibre-optic-guided left-sided endobronchial intubation avoiding tracheostomy. It facilitates a valid separation of the two lungs despite variability in bronchial and carinal anatomy. No clinical complications that could be related to its use have been seen.

[Monitoring of the oxygen saturation of horses during halothane anesthesia using pulse oximetry in the nasal septum]. / Uberwachung der Sauerstoffsättigung von Pferden in Halothannarkose durch Pulsoximetrie am Septum Nasale.

The use of pulse oximetry for on-line monitoring of oxygen saturation of arterial blood using a probe on the nasal septum is described in the horse. When compared to the results of blood gas analysis an excellent correlation between the two methods for measuring oxygen saturation is found. Nevertheless a discrepancy between the values for oxygen saturation provided by either method is found. This can lead to misinterpretation of oxygen saturation values generated by the pulse oximeter. The cause of this discrepancy is not clear but differences in measuring principle, presence of dyshemoglobins and differences in absorption characteristics of hemoglobin are to be ruled out as major contributors. Contrary to findings in several other animal species occasionally double counting of pulse frequency by the pulse oximeter is seen.

Surgical repair of a cranial meningocele in a calf.

A meningocele at the anterior fontanelle was diagnosed in a two-and-a-half month old Meuse-Rhine-Yssel calf. After radiographic and ultrasonographic evaluation, the meningocele was successfully repaired surgically.

[Application of the shuttle system in inhalation anesthesia in horses and ponies]. / De toepassing van het pendelsysteem bij de inhalatie-anesthesie van paarden en pony's.

Using the semi-closed to-and-fro system, oxygen-halothane anaesthesia was produced in horses and ponies. It is concluded that the system described enables the equine veterinary practitioner to carry out moderately prolonged surgery (up to approximately 120 minutes) by a justifiable and relatively safe method.

[Lumbosacral epidural anesthesia in dogs]. / De lumbo-sacrale epiduraal anesthesie bij de hond.

The technique of epidural anaesthesia is described in the present paper. In addition, the relationship between external features and the dosage of the local anaesthetic was studied. In this study, epidurography was used to visualise the cranial extension of the anaesthetic solution in the vertebral canal. It is concluded that the most reliable parameter is the length of the spinal column. Cases in which the use of epidural anaesthesia is indicated, are stated.

Volume-regulated arginine vasopressin release in conscious dogs. Evidence for dopaminergic or opioid modulation?

The possibility of a dopaminergic and/or opioid modulation of the volume-regulated release of AVP was investigated in conscious dogs. Either bromocriptine, 10 micrograms/kg body weight po, or naloxone, 0.1 mg/kg body weight iv, was administered prior to induction of nonhypotensive hypovolemia. Volume contraction of 15 ml/kg body weight was induced gradually, over a period of 30 min. Basal plasma AVP levels in the bromocriptine group were not significantly different from control group values. Bromocriptine administration significantly augmented AVP release following volume contraction. Mean arterial pressure in the bromocriptine group decreased to a slightly, but significantly, lower level than that in the control group. Mean arterial pressures, however, did not adequately explain the magnitude of the AVP response in the bromocriptine group. In the naloxone group, neither baseline levels, nor AVP values following volume contraction, differed significantly from respective control group values. In conclusion, the results suggest the possibility of a stimulatory role for endogenous dopamine in the volume-regulated, but not the basal, release of AVP in conscious dogs.

Osmolality-regulated arginine vasopressin release in conscious dogs. Evidence for dopaminergic or opioid modulation?

Experiments were performed in conscious dogs, in order to study the possibility of dopaminergic or opioid modulation of the osmolality-regulated release of AVP. Hypertonic saline (20%), infused during a period of 2 h at a rate of 0.03 ml.kg-1.min-1, induced a significant AVP response, which was not influenced by prior administration of bromocriptine or naloxone. Data presented in this report, therefore, are not in support of a dopaminergic or opioid modulation of the osmolality-regulated AVP release in dogs. The results demonstrate a great consistency in individual plasma osmolality-plasma AVP relationships, next to a large inter-individual variation.

Hypnoanalgesia with R 8110/fentanyl in the dog: pharmacodynamic and pharmacokinetic interactions.

The pharmacokinetics and clinical effects of the short-acting hypnotic R 8110 and of the narcotic analgesic fentanyl were studied in the dog. The effects of separate intravenous (i.v.) injections of R 8110 (4 mg/kg) and fentanyl (0.015 mg/kg) and of concurrent i.v. injection of the two were studied. After administration of R 8110, induction of hypnosis occurred within 1 min, maximal depth of anaesthesia and satisfactory analgesia and muscle relaxation were obtained after 5 min. The effects had decreased within 15 min and full recovery occurred within 30 min. Fentanyl alone produced neither hypnosis nor muscle relaxation. When fentanyl and R 8110 were given simultaneously, the duration of hypnosis was doubled in comparison with R 8110 alone. Moreover, markedly improved and longer lasting analgesia and muscle relaxation were observed with the combination. When the drugs were injected together, the plasma concentrations of R 8110 were initially much higher than after separate injection of R 8110, but they became similar after 30 min. Although statistically non-significant, fentanyl reduced the total plasma clearance of R 8110 (31.1 +/- 6.9 vs. 21.9 +/- 2.3 ml/kg/min) and decreased the volume of distribution (3.78 +/- 1.83 vs. 2.23 +/- 0.90 l/kg, P less than 0.05). Fentanyl did not alter the elimination half-life of R 8110. R 8110 had no apparent influence on the pharmacokinetics of fentanyl.

Comparison of the effects of etomidate and its fluoro analogue, R 8110, on plasma cortisol, 11 beta-deoxycortisol, 17 alpha-hydroxyprogesterone and testosterone concentrations in dogs.

R 8110, an imidazole derivative, was shown to be clinically superior to etomidate for induction and maintenance of anaesthesia in dogs. The present study compared the effects of intravenous (i.v.) R 8110, etomidate and Ringer solution on cortisol biosynthesis by the adrenal gland in seven male labradors. A tetracosactide challenge was carried out 30 min after the i.v. injection of 3 mg/kg of both drugs and after i.v. Ringer solution (1 ml/kg). Etomidate and R 8110 both suppressed the cortisol response to tetracosactide almost completely and increased the plasma 11 beta-deoxycortisol levels more than 20 fold. Maximal 11 beta-deoxycortisol values were reached 120 min after R 8110, and not less than 300 min after etomidate. Plasma 17 alpha-hydroxyprogesterone and testosterone concentrations did not differ between placebo and R 8110 treatment, but they decreased after etomidate. These results indicate that the effects of R 8110 on steroid biosynthesis in dogs are less pronounced than those of etomidate and are largely limited to a temporary inhibition of the 11 beta-hydroxylase in the adrenal gland.

Clinical, cardiovascular and respiratory effects of R8110 in premedicated dogs.

The clinical, cardiovascular and respiratory effects after i.v. administration of R8110, a fluoro analogue of etomidate (Fig. 1), were studied in pre-medicated dogs. The clinical observations were made at doses of 3 and 4 mg/kg body weight (BW) injected slowly i.v., whereas cardiovascular and respiratory studies were carried out at a dose rate of 3 mg/kg R8110 i.v. Induction and recovery were smooth and no significant side-effects were observed. The cardiovascular system was slightly influenced, but respiration was hardly affected. The effect of pre-medication on respiration and the cardiovascular system was hardly potentiated by R8110. Although there were significant changes in cardiovascular and biochemical parameters, all values remained within physiological limits. R8110 appeared to be a safe and reliable induction agent.

R 8110, a new short-acting hypnotic in dogs.

The clinical, respiratory and cardiovascular effects of intravenous injections of R 8110, a fluor analogue of etomidate, were studied in unpremedicated dogs. Clinical observations were carried out after intravenous injections of 3 and 4 mg kg-1 of R 8110. Cardiovascular studies were conducted after an intravenous injection of 3 mg kg-1. The drug proved to be a safe and reliable agent for induction and produced a short-lasting hypnosis and some analgesia. Both induction and recovery were smooth and rapid. Heart rate and systolic and diastolic blood pressure decreased significantly (P less than or equal to 0.05) 10 minutes after injection; the influence on arterial blood parameters was minimal.

The twitch in horses: a variant of acupuncture.

The twitch procedure in horses attenuates the increase in the heart rate evoked by pain-inducing stimuli and the reaction of the animals to such stimuli. Endorphin systems are probably involved in the effectiveness of the twitch, since its action is blocked by naloxone and its application increases plasma concentrations of immunoreactive beta-endorphin. The mode of action of the twitch cannot be explained by the generally accepted theory of divertive pain and may resemble that of classical acupuncture.

Quality comparison of different CO2 absorbents.

A comparative investigation into the quality and properties of six brands of CO2 absorbent was performed. The six investigated brands were: Aldalime, Baralyme, Drägersorb, Durasorb, Medisorb and Sodasorb. The following quality factors were investigated: the CO2 absorption capacity, dust content, the flow resistance and the colourshift. The circumstances under which the absorbent is used in the clinic were simulated as much as possible.

[Changes in the spinal fluid pressure due to spinal anaesthesia in the sheep (author's transl)]. / Liquordruckänderungen infolge Spinalanaesthesie beim Schaf.

In five adult horned sheep long-term cerebro-spinal fluid pressure measurements were carried out before, during and after ten spinal-anaesthetics with the help of a canula which was fixed on the skull. In the lumbar subarachnoid space the pressure-course was recorded also, before and after the injection of local-anaesthetic. The following reproducible phenomena were found: Rapid pressure increase after injection, increase of cardio-respiratory oscillations and in two cases a sharp decrease of pressure after removal of the spinal needle. A connection between this and the cause of postural headache in man is postulated.