Materials and Methods

Animals—Six heartworm-negative adult dogs (3 males and 3 females), each of which weighed between 8 and 12 kg, were subjects in the experiments reported here. Dogs were deemed healthy on the basis of anticipated results for a CBC and physical examination, which included thorough auscultation of the thorax. The experiments were approved by the Animal Care and Use Committee of The Ohio State University.

Equipment and calibration—Oxygen and carbon dioxide were delivered via a circle anesthetic breathing system. Inhalant anesthetics were delivered by out-of-circle, calibrated halothane^a or isoflurane^b vaporizers. End-tidal concentration of inhaled anesthetic, FIO₂, and FICO₂ were measured by use of a respiratory gas analyzer.^c The gas analyzer was calibrated before each experiment with known concentrations of halothane (or isoflurane), oxygen, and carbon dioxide.

Airflow and airway pressures were measured by use of a wire-mesh pneumotachograph flowmeter and an attached differential pressure transducer^d positioned between the dog and breathing system. The pressure transducer was calibrated at 0 and 10 cm H₂O. Calibration of equipment was performed at the start of each experiment. Changes in V_T were obtained by electrical integration of the flow signal.^e Gas flow, RR, V_T, V_M, intratracheal pressure, and an ECG were simultaneously recorded by use of a computer-based data acquisition system.^f

The IOCP was determined by temporarily rapidly inflating a pneumatically controlled balloon^g positioned in the inspiratory limb of the breathing circuit.⁴ This technique permitted the measurement of IOCP_max reached during occlusion throughout a complete respiratory cycle. The inflated balloon completely occluded the inspiratory limb of the breathing circuit, which prevented gas flow.

Experimental protocol—The study was conducted in accordance with a randomized crossover design. Each dog was subjected to 4 treatments. Dogs were anesthetized by administration of halothane and injected IV with saline (0.9% NaCl) solution or medetomidine (5 μg/kg) and similarly were anesthetized by administration of isoflurane and injected IV with saline solution or medetomidine (5 μg/kg). Order of treatments for each dog was randomized. There was a minimum of 7 days between successive treatments for each dog.

MAC determination—In a preliminary study, we found that IV administration of medetomidine at a dosage of 5 μg/kg to dogs maintained at a constant inhalant anesthetic concentration of 1.4 MAC eliminated all response to an increase in inspired carbon dioxide, which suggested marked depression of central respiratory drive. Medetomidine has potent anesthetic-sparing qualities, which result in a marked reduction in the amount of isoflurane required to induce a surgical plane of anesthesia.⁶ Therefore, in preliminary experiments, we determined the MAC required to maintain anesthesia with halothane or isoflurane before and after the IV administration of medetomidine at a dosage of 5 μg/kg. The noxious stimulus used to determine MAC in those preliminary experiments was continuous airway occlusion for 60 seconds while breathing FICO₂ of 15%.⁷ This amount of carbon dioxide was selected because it was greater than the maximum FICO₂ administered to dogs during the study reported here. Gross purposeful movements, including but not limited to paddling, head lifting, and gagging on the endotracheal tube, were considered positive responses to this noxious stimulus; MAC was determined immediately before and 5, 30, and 60 minutes after administration of medetomidine (5 μg/kg, IV).

Collection of respiratory and IOCP data—Anesthesia was induced by administering inhalant anesthetic via a tight-fitting face mask attached to the circle system with the vaporizer set at 5%. After induction, dogs were orotracheally intubated with an appropriately sized, cuffed endotracheal tube. Dogs were placed on a circulating warm water blanket during anesthesia. During the experiments, dogs breathed specific concentrations of oxygen and carbon dioxide and were administered halothane or isoflurane at selected multiples of MAC. Two gas flow meters were independently used to control delivery of oxygen and carbon dioxide into the breathing circuit and to adjust the required FICO₂ to steady-state values. Equilibration to achieve steady-state carbon dioxide values required approximately 10 minutes. Steady-state values for RR, gas flow, and end-tidal carbon dioxide concentration were confirmed before data collection and were achieved ≤ 5 minutes after carbon dioxide had equilibrated. Respiratory variables (RR, V_T, and V_M) and IOCP_max were recorded 15 minutes before administration of medetomidine or saline solution. Ten to 20 minutes was required for the amount of carbon dioxide and any concomitant increase in sympathetic tone to return to preadministration steady-state values.

Fifteen minutes after baseline values were obtained, medetomidine or saline solution was administered IV. Time of IV injection was designated as time 0. Respiratory variables were then recorded 5, 30, and 60 minutes after administration of medetomidine or saline solution. All recorded variables were obtained at an FICO₂ of 0% followed by an FICO₂ of 10% to generate response data for changes in carbon dioxide concentrations. Order of delivery for FICO₂ was not randomized (ie, dogs were always subjected to an FICO₂ of 0% and then subsequently to an FICO₂ of 10%). Inhalant concentration for halothane and isoflurane was 1.4 MAC at baseline and at all times after administration of saline solution. Inhalant concentration for halothane and isoflurane was 0.4, 0.6, and 1.2 MAC at 5, 30, and 60 minutes after administration of medetomidine, respectively. Respiratory data were recorded beginning 5 breaths before the first of 3 airway occlusions, which were performed at each time point and each value of FICO₂.

The IOCP_max was directly measured after the experiments from computer printouts of tracheal pressure recorded during each occlusion. The IOCP_max and V_M were plotted as functions of FICO₂. The slope of the line was calculated for each time point in accordance with the following equations: (IOCP_max at FICO₂ of 10% − IOCP_max at FICO₂ of 0%)/(FICO₂ of 10% − FICO₂ of 0%) and (V_M at FICO₂ of 10% − V_M at FICO₂ of 0%)/(FICO₂ of 10% − FICO₂ of 0%).

Statistical analysis—Data for IOCP and respiratory variables were analyzed by use of a 2-way ANOVA for repeated measures. Data were analyzed by use of a statistical software package.^h Differences within and among groups were detected by use of the Tukey multiple comparison post hoc test. Results were considered significant at values of P ≤ 0.05.

Results

Effects of medetomidine on MAC of halothane and isoflurane—Mean ± SD MAC was 0.9 ± 0.1% for halothane and 1.2 ± 0.1% for isoflurane. Mean MAC multiple (ie, actual percentage of inhalant divided by the MAC) required to result in no physical response (ie, no gross purposeful movement) to the noxious stimulus in all dogs was 1.4 ± 0.1% for both inhalant anesthetics (1.3 ± 0.1% for halothane and 1.7 ± 0.1% for isoflurane). There were no differences in the reduction of MAC after administration of medetomidine between halothane- and isoflurane-anesthetized dogs at any of the time points; therefore, MAC data were combined for analysis. Administration of medetomidine reduced the mean MAC multiple required to induce anesthesia in all dogs to 0.4 ± 0.1, 0.6 ± 0.1, and 1.2 ± 0.1 at 5, 30, and 60 minutes after injection, respectively. This corresponded to mean inhalant anesthetic concentrations of 0.4 ± 0.1%, 0.5 ± 0.1%, and 1.1 ± 0.1% for halothane and 0.5 ± 0.1%, 0.7 ± 0.1%, and 1.4 ± 0.1% for isoflurane.

Anesthesia—Total duration of anesthesia for collection of respiratory data (including IOCP_max) was approximately 2.5 hours, which included stabilization of each dog, baseline measurements, a 15-minute interval before administration of medetomidine or saline solution, and 60 minutes after injection. Body temperature did not decrease significantly during this time, and dogs were extubated within 15 minutes after the end of each experiment.

Effects of FICO₂ on respiratory variables— Anesthesia achieved by use of isoflurane or halothane at 1.4 MAC did not significantly alter RR, compared with an RR of 18 breaths/min in conscious dogs; however, administration of isoflurane did cause RR to have slightly lower values. The RR was generally higher when dogs were anesthetized with halothane, compared with RR for dogs when anesthetized with isoflurane. Breathing an FICO₂ of 10% did not significantly increase RR, compared with the RR when breathing an FICO₂ of 0% (Table 1).

Anesthesia achieved by administration of halothane and isoflurane resulted in a significant decrease in mean ± SD V_T (halothane, 91 ± 8 mL; isoflurane, 109 ± 6 mL), compared with V_T for conscious dogs (171 ± 11 mL). Regardless of inhalant, V_T always increased significantly when dogs breathed an FICO₂ of 10%, compared with V_T when they breathed an FICO₂ of 0% (Table 1).

The V_M was generally lower in dogs anesthetized by administration of isoflurane, compared with the V_M for dogs anesthetized by use of halothane, regardless of FICO₂, and was significantly lower at an FICO₂ of 10% (Figure 1). The V_M was always significantly increased when dogs breathed an FICO₂ of 10%, compared with V_M when dogs breathed an FICO₂ of 0%. Slope of the V_M/FICO₂ response line was significantly greater when dogs were anesthetized by administration of halothane, compared with the slope for the response line when dogs were anesthetized by administration of isoflurane.

Effects of medetomidine on respiratory variables—The RR was decreased after the IV administration of medetomidine in halothane- and isoflurane-anesthetized dogs. The RR was not affected by medetomidine when the FICO₂ was 10% (Table 1).

Medetomidine did not significantly decrease V_T from the baseline value. Regardless of treatment group, V_T always increased significantly when dogs breathed an FICO₂ of 10%, compared with V_T when dogs breathed an FICO₂ of 0%. Regardless of the FICO₂, V_T was greater in dogs anesthetized by administration of halothane and administered medetomidine, compared with V_T for all other treatment groups.

In isoflurane-anesthetized dogs, administration of medetomidine caused little change in the slope of V_M in response to an FICO₂ of 10%, compared with the slope of V_M in response to an FICO₂ of 10% when dogs were administered saline solution. The slope of the V_M/FICO₂ line for halothane-anesthetized dogs administered medetomidine was greater than the slope of the V_M/FICO₂ response line in halothane-anesthetized dogs administered saline solution or isoflurane-anesthetized dogs administered medetomidine or saline solution (Figure 1).

Effects of inhalants, FICO₂, time, and medetomidine on IOCP_max—The IOCP_max (range, 12.4 to 19.2 mm H₂O) did not differ significantly when dogs breathed an FICO₂ of 0%, regardless of inhalant anesthetic and administration of medetomidine (Figure 2). The IOCP_max always increased in response to an increase in FICO₂, regardless of inhalant or administration of medetomidine. The IOCP_max and slope of IOCP_max/FICO₂ response line were generally lower when dogs were anesthetized by administration of halothane (IOCP_max range, 32.1 to 38.1 mm H₂O), compared with results when dogs were anesthetized by administration of isoflurane (IOCP_max range, 31.9 to 48.2 mm H₂O). Administration of medetomidine did not affect slope of the IOCP_max/FICO₂ response line in halothane-anesthetized dogs, whereas administration of medetomidine generally decreased, although not significantly, the slope of the IOCP_max/FICO₂ response line in isoflurane-anesthetized dogs.

Effects of time on IOCP_max and V_M—The IOCP_max did not differ significantly over time when dogs were anesthetized by administration of halothane, regardless of administration of medetomidine or FICO₂. The pattern by which IOCP_max decreased over time when dogs anesthetized by use of halothane were administered medetomidine and breathed FICO₂ of 10% was not significant (Figure 3).

The IOCP_max did not differ significantly over time when dogs anesthetized by use of isoflurane were administered saline solution, regardless of FICO₂, and when isoflurane-anesthetized dogs were administered medetomidine and breathed an FICO₂ of 10%. The increase in IOCP_max after dogs anesthetized by use of isoflurane were administered medetomidine and breathed an FICO₂ of 0% was significantly different 30 minutes after injection of medetomidine, compared with IOCP_max at baseline. By 60 minutes after injection, IOCP_max in this group was decreasing and was no longer significantly different from the baseline value.

The V_M did not differ significantly over time when dogs were anesthetized by administration of halothane and breathed an FICO₂ of 0%, regardless of administration of medetomidine. The V_M also did not change significantly over time when halothane-anesthetized dogs were administered saline solution and breathed an FICO₂ of 10%. There was a significant decrease in V_M at 60 minutes when halothane-anesthetized dogs were administered medetomidine and breathed an FICO₂ of 10%, compared with the V_M at baseline. There was no difference in V_M over time in dogs anesthetized by administration of isoflurane, regardless of FICO₂ or administration of medetomidine.

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Response of V_M to FICO₂ at baseline (A) and 5 (B), 30 (C), and 60 (D) minutes after IV injection of medetomidine (5 μg/kg) or saline (0.9% NaCl) solution to dogs anesthetized by administration of halothane or isoflurane. Baseline values were obtained 15 minutes before IV injection of medetomidine or saline solution. Time of IV injection of medetomidine or saline solution was designated as time 0. Responses were recorded for dogs administered isoflurane at 1.4 MAC and saline solution IV (black circles), halothane at 1.4 MAC and saline solution IV (white circles), isoflurane at 1.4 MAC and medetomidine IV (black squares), halothane at 1.4 MAC and medetomidine IV (white squares), isoflurane at 0.4 MAC and medetomidine IV (black triangles), halothane at 0.4 MAC and medetomidine IV (white triangles), isoflurane at 0.6 MAC and medetomidine IV (black diamonds), halothane at 0.6 MAC and medetomidine IV (white diamonds), isoflurane at 1.2 MAC and medetomidine IV (crosses), and halothane at 1.2 MAC and medetomidine IV (plus signs). *Within a treatment group, value for V_M differs significantly (P ≤ 0.05) from value for V_M at an FICO₂ of 0%. †Slope of the line for dogs administered halothane and medetomidine IV differs significantly (P ≤ 0.05) from the slope of the line for dogs administered the corresponding MAC of isoflurane and medetomidine IV. ‡Slope of the line for dogs administered halothane and medetomidine IV differs significantly (P ≤ 0.05) from the slope of the line for dogs administered the corresponding MAC of halothane and saline solution IV.

Citation: American Journal of Veterinary Research 67, 5; 10.2460/ajvr.67.5.782

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Response of IOCP to FICO₂ at baseline (A) and 5 (B), 30 (C), and 60 (D) minutes after IV injection of medetomidine (5 μg/kg) or saline solution to dogs anesthetized by administration of halothane or isoflurane. *Within a treatment group, value for IOCP differs significantly (P ≤ 0.05) from value for IOCP at an FICO₂ of 0%. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 67, 5; 10.2460/ajvr.67.5.782

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Discussion

In the study reported here, increases in FICO₂ generally caused increases in V_T, V_M, and IOCP_max in isoflurane- and halothane-anesthetized dogs. These changes reflect an increase in output from the respiratory centers in the brain in response to a known stimulus of those respiratory centers.⁸ Administration of a relatively low dose of medetomidine (5.0 μg/kg, IV) caused decreases in the concentration of inhalant anesthetic required to maintain anesthesia and, in general, had minimal effects on respiratory variables. Duration of anesthesia played a minor role, with changes in respiratory variables being closely related to the decrease in inhalant concentration when medetomidine was administered.

Dogs anesthetized with halothane or isoflurane, and without other drugs, had an increase in RR, V_T, V_M, and IOCP_max in response to an increase in FICO₂. However, dogs administered isoflurane had a lower RR, similar V_T, and decreased V_M, compared with results for halothane-anesthetized dogs. The V_M in response to carbon dioxide challenge was greater with halothane than isoflurane. This finding is consistent with other studies involving dogs and people^9,10 in which isoflurane had a greater respiratory depressant effect than was evident for halothane.

Repetitive measurement of IOCP is the current standard for quantification of electrical output of the respiratory centers.⁵ When the airway of a spontaneously breathing unconscious animal is occluded at the end of expiration, there is no flow or volume change during the inspiratory phase that follows, and a negative pressure is generated in the airway when the subject attempts to inspire.¹¹ Because there is no lung inflation, there is a lack of sensory input from pulmonary stretch receptors.¹² Inspiratory occlusion also allows the measurement of output under an infinite load.¹³ This method of evaluating the control signal is independent of resistance (because there is no flow) and elastance (because there is no change in volume). This results in uniform impedance allowing comparisons of IOCP, and therefore respiratory center output, among subjects and experimental conditions.⁵ However, there are limitations to this technique. The act of occlusion is not a typical action of the muscles of respiration, and this may trigger reflexes that are not generally associated with breathing. Also, diseases of the lungs, muscles of respiration, or chest wall can alter pressures generated within the airways through changes in resistance.

Increases in FICO₂ stimulate the respiratory centers by activating central chemoreceptors located in the brainstem, with approximately 20% of sensory input originating from peripheral chemoreceptors leading to increases in RR, V_T, and V_M.^14,15 Stimulation of α₂-adrenoreceptors in the locus coeruleus causes sedation in dogs, and experimentally created lesions within the loci coerulei of cats attenuated the response of cranial blood flow during hypercapnia.¹⁶ Nerve fibers extend from the locus coeruleus to other parts of the brainstem and are generally excitatory in nature because they have norepinephrine as a neurotransmitter.¹⁷ Inhibition of excitatory stimuli to the respiratory centers would explain the decreased ventilatory drive detected after the administration of medetomidine.

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Response of IOCP (A and B) and V_M (C and D) during anesthesia induced by administration of halothane (A and C) or isoflurane (B and D) with IV administration of medetomidine (circles) or saline solution (squares) and an FICO₂ of 0% (solid lines) or 10% (dashed lines). ^a,bWithin a time point and FICO₂ percentage, values with different letters differ significantly (P ≤ 0.05).

Citation: American Journal of Veterinary Research 67, 5; 10.2460/ajvr.67.5.782

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Halothane is considered to be unique among the currently available inhalant anesthetics because, contrary to other inhalants, V_M and IOCP are not reduced in humans.^10,18,19 When medetomidine was not administered, V_M was consistently lower when dogs were anesthetized with isoflurane, compared with V_M when dogs were administered halothane at equipotent doses (ie, similar MAC multiples). In 1 study²⁰ in which investigators evaluated respiratory variables in dogs anesthetized with isoflurane and halothane, V_T tended to be higher in isoflurane-anesthetized dogs and RR was consistently higher in halothane-anesthetized dogs (V_M was not measured or calculated). Calculation of minute ventilation from the mean V_T and RR values reported in that study²⁰ reveals that V_M was consistently higher in the halothane-anesthetized dogs, compared with V_M for the isoflurane-anesthetized dogs. However, it is not possible to determine whether these values differed significantly without examination of the original data. Compared with effects of halothane,¹⁰ isoflurane causes greater depression of ventilation (V_T and V_M) in humans, as determined by end-tidal carbon dioxide concentration and IOCP. In humans, halothane allows increases in central respiratory output in response to the increase in carbon dioxide, compared with increases in response to isoflurane.¹⁰ Analysis of these data suggests that isoflurane causes more respiratory depression than does halothane.

At an FICO₂ of 0%, IOCP_max did not differ among groups. For anesthesia achieved by administration of isoflurane, baseline IOCP_max was greater when dogs were administered saline solution, compared with values when dogs were administered medetomidine. This was attributable to an outlier for the medetomidine injection in which the measured IOCP_max was 19 mm H₂O for 1 dog. Values for the remainder of the dogs ranged from 33 to 43 mm H₂O. In general, IOCP_max values were consistent for each dog. We have no explanation for the outlier. The values of IOCP_max were higher with isoflurane-induced anesthesia, compared with values of IOCP_max for halothane-induced anesthesia, when FICO₂ was 10%. Similarly, the slope of IOCP_max/FICO₂ was significantly higher when dogs were anesthetized with isoflurane, compared with the slope of IOCP_max/FICO₂ when they were anesthetized with halothane.

Slope of the inspiratory pressure wave, although consistent during undisturbed conscious breathing, can be affected by anesthetic agents.⁵ Halothane can have varying effects on ventilatory drive and can cause a steeper initial portion of the occlusion pressure wave-form in dogs and people,^18,21 compared with effects for enflurane, the stereoisomer of isoflurane. The lower RR evident during isoflurane-induced anesthesia may allow more time for inspiration and thus more time to generate a higher IOCP. In the study reported here, RR at an FICO₂ of 10% was significantly higher in halothane-anesthetized dogs, which may account for the difference we observed in IOCP_max.

Marked inhalant anesthetic-sparing effects were evident after IV administration of medetomidine (5 μg/kg) to isoflurane- or halothane-anesthetized dogs, resulting in large reductions in the concentration of inhalant required to maintain a similar depth of anesthesia 5 and 30 minutes after medetomidine injection. By 60 minutes after medetomidine injection, MAC was much closer to baseline values. This can be explained by the waning effects of medetomidine because temperature, which has a major impact on MAC,¹ did not change significantly in the dogs of our study. In another study,²² it was reported that medetomidine administered to conscious dogs at a dosage of 5 μg/kg has a clinical effect for 60 minutes, which is similar to values reported here.

Medetomidine has an MAC-sparing effect in isoflurane-anesthetized dogs.⁶ Because we did not determine MAC by any other method, we cannot reach conclusions about comparisons with other techniques used for determining MAC. Also, it was not possible to determine MAC with absolute accuracy within 5 minutes after medetomidine administration because periods of stability between stimulation were required. We partially extrapolated backward from the MAC at 30 minutes after medetomidine administration (0.6 MAC) to obtain a MAC multiple that was practical and consistent for collecting respiratory data at the time point 5 minutes after medetomidine administration.

Sensitivity of respiratory centers can be assessed by evaluating the slope of the curve of any respiratory variable (such as RR or IOCP_max) plotted against changes in carbon dioxide concentration. A decrease in the slope from baseline values indicates a decrease in sensitivity of the respiratory centers, assuming that the lungs and nervous system are not affected by disease.²³ Administration of medetomidine increased V_M in halothane-anesthetized dogs, compared with results for control dogs, but did not affect the slope of the IOCP_max/FICO₂ response. The percentage of inhalant had been decreased at that time, which likely accounted for this difference. Interestingly, this is in contrast to the results we observed when isoflurane was administered but no change was seen in V_M despite the decrease in inhalant. There is evidence that dogs anesthetized with halothane have an increase in the contribution of rib cage expansion during rebreathing, which may allow dogs to compensate through recruitment of parasternal intercostal muscles for decreases expected as a result of inhalant anesthesia.²⁴ The exact mechanism for this compensation is unclear, but because separate groups of medullary inspiratory and expiratory neurons drive the motor neurons of inspiratory and expiratory muscle groups, a differential effect of halothane on central respiratory centers may explain such recruitment.

Medetomidine is licensed for use as a sedative and preanesthetic medication in dogs at a dosage of 40 μg/kg, although analysis of data suggests avoiding such high doses during anesthesia, particularly in older dogs.^25,26 The decrease in respiratory center output resulting from the administration of medetomidine at a dosage of 5 μg/kg in isoflurane-anesthetized dogs reveals that medetomidine contributes to respiratory depression. The decreased MAC required to maintain a similar depth of anesthesia for halothane- and isoflurane-anesthetized dogs in the study reported here emphasizes the MAC-sparing properties of medetomidine. It is advisable to closely monitor anesthetic depth, respiration, and pulmonary gas exchange in halothane- or isofluraneanesthetized dogs administered medetomidine.

Analysis of the data reported here emphasizes the inhalant anesthetic-sparing effects of medetomidine in dogs. Evaluation of differences between the effects of halothane and isoflurane on respiratory variables suggests they are likely a result of differences in drug effect on central respiratory centers. Additional studies are required to fully elucidate the precise differences for the mechanisms involved.