The activity of the CNS as well as the ANS is influenced by general anesthesia. By use of clinical variables such as respiratory rate, heart rate, and blood pressure, information derived from the ANS is commonly incorporated into the assessment of anesthetic depth.1 Objective quantification of ANS activity might improve determination of anesthetic depth. An established method for quantification of ANS activity is HRV analysis,2,3 which is an analysis of the variability of R-R intervals in ECG recordings. Commonly derived HRV variables reflect the influence of sympathetic and parasympathetic activity. The time domain variable SDNN is regarded as an estimate of overall HRV, reflecting the total power, whereas, for example, RMSSD represents short-term variations and the parasympathetic activity.4,5 The frequency analysis variable HF power also reflects mainly the parasympathetic influence, and both autonomic branches influence the LF power variable.6
Several studies reported in the veterinary literature have used HRV for the assessment of heart disease5 and pacing-induced heart failure in dogs,7 differences in autonomous regulation among breeds of dogs,8 or pain in horses.9 In humans, a dose-related decrease in total ANS activity during isoflurane anesthesia has been described,10 whereas sevoflurane did not change cardiac parasympathetic tone.11 In addition, HRV variables were able to differentiate the awake state from anesthesia achieved with propofol and remifentanil.12 After various drugs were tested in humans in multiple situations, it became clear that HRV greatly depends on the given medication and not on anesthetic depth.13
However, the interaction of anesthetic depth, type of anesthetic drug or adjuvant drugs administered, and nociception can be complex and will influence the response of the ANS. Therefore, the objectives of the study reported here were to determine the influence of 3 anesthetic protocols (each commonly used in veterinary anesthesia) and multiples of MAC with and without nociceptive stimulation on HRV in dogs. The intent was to evaluate the influence of nociceptive stimulation on HRV in dogs as well as assess the ability of HRV to discriminate levels of anesthetic depth as defined by the MAC concept.
Materials and Methods
Six adult Beagles (3 sexually intact females, 1 castrated female, and 2 castrated males) were selected for use in the study. For these dogs, mean ± SD body weight was 16.3 ± 1.0 kg and mean age was 4.0 ± 2.7 years. The dogs were housed in separate kennels and were fed commercial dry adult maintenance dog food. They were considered healthy on the basis of results of physical examination and hematologic and biochemical analyses. The dogs were vaccinated and dewormed on a regular basis. Food but not water was withheld for 6 to 8 hours prior to each anesthetic session. The study protocol was approved by the Animal Care and Use Committee of the local district government of Lower Saxony, Germany.
Experimental design—Each dog underwent 3 anesthetic sessions (each involving a different anesthetic protocol) with at least a 1-week washout interval between sessions. The 3 protocols involved isoflurane alone, IsoD, and IsoR.
Each anesthetic session was undertaken at the same time of the day (afternoon). In each session, anesthesia was induced with isoflurane in oxygen (5 vol%; 5 L/min) via a face mask until endotracheal intubation of the dog was possible. In the isoflurane session, each dog received isoflurane only for maintenance of anesthesia. In the IsoD session, each dog was given a loading dose of dexmedetomidine (3 μg/kg, IV) delivered via a syringe pump over a period of 10 minutes, followed by induction of anesthesia; anesthesia was maintained via inhalation of isoflurane in oxygen and a CRI of dexmedetomidine (3 μg/kg/h, IV).14 In the IsoR session, a remifentanil CRI (18 μg/kg/h, IV)a was initiated without a loading dose before isoflurane in oxygen was administered for induction and maintenance of anesthesia. Both drugs used for the CRIs were diluted in saline (0.9% NaCl) solution. Among the 6 dogs, the IsoD and IsoR sessions followed the isoflurane session in randomized order.
Instrumentation—In each anesthetic session, an instrumentation and stabilization period of at least 1 hour was allowed after intubation of each dog. During that period, the dog was maintained at the expected ETISO of 1 MAC (ie, 1.7, 1.0, and 1.0 vol% of isoflurane for the isoflurane, IsoD, and IsoR sessions, respectively). The endotracheal tube was connected to a circle breathing system operated in a semiclosed mode with an oxygen flow rate of 1 L/min. Each dog was placed in right lateral recumbency and mechanically ventilated, with settings adjusted to maintain eucapnia (35 to 45 mm Hg). Esophageal temperature was kept constant (37.6° [± 0.5°C]) with a warm air blanket. An indwelling IV catheter was placed in 1 cephalic vein, and balanced electrolyte solution was infused at 5 mL/kg/h with a volumetric pump.
To assess hemodynamic responses, arterial blood pressure variables were monitored via an arterial catheter placed in a dorsal pedal artery connected to a pre-calibrated pressure transducer via noncompliant tubing. The level of the sternal manubrium was used as the zero reference point. Arterial blood samples (1.3 mL each) were collected periodically into syringes containing heparin for blood gas analysis; these samples were corrected to esophageal temperature and analyzed immediately to verify eucapnia and adjust ventilator settings (respiratory frequencies varied between 9 and 11 cycles/min). Gas samples for the analysis of ETISO and end-tidal carbon dioxide concentration were collected from the tracheal end of the endotracheal tube. Gas samples were constantly analyzed via an infrared technique with a multivariable anesthesia monitor, which, before each anesthetic session, was calibrated with a reference gas mixture containing 5.00% CO2, 33.0% N2O, 2% desflurane, and N2 as a balance gas. Peripheral oxygen saturation was monitored by pulse oximetry with the same anesthesia monitor. Four surface electrodes, fixed bilaterally to the lateral thoracic and abdominal wall, were connected to a telemetric ECG.b The signal was transmitted wirelessly and recorded by commercial softwarec on a laptop. For nociceptive stimulation, 2 stimulation electrodes were placed subcutaneously, 4 to 5 cm apart, on the middle third of the medial side of the ulna of the right thoracic limb. The electrodes were connected to a square pulse stimulatord that was set at 50 V, 50 Hz, and 10 milliseconds.
MAC determination—After the instrumentation and stabilization period, standardized anesthetic levels were obtained by individual MAC determinations (always observed by the same investigator [AMV]) in the first phase of each anesthetic session for each protocol and individual dog. The supramaximal electrical stimulation protocol consisted of 2 single stimuli and 2 continuous stimuli (applied over a 3-second period), with pauses of 5 seconds’ duration between each stimulus.15 A positive reaction was defined as gross purposeful movement of the head, limbs, or tail and caused the observer to terminate the stimulation protocol. Negative reactions were accelerated breathing, swallowing, or chewing. For each level of ETISO, a 15-minute equilibration period was allowed before nociceptive stimuation.16,17 To determine the individual MAC for each anesthetic protocol, the bracketing study design was applied with steps of 0.2 to 0.1 vol% of isoflurane.18 One MAC was calculated as the arithmetic mean of the ETISO concentrations with which movement after supramaximal stimulation was just enabled and just prevented. In addition to 1.0 MAC, the anesthetic levels of 0.75 and 1.5 MAC were attained in randomized order.
The same stimulation protocol as for MAC determination was used for nociceptive stimulation at the different MAC multiples after allowing 15 minutes of equilibration. Time points for nociceptive stimulation were independent of MAC determination.
After completion of the experiments, all catheters were removed. The dogs were allowed to recover from anesthesia and received a single injection of carprofen (4 mg/kg, SC).
Data analysis—Each ECG recording was visually checked for the presence of arrhythmias. In instances of multiple atrioventricular blocks, the ECGs were to be excluded from the further analysis. Offline analysis of the ECG signal consisted of an automatic R-peak detection, which was visually verified or manually corrected.c The R-R intervals were exported and transferred to an HRV analysis program.e Trend components were removed by a method with smoothing priors and a Λ = 500 (cutoff frequency = 0.035 Hz). The R-R series were interpolated at 4 Hz. The autoregressive model of order 16 without factorization was chosen for analysis of the power spectra. For frequency domain variables that had predefined band thresholds such as an LF power of 0.04 to 0.1 Hz and HF power of 0.1 to 0.6 Hz19 in absolute (milliseconds2) and normalized values (ie, LF power [nu] = LF/[LF + HF]), the LF:HF ratio, heart rate, and selected time domain variables SDNN and RMSSD were analyzed offline in 2-minute intervals both directly before and after nociceptive stimulation.4
Statistical analysis—Statistical analysis was performed with commercial software.f–h Because data from 6 dogs were insufficient to prove normal distribution, matched-pair signed rank tests were used to compare HRV variables before and after nociceptive stimulation and among different MAC levels. For assessing correlation of HRV variables with MAC multiples, Spearman rank correlation and linear regression analysis were used. The probability for a type I error (comparison-wise error rate α) was set to P < 0.05 or 5% for each comparison (and variable).
Results
After the stabilization period, the mean ± SD time to determine 1.0 MAC in the isoflurane, IsoD, and IsoR sessions was 73 ± 26 minutes, 53 ± 8 minutes, and 50 ± 8 minutes, respectively. Mean ETISO of 1.0 MAC ± SD for the isoflurane, IsoD, and IsoR sessions was 1.7 ± 0.3 vol%, 1.0 ± 0.1 vol%, and 1.0 ± 0.1 vol% of isoflurane, respectively. These values indicated that there was an MAC reduction of 41% achieved by administration of dexmedetomidine and remifentanil, compared with findings for isoflurane alone.
Overall, 6 ECG recordings derived from 2 dogs (1 dog in the IsoD session and 1 dog in the IsoR session) had evidence of multiple second-degree atrioventricular blocks. These recordings were excluded from HRV analysis.
The isoflurane session had the overall lowest prestimulation SDNN values combined with the highest LF power nu values at all 3 MAC multiples. The IsoD session had higher median prestimulation SDNN values at 0.75 and 1.0 MAC than did the isoflurane and IsoR sessions. Higher median prestimulation HF power prevailed in data for the IsoD and IsoR sessions over all MAC multiples, compared with values for isoflurane alone (Table 1). In all sessions, prestimulation values of SDNN, RMSSD, HF power (milliseconds2), and LF power (milliseconds2) decreased significantly from 0.75 or 1.0 to 1.5 MAC.
Values of selected HRV variables determined at 2-minute intervals before and after stimulation in 6 adult Beagles during anesthesia with isoflurane alone, with IsoD (3 μg/kg/h, IV), and with IsoR (18 μg/kg/h, IV) and at multiples of MAC.
| 0.75 MAC | 1.0 MAC | 1.5 MAC | |||||
|---|---|---|---|---|---|---|---|
| Variable | Session | Prestimulation | Poststimulation | Prestimulation | Poststimulation | Prestimulation | Poststimulation |
| Heart rate (beats/min) | Isoflurane | 109 (85; 128] | 127* (106; 155) | 113 (77; 129) | 122* (106; 163) | 119 (101; 128) | 125* (111; 151) |
| IsoD | 63 (51; 69] | 67 (46; 103) | 71 (51; 82) | 75 (51; 90) | 82† (62; 107) | 84 (63; 103) | |
| IsoR | 78 (64; 92) | 93* (79; 105) | 69 (57; 97) | 93* (73; 116) | 81† (62; 101) | 92* (69; 104) | |
| SDNN (ms) | Isoflurane | 11.9 (1.1; 21.5) | 3.8 (2.2; 5.7) | 4.2 (1.3; 22.2) | 3.5 (2.9; 16.0) | 1.4†‡ (1.0; 1.7) | 2.3* (1.5; 3.4) |
| IsoD | 137.1 (126.6; 207.4) | 175.3* (126.6; 230.1) | 62.5 (35.4; 143.3) | 85.6* (40.1; 188.2) | 11.0‡ (1.8; 74.2) | 55.1 (17.9; 230.1) | |
| IsoR | 68.7 (47.7; 88.8) | 44.3 (37.0; 75.5) | 51.7 (29.5; 71.5) | 50.8 (18.7; 65.0) | 40.1‡ (22.9; 72.6) | 43.4 (31.9; 66.9) | |
| RMSSD (ms) | Isoflurane | 15.3 (1.5; 28.1) | 2.8 (1.9; 4.0) | 3.1 (1.8; 26.1) | 2.2 (1.8; 22.2) | 1.7† (1.4; 2.1) | 1.7 (1.4; 3.0) |
| IsoD | 232.8 (118.7; 393.1) | 237.9 (164.0; 306.2) | 90.8 (28.4; 238.7) | 120.7* (40.9; 315.3) | 17.6†,‡ (1.6; 111.7) | 80.4 (23.9; 233.3) | |
| IsoR | 94.4 (53.4; 132.1) | 63.3 (43.7; 102.9) | 73.4 (31.1; 107.7) | 66.4 (17.6; 89.2) | 49.9‡ (27.4; 82.7) | 55.3 (38.1; 82.4) | |
| HF power (ms2) | Isoflurane | 107.1 (0.6; 330.0) | 2.7 (1.6; 6.8) | 9.6 (0.7; 497.7) | 4.8 (1.1; 161.9) | 0.8†‡ (0.5; 1.7) | 1.0 (0.6; 3.2) |
| IsoD | 16,501.4 (4,945.8; 28,980.8) | 26,635.9* (11,747.0; 41,979.7) | 4,306.4 (1,078.8; 17,451.7) | 8,352.6* (1,543.9; 26,158.8) | 49.3†‡ (2.8; 4,475.3) | 2,673.8 (303.7; 41,979.7) | |
| IsoR | 4,210.7 (1,786.8; 7,594.0) | 1,554.7 (737.2; 4,417.7) | 1,940.8 (506.2; 4,037.2) | 2,186.5 (195.2; 3,159.8) | 1,487.2‡ (314.6; 4,984.6) | 1,469.3 (584.0; 3,353.2) | |
| HF power (nu) | Isoflurane | 84.0 (79.0; 88.8) | 46.6* (31.7; 85.0) | 83.2 (33.4; 95.2) | 57.8 (20.2; 90.8) | 79.7 (54.2; 93.3) | 52.6 (19.3; 67.4) |
| IsoD | 98.6 (71.7; 99.5) | 90.7* (79.7; 96.5) | 98.5 (97.2; 99.20 | 95.6 (91.1; 99.4) | 96.1 (81.9; 99.3) | 95.7 (79.7; 98.7) | |
| IsoR | 96.5† (89.2; 99.2) | 90.6* (75.3; 96.6) | 93.3 (71.7; 96.9) | 92.1 (70.6; 96.6) | 95.1 (86.0; 99.2) | 94.0 (91.7; 96.8) | |
| LF power (ms2) | Isoflurane | 17.5 (0.2; 54.6) | 4.3 (0.3; 13.4) | 1.3 (0.2; 123.6) | 4.0 (1.2; 16.4) | 0.2‡† (0.1; 0.4) | 1.2* (0.5; 5.4) |
| IsoD | 116.5 (62.2; 239.3) | 2,265.7* (735.5; 10,678.0) | 146.8 (67.9; 702.8) | 489.2* (9.5; 2,058.4) | 43.9‡† (36.6; 64.9) | 102.4* (10.9; 10,678.0) | |
| IsoR | 190.7 (50.0; 373.2) | 199.2 (57.2; 747.3) | 66.9 (8.7; 222.9) | 168.5 (77.1; 290.4) | 8.4†‡ (0.1; 101.1) | 70.1 (42.4; 303.2) | |
| LF power (nu) | Isoflurane | 16.0 (11.2; 21.0) | 53.4* (15.0; 68.3) | 16.8 (4.8; 66.6) | 42.2 (9.2; 80.0) | 20.3 (6.7; 45.8) | 47.4 (32.6; 80.7) |
| IsoD | 1.4 (0.5; 2.9) | 9.3* (3.5; 20.3) | 1.5 (0.8; 2.8) | 4.4 (0.6; 8.9) | 4.0 (0.7; 18.1) | 4.4 (1.3; 20.3) | |
| IsoR | 3.5† (0.8; 10.8) | 9.5* (3.4; 24.7) | 6.8 (3.1; 28.3) | 8.0 (3.4; 29.4) | 4.9 (0.8; 14.0) | 6.0 (3.2; 8.3) | |
| LF/HF power | Isoflurane | 0.19 (0.13; 0.27) | 1.30* (0.18; 2.15) | 0.20 (0.05; 1.94) | 0.80 (0.10; 3.99) | 0.26 (0.07; 0.84) | 0.92 (0.49; 4.18) |
| IsoD | 0.02 (0.01; 0.03) | 0.10* (0.04; 0.25) | 0.02 (0.01; 0.03) | 0.05 (0.01; 0.10) | 0.04 (0.01; 0.22) | 0.05 (0.01; 0.25) | |
| IsoR | 0.04† (0.01; 0.12) | 0.11* (0.04; 0.33) | 0.07 (0.03; 0.40) | 0.09 (0.04; 0.42) | 0.05 (0.01; 0.16) | 0.06 (0.03; 0.09) | |
Data are reported as median (minimum; maximum). Each dog was anesthetized 3 times, and individual MAC was determined via supramaximal electrical stimulation. Sinus rhythm–derived R-R intervals were exported from ECG recordings. Selected HRV time and frequency domain variables were obtained (at 2-minute intervals) and analyzed offline with signed rank tests before and after stimulation at 0.75, 1.0, and 1.5 MAC for each anesthetic session. Overall, 6 ECG recordings derived from 2 dogs (1 dog in the IsoD session and 1 dog in the IsoR session) had evidence of multiple second-degree atrioventricular blocks, and these recordings were excluded from HRV analysis.
Within a session, value is significantly different (P < 0.05) from the corresponding prestimulation value.
Within a session, value is significantly different (P < 0.05) from prestimulation value at 1.0 MAC.
Within a session, value is significantly different (P < 0.05) from the prestimulation value at 0.75 MAC.
Heart rate increased significantly with nociceptive stimulation at all MAC levels in the isoflurane and IsoR sessions, but not in the IsoD session. In the IsoD session, SDNN increased with stimulation, an increase that was significant at 0.75 and 1.0 MAC (Table 1).
The strongest inverse correlations of SDNN and HF power (milliseconds2) with increasing MAC were evident during the IsoD session (Figure 1). The Spearman rank correlation coefficients for LF power (milliseconds2) were −0.58 (P = 0.012), −0.72 (P < 0.001), and −0.63 (P = 0.005) for the isoflurane, IsoD, and IsoR sessions, respectively.
Values of SDNN (milliseconds; A) and HF power (milliseconds2; B) determined in 6 adult Beagles during anesthesia with isoflurane alone (circles), with IsoD (3 μg/kg/h, IV; diamonds), and with IsoR (18 μg/kg/h, IV; triangles) and at multiples of MAC. Each dog was anesthetized 3 times, and individual MAC was determined via supramaximal electrical stimulation. Sinus rhythm–derived R-R intervals were exported from ECG recordings. Selected HRV time and frequency domain variables were obtained (at 2-minute intervals) and analyzed offline with signed rank tests before and after stimulation at 0.75, 1.0, and 1.5 MAC for each anesthetic session. Overall, 6 ECG recordings derived from 2 dogs (1 dog in the IsoD session and 1 dog in the IsoR session) had evidence of multiple second-degree atrioventricular blocks, and these recordings were excluded from HRV analysis. For SDNN data, the correlation coefficients were −0.58 (P = 0.012), −0.77 (P < 0.001), and −0.58 (P = 0.012), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −13 (r2 = 0.31; P = 0.016) in the isoflurane session, −140 (r2 = 0.55; P < 0.001) in the IsoD session, and −35 (r2 = 0.35; P = 0.010) in the IsoR session. For HF power, the correlation coefficients were −0.63 (P = 0.005), −0.77 (P < 0.001), and −0.50 (P = 0.035), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −186 (r2 = 0.17; P = 0.094) in the isoflurane session, −18,850 (r2 = 0.44; P = 0.003) in the IsoD session, and −2,984 (r2 = 0.24; P = 0.040) in the IsoR session.
Citation: American Journal of Veterinary Research 74, 5; 10.2460/ajvr.74.5.665
Values of SDNN (milliseconds; A) and HF power (milliseconds2; B) determined in 6 adult Beagles during anesthesia with isoflurane alone (circles), with IsoD (3 μg/kg/h, IV; diamonds), and with IsoR (18 μg/kg/h, IV; triangles) and at multiples of MAC. Each dog was anesthetized 3 times, and individual MAC was determined via supramaximal electrical stimulation. Sinus rhythm–derived R-R intervals were exported from ECG recordings. Selected HRV time and frequency domain variables were obtained (at 2-minute intervals) and analyzed offline with signed rank tests before and after stimulation at 0.75, 1.0, and 1.5 MAC for each anesthetic session. Overall, 6 ECG recordings derived from 2 dogs (1 dog in the IsoD session and 1 dog in the IsoR session) had evidence of multiple second-degree atrioventricular blocks, and these recordings were excluded from HRV analysis. For SDNN data, the correlation coefficients were −0.58 (P = 0.012), −0.77 (P < 0.001), and −0.58 (P = 0.012), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −13 (r2 = 0.31; P = 0.016) in the isoflurane session, −140 (r2 = 0.55; P < 0.001) in the IsoD session, and −35 (r2 = 0.35; P = 0.010) in the IsoR session. For HF power, the correlation coefficients were −0.63 (P = 0.005), −0.77 (P < 0.001), and −0.50 (P = 0.035), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −186 (r2 = 0.17; P = 0.094) in the isoflurane session, −18,850 (r2 = 0.44; P = 0.003) in the IsoD session, and −2,984 (r2 = 0.24; P = 0.040) in the IsoR session.
Citation: American Journal of Veterinary Research 74, 5; 10.2460/ajvr.74.5.665
Values of SDNN (milliseconds; A) and HF power (milliseconds2; B) determined in 6 adult Beagles during anesthesia with isoflurane alone (circles), with IsoD (3 μg/kg/h, IV; diamonds), and with IsoR (18 μg/kg/h, IV; triangles) and at multiples of MAC. Each dog was anesthetized 3 times, and individual MAC was determined via supramaximal electrical stimulation. Sinus rhythm–derived R-R intervals were exported from ECG recordings. Selected HRV time and frequency domain variables were obtained (at 2-minute intervals) and analyzed offline with signed rank tests before and after stimulation at 0.75, 1.0, and 1.5 MAC for each anesthetic session. Overall, 6 ECG recordings derived from 2 dogs (1 dog in the IsoD session and 1 dog in the IsoR session) had evidence of multiple second-degree atrioventricular blocks, and these recordings were excluded from HRV analysis. For SDNN data, the correlation coefficients were −0.58 (P = 0.012), −0.77 (P < 0.001), and −0.58 (P = 0.012), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −13 (r2 = 0.31; P = 0.016) in the isoflurane session, −140 (r2 = 0.55; P < 0.001) in the IsoD session, and −35 (r2 = 0.35; P = 0.010) in the IsoR session. For HF power, the correlation coefficients were −0.63 (P = 0.005), −0.77 (P < 0.001), and −0.50 (P = 0.035), with increasing MAC for the isoflurane, IsoD, and IsoR sessions, respectively. The slopes of the best-fit linear regression lines were −186 (r2 = 0.17; P = 0.094) in the isoflurane session, −18,850 (r2 = 0.44; P = 0.003) in the IsoD session, and −2,984 (r2 = 0.24; P = 0.040) in the IsoR session.
Citation: American Journal of Veterinary Research 74, 5; 10.2460/ajvr.74.5.665
Discussion
In the dogs of the present study, the differential influence of drugs on HRV was apparent. Similar findings have been reported in a review13 of several human studies involving different anesthetic protocols in which HRV variables were influenced to a large extent by the drugs administered.
Dogs treated with the α2-adrenoceptor agonist dexmedetomidine had the highest SDNN values at the lower MAC levels in the present study. This high variability may be attributed to dexmedetomidine, which induces sinus arrhythmia and atrioventricular blocks.20 Administration of dexmedetomidine results in an increase of systemic vascular resistance via activation of vascular α2β-adrenoceptors,21 followed by a baroreceptor-related decrease of heart rate and a central α2-adrenoceptor–related reduction of sympathetic tone.22 These effects were apparent in the study dogs, as evidenced by low heart rate and LF power nu values that are due to the corresponding increase of vagal influence, and were identified by the proportionately greater extent of the increase in HF power values. With increasing MAC, prestimulation SDNN decreased and prestimulation heart rate increased in the IsoD session. Because the dexmedetomidine dose was not changed, this effect could be a result of an increasing influence of isoflurane, which counteracted the dexmedetomidine-induced vasoconstriction.23
Isoflurane alone resulted in high LF power nu values, low HF power nu values, low SDNN values, and high heart rates in a dose-dependent manner. These findings were indicative of a high sympathetic tone or stress response and might be a result of a reflex increase in sympathetic tone in response to a dose-dependent reduction of systemic vascular resistance that may occur with low isoflurane concentrations such as those used in this study.24,25 In contrast, very high isoflurane concentrations are able to depress sympathetic activity.25,26 In the present study, data from awake dogs were not obtained; therefore, it cannot be determined whether isoflurane alone resulted in actual sympathetic stimulation or whether the higher sympathetic activity in the isoflurane session was just an effect of the dogs not being treated with a sympatholytic or vagomimetic drug.
The extent to which the prestimulation heart rate was reduced (compared with findings for the isoflurane session) in the IsoR session was similar to that detected in the IsoD session. A common adverse effect of remifentanil is bradycardia induced by increased vagal tone.27 In contrast to such findings in the IsoR session, heart rate did not increase significantly with nociceptive stimulation at the same MAC-defined levels of anesthetic depth in the IsoD session. Dexmedetomidine might have blunted the dogs’ heart rate response by the aforementioned blockade of the sympathetic branch of the ANS or by an increase in systemic vascular resistance and baroreceptor-related blunting of heart rate increase. Therefore, heart rate as an autonomic indicator of nociception might have limitations in the presence of dexmedetomidine or other α2-adrenoceptor agonists. In contrast, the SDNN increased with nociceptive stimulation in dogs during the IsoD session despite the lack of a heart rate response. This finding is in agreement with results of a studyi in isoflurane-anesthetized pigs in which SDNN also proved to be useful as an indicator of nociceptive stimulation. The SDNN is of interest because it represents interbeat variability; therefore, it provides more information than just the heart rate. However, when the dogs received isoflurane only, SDNN did not indicate nociceptive stimulation at all multiples of MAC despite a heart rate response. Thus, change in SDNN as an indicator of nociception is questionable.
The time domain variable SDNN and the frequency domain variables HF power (milliseconds2) and LF power (milliseconds2) had inverse correlations with increasing MAC levels within the anesthetic sessions. In addition, there was a large overlap of values among the anesthetic protocols and among individual dogs, restricting the clinical usefulness of these variables as indicators of anesthetic depth. In humans, a prediction probability of 0.91 was reached by absolute HF power values for the differentiation of the awake state and isoflurane anesthesia, as derived from 1,024 data points via FFT.3 A correlation of HF power with anesthetic depth was also found in a study28 involving propofol-anesthetized children undergoing eye surgery. In adults, various HRV variables were able to discriminate the awake state from anesthesia with propofol and remifentanil, but not able to discriminate among different states of hemodynamic responses.12 In those human studies, anesthetic depth was defined by clinical hemodynamic criteria or bispectral index values. In the present study in dogs, anesthetic depth was defined by MAC, a standardized index of immobility.29 Therefore, a direct comparison of results of the human studies with those of the the present study is not possible. The multiples of MAC used in the present study were within a clinically useful range, and differences were small. In addition, differentiation between the awake state and anesthesia was not performed in the dogs; doing so should have resulted in more pronounced changes in autonomous regulation than in the different anesthetic states.
Because the concept of MAC is the state-of-the-art method for the comparison of anesthetic potency,29 it was used in the present study to attain quantitatively comparable anesthetic levels. In 1 study15 in dogs, the technical influences of traditional clamping versus electrical stimulation were not significant when both techniques were evaluated as supramaximal stimuli, but a tendency toward higher MAC values with the electrical stimulation was noticed, which might also explain the rather high values of isoflurane MAC in the present study. In the present study, the same concentrations of isoflurane were administered to the dogs in the IsoR and IsoD, as evidenced by the fact that remifentanil and dexmedetomidine in the administered dose rates resulted in exactly the same reduction in isoflurane MAC (ie, 41% reduction). An even greater MAC reduction (59% reduction) has been observed after administration of either of these drugs to dogs in previous studies.14,a Opioids and α2-adrenoceptor agonists are known for their MAC-sparing effects in dogs and also in other species, such as humans or rats. Remifentanil exerts a strong analgesic effect via supraspinal μ1-opioid receptors,30 whereas dexmedetomidine reduces the MAC, probably by binding to α2-adrenoceptors at the spinal level and in the locus coeruleus, which are the major sites for its analgesic and sedative-hypnotic action.31,32
Drug accumulation as a result of prolonged anesthesia can be considered minimal. A metabolism rate of only 0.2% has been reported for isoflurane in humans because it is primarily eliminated via the lungs.33 In dogs, remifentanil has a blood-brain equilibration half-life of 2.3 to 5.2 minutes, which, in dogs as well as in humans, is independent of the duration of an infusion because it is rapidly metabolized by nonspecific esterases in blood and tissue.34,35 In a pharmacokinetic study14 involving isoflurane-anesthetized Beagles, no cumulative effects of dexmedetomidine were detected; following administration of a 7-hour CRI of dexmedetomidine at the same dose rate as that used in the present study, a steady-state serum dexmedetomidine concentration of approximately 2 ng/mL was reached.
The present study had several limitations. The low number of dogs was not sufficient to truly evaluate the overall reliability of the evaluated variables. A post hoc power analysis for prestimulation SDNN values revealed a power of 75% for the isoflurane session and 60% for both the IsoD and IsoR sessions. Evaluation of at least 9 dogs would have been needed to reach a power of 80% with a confidence interval of 95%.
The results from the present study may be valid only for Beagles because HRV differs among different breeds.5,8 Additionally, an influence of age and sex on HRV variables may also be present.36 Furthermore, the electrical stimulation used in the present study was transient and did not fully represent a continuous surgical stimulus; these conditions might have impaired the detection of prolonged changes in sympathetic tone. Stable variability during the measurement period is required for HRV analysis,37,38 but this is not realistic after short-term noxious stimulation because changes in variability are also very short-lived; this influence is a limitation of the reliability of the spectral powers.39 Because intervals with modest second-degree atrioventricular blocks were, if possible, artifact corrected and included in the analysis in the present study, the variability of the sinus rhythm–derived beats was high.
Direct comparison of the present study findings with those of other HRV studies is difficult because of variable frequency band definitions and calculation methods. For analysis of HRV frequency domain variables, the FFT procedure and the autoregressive model are 2 common techniques.38 Compared with the FFT procedure, the autoregressive model has a better spectral resolution for short frames of data, requires no windowing procedures, and is independent of the number of samples (eg, R-R intervals).38 These characteristics are desirable for short periods of evaluation, such as those in the present study.40 Given that human and dog spectra and HRV values appear similar,40,41 the same model order can be selected and dogs might provide the basis of experimental models for humans. However, in contrast to human medicine, there are no state-of-the-art definitions of HRV frequency bands for dogs in veterinary medicine. Therefore, the ranges defined for a previous study19 in Beagles were chosen for the present study. With regard to the statistical analysis, the multiplicity of the test conducted has to be considered in the interpretation of the results. Thus, any single value of P < 5% should not be completely interpreted as final evidence of a significant or even clinically important effect.
In the present study, HRV variables in anesthetized dogs were significantly influenced by the drugs given. In dogs, SDNN and HF power appeared to best differentiate anesthetic levels between 0.75 and 1.5 MAC within the same anesthetic protocol, although there was a large overlap of values among protocols. Changes in SDNN did not prove to be a reliable indicator of nociception. Therefore, clinical usefulness of the standard HRV variables evaluated in the present study for assessment of anesthetic depth and nociception in dogs is highly questionable.
ABBREVIATIONS
| ANS | Autonomic nervous system |
| CRI | Constant rate infusion |
| ETISO | End-tidal concentration of isoflurane |
| FFT | Fast Fourier transform |
| HF | High frequency |
| HRV | Heart rate variability |
| IsoD | Isoflurane and a constant rate infusion of dexmedetomidine |
| IsoR | Isoflurane and a constant rate infusion of remifentanil |
| LF | Low frequency |
| MAC | Minimum alveolar concentration |
| nu | Normalized units |
| RMSSD | Square root of the mean of the sum of the squares of differences between adjacent R-R intervals |
| SDNN | SD of all R-R intervals |
Monteiro ER, Teixeira Neto FJ, Campagnol D, et al. Effects of remifentanil on the minimum alveolar concentration of isoflurane in dogs (abstr), in Proceedings. 10th World Cong Vet Anaesth 2009;132.
Televet 100, provided by Rösch & Associates Information Engineering GmbH, Frankfurt am Main, Germany.
Televet 100, version 4.2.0, provided by Rösch & Associates Information Engineering GmbH, Frankfurt am Main, Germany.
Grass S48 Square Pulse Stimulator, Astro-Med, West Warwick, RI.
Kubios HRV, version 2.0, Biosignal Analysis and Medical Imaging Group, Department of Physics, University of Kuopio, Kuopio, Finland.
SAS, version 9.1.3 service pack 1, SAS Institute Inc, Cary, NC.
GraphPad Prism, version 5.0, GraphPad Software, La Jolla, Calif.
GraphPad StatMate, version 2.0, GraphPad Software, La Jolla, Calif.
Haga HA, Ranheim B, Spadavecchia C. Heart rate variability responses to nociception during isoflurane anaesthesia in pigs (abstr), in Proceedings. Spring Meet Assoc Vet Anaesthetists - Eur Coll Vet Anaesth Analg 2008;26.
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