Comparative evaluation of halothane and isoflurane maintenance anesthesia in water buffaloes (Bubalus bubalis)

Abstract

Halothane (HAL) and isoflurane (ISO) maintenance anesthesia in 12 water buffaloes was compared on the basis of certain cardiopulmonary, hemodynamic, acid base, electrolyte, and recovery parameters. Buffaloes were randomly divided into two groups: HAL (n = 6) and ISO (n = 6), premedicated with midazolam (0.2 mg/kg) intravenously, and 5 min later, induced to anesthesia by intravenous thiopentone sodium (10 mg/kg IV, given till effect). Anesthesia was maintained with halothane or isoflurane in oxygen at a total gas flow rate of 4–6 lt/min. Results revealed that recovery with isoflurane was more rapid than halothane with a significant (p < 0.05) difference in recovery quality between two groups. Heart rate, respiration rate, rectal temperature, systolic blood pressure, diastolic blood pressure, and mean arterial pressure decreased significantly (p < 0.01) in HAL group. Values of pH_a decreased significantly (p < 0.01) and PaCO₂ increased significantly (p < 0.01) in HAL group. There were no significant (p > 0.05) differences in PaO₂, HCO₃, and plasma electrolytes between two groups. Values of SpO₂ decreased significantly (p < 0.05) in HAL group. Isoflurane maintenance anesthesia contributes to a shorter, safer recovery from anesthesia with maximum preservation of cardiopulmonary and hemodynamic functions along with minimal changes in acid base parameters in water buffaloes. Thus, isoflurane maintenance anesthesia is better than halothane maintenance anesthesia in water buffaloes.

Keywords:

• halothane
• isoflurane
• maintenance
• anesthesia
• water buffaloes

1. Introduction

A variety of surgical procedures in bovine is usually performed under combination of physical restraint, sedation, and local or regional anesthesia. But, for complex surgical procedures like repair of diaphragmatic hernia, ventral hernia, prepubic tendon rupture, and orthopedic surgery maintenance with an inhalant anesthetic agent is must (Riazuddin et al. 2004). Inhalant anesthetics offer quicker changes in anesthetic depth, quicker reversal of central nervous system (CNS) depression, lesser risk of overdosing, and a faster recovery from anesthesia compared to injectable anesthetics. Halothane (HAL) has long been used as the volatile anesthetic agent of choice for both induction and maintenance of general anesthesia in cattle (Rugh et al. 1985; Matthews et al. 1986; Riazuddin et al. 2004; Gibson et al. 2009), cattle calves (Steffey & Howland 1979), and water buffaloes (Bose & Kohli 1983; Bodh 2011). The introduction of isoflurane (ISO) as a maintenance agent during general anesthesia offers certain advantages over halothane. Isoflurane anesthesia has been investigated by a number of workers in cattle (Cantalapiedra et al. 2002; Riazuddin et al. 2004), calves (Kerr et al. 2007; Vesal et al. 2011; Offinger et al. 2012), African buffalo (Stegmann 2004), and water buffalo (Bodh 2011; Singh et al. 2013). Persual of literature reveals limited information on halothane and isoflurane maintenance anesthesia in water buffaloes. Considering the importance of this species and scarcity of information available, present study was conducted with the objective to evaluate and compare the effects of halothane and isoflurane maintenance anesthesia on certain cardiopulmonary, hemodynamic, acid base, and electrolyte parameters along with recovery characteristics in water buffaloes undergoing a variety of surgical procedures in a clinical setting.

2. Materials and methods

2.1. Animals and anesthetic procedure

The present study was conducted on 12 clinical cases of water buffaloes of either sex, aged 3–5 years, and weighing 350–450 kg. The selection of buffaloes for inclusion in the study was based on the requirement for general anesthesia and owners consent. A preoperative history was obtained and a routine pre-anesthetic physical examination was performed on each buffalo. The buffaloes were kept off feed for 24 h and water was withheld for 12 h prior to induction of anesthesia. Left jugular vein and left auricular artery were aseptically prepared for anesthetic/intravenous fluid administration and blood collection, respectively. All buffaloes presented for various surgical affections requiring general anesthesia (Table 1) were randomly divided into two groups: HAL (n = 6) and ISO (n = 6). The buffaloes were restrained in lateral recumbency prior to administration of any drug, i.e., before premedication, left undisturbed and 5 min later, administered intravenous midazolam (0.2 mg/kg) (5 mg/ml; Mezolam, Neon Laboratories, Mumbai, India) in both groups. Induction of anesthesia was done by intravenous thiopentone sodium (5%, 10 mg kg⁻¹ given “till effect”) (Thiosol sodium, Neon Laboratories, Mumbai, India) 5 min after midazolam administration in both groups. Following the disappearance of swallowing reflex (judged by flaccid/no tongue movement, inability to retract tongue inside the mouth, and no resistance when endotracheal tube was advanced toward the larynx), blind tracheal intubation with an endotracheal tube (16–22 mm internal diameter) was performed and tube was connected to the anesthesia machine. The transfer time to inhalant anesthesia after induction with thiopentone sodium on an average ranged between 2 and 3 min in all the animals. Maintenance of anesthesia was done by halothane (2.5–4%) and isoflurane (2.5–4%) in 100% oxygen in HAL and ISO group, respectively, via a semi-closed re-breathing system at a total gas flow rate of 4–6 lt/min. The vaporizer setting was adjusted according to depth of anesthesia after monitoring animal's response to various reflexes and by lack of painful response to surgery. The surgical procedures were completed between 60 and 70 min duration and inhalation anesthesia was maintained up to 70 min interval in all the cases. However, rest of the parameters were recorded till 120 min. Soon after the completion of surgical procedure, anesthetic vaporizer dial setting was abruptly reduced to zero and all buffaloes were allowed to breathe 100% oxygen delivered through oxygen flush valve (4–6 lt /min) for a period of 10 min in both the groups. The endotracheal tube with its cuff inflated was left undisturbed until the return of swallowing reflex (i.e., when animal started to chew, moved its tongue and retracted it when pulled outwards, showed evidence of return of good laryngeal control), following which the tube was disconnected from the anesthesia machine and buffaloes were allowed to breathe room air. The buffaloes were monitored continuously during the recovery period until they were standing unassisted. Throughout the period of anesthesia, buffaloes were administered intravenous normal saline solution in both the groups.

Table 1. List of surgical procedures performed in animals of HAL (n = 6) and ISO (n = 6) group.

Group	Sex	No. of animals	Surgical procedure	Recumbency
HAL	Male	2	Repair of prepucial prolapse	Lateral
	Female	2	Repair of diaphragmatic hernia	Dorsal
	Female	2	Repair of ventral hernia	Dorsal
ISO	Female	1	Repair of teat laceration	Lateral
	Female	1	Repair of ventro-lateral hernia	Dorso-lateral
	Female	1	Repair of diaphragmatic hernia	Dorsal
	Male	2	Limb amputation	Lateral
	Male	1	Removal of massive growth from neck	Lateral

2.2. Parameters measured

Heart rate (beats per min) was recorded with the stethoscope, respiratory rate (breaths per min) by counting chest excursions, and rectal temperature (°C) by means of a digital thermometer. Hemoglobin oxygen saturation (SpO₂) (%) value was obtained by applying the sensor of pulse oximeter (Nonin Medical, Inc., Plymouth, MN, USA) on the anal fold of animal. Systolic blood pressure (SBP) (mm of Hg), diastolic blood pressure (DBP) (mm of Hg), and mean arterial pressure (MAP) (mm of Hg) were measured with the help of a non-invasive blood pressure monitor (SurgiVet^®, Smiths Medicals, North America, Waukesha, WI, USA) whose cuff was applied around the base of the tail of animal. All the parameters were recorded at base (0 min), 5 min after premedication and at 5, 15, 30, 45, 60, 90, and 120 min after induction of anesthesia.

For acid base and electrolyte studies, 1 ml arterial blood was collected by needle puncture from the auricular artery in 2 ml heparinized disposable syringes at same time intervals and analyzed immediately. Arterial oxygen and carbon dioxide partial pressures (PaO₂, PaCO₂), arterial pH (pH_a), bicarbonate (HCO₃), and sodium (Na⁺), potassium (K⁺), calcium (Ca⁺⁺), and chloride (Cl⁻) were measured with the help of an automated blood gas analyzer (Radiometer make, ABL-80, fully automatic, EM Technologies, India Pvt. Limited, Ghaziabad, UP, India).

The recovery period was divided into four intervals: (a) time to extubation (TE; time in minutes from end of halothane/isoflurane administration to recovery of voluntary swallowing reflex); (b) time to first head movement (time in minutes from TE till first spontaneous movement of head); (c) time to sternal recumbency (TSR; time in minutes from TE until voluntary return to sternal recumbency); and (d) time to standing (TS; time in minutes from TE to standing up). The recovery quality was graded as 1 = poor (prolonged struggling, premature attempts to stand); 2 = moderate (transient excitement along with some struggling); 3 = good (smooth, easy transition to alertness, resumes sternal position); 4 = excellent (smooth, excitement free, animal stands of its own) [modified from Prassinos et al. (2005)].

The extent of salivation/lacrimation was graded as 1 = no salivation/lacrimation; 2 = mild salivation/lacrimation; 3 = moderate salivation/lacrimation; and 4 = profuse salivation/lacrimation. The extent of regurgitation was scored as mild = presence of small amount of ruminal contents within oral cavity; severe = large volume of rumen contents coming out through mouth or nostrils [modified from Lin et al. (1997) and Carroll et al. (1998)].

2.3. Statistical analysis

Analysis of variance and Duncan's multiple range test were used to compare the means at different time intervals among the two groups. Paired “t” test was used to compare the mean values at different intervals with their base values in each group (Snedecor & Cochran 1980). Values of p < 0.05 and p < 0.01 were considered significant.

3. Results and discussion

In the present study, we found both anesthetics to be safe and reliable for use in water buffaloes. However, isoflurane was better than halothane for maintenance of general anesthesia due to certain features highly desirable in large ruminant anesthesia. There was a significant (p < 0.01) decrease in heart rate in HAL group. A statistically significant (p < 0.01) difference in heart rate from 15 to 60 min was observed between the two groups (Table 2), indicating that heart rate differs during halothane and isoflurane maintenance anesthesia in water buffaloes. Varying responses regarding heart rate during halothane and isoflurane anesthesia have been reported for different species or ages. An increase in heart rate in dogs (Frink et al. 1992), horses (Steffey et al. 1987), and bulls (Greene et al. 1998) and a decrease in heart rate in piglets (Schieber et al. 1986), lambs, and piglets (Brett et al. 1987) were observed. It has been seen that heart rate either remains constant or increases with an increase in alveolar inhalation anesthetic concentration in dogs (Steffey & Howland 1979) or decreases with an increase in anesthetic depth due to increased vagal tone (Hall & Clarke 1983). Our results were in accordance with the latter view, where in HAL group, a decrease in heart rate was associated with an increase in anesthetic depth. The cardiovascular depressant effects of halothane are dose-dependent, resulting in a decrease in arterial blood pressure with increased dose (Steffey & Mama 2007). Non-significant (p > 0.01) increase in heart rate in ISO group was in conflict with the results of previous studies performed in cats (Hikasa et al. 1997) and goats (Hikasa et al. 1998). The increase in heart rate during isoflurane maintenance anesthesia could be due to several potential causes such as an inadequate level of anesthesia, differences in the amount of sympathetic stimulation within the two groups or it may be compensatory to a greater decrease in peripheral resistance or a stronger negative inotropic effect with isoflurane, taking into account similar arterial blood pressures. Causes of these differences might be either due to elevated arterial PaCO₂ or the anesthetic agent itself. Since isoflurane does not usually reduce the heart rate, it may have an advantage over halothane in maintaining adequate cardiac output during certain anesthetic conditions in water buffaloes.

Table 2. Cardiopulmonary and hemodynamic changes during halothane and isoflurane maintenance anesthesia in water buffaloes (n = 12).

Parameters		Base	P5	I5	15	30	45	60	90	120
Heart rate (beats/min)	HAL	68.25^ab ± 3.92	66.75^b ± 3.76	62.50^ab ± 3.59	58.12^a** ± 4.68	47.25^a ** ± 4.28	42.20^a* ± 5.41*	53.14^ab ± 3.12	62.23^ab ± 3.36	66.50^ab ± 4.31
	ISO	65.50^a ± 3.59	62.50^bc ± 4.11	59.75^a ± 4.60	68.50^bc ± 3.40	70.32^c ± 3.55	75.50^c ± 2.62	72.50^c ± 4.57	69.50^a ± 3.86	67.50^b ± 3.59
Respiratory rate (breaths/min)	HAL	22.50 ± 3.41	20.50 ± 3.59	16.50 ± 2.87	14.50^a** ± 2.62	15.50^ab** ± 2.21	18.00^ac ± 0.00	21.06 ± 2.16	22.24 ± 1.73	20.50 ± 2.87
	ISO	24.50 ± 2.06	22.44 ± 2.08	18.50 ± 1.70	20.25^c ± 1.50	21.50^c ± 1.91	22.25^a ± 1.41	20.50 ± 1.70	22.50 ± 2.06	24.12 ± 1.63
Rectal temperature (°C)	HAL	38.71 ± 0.18	38.72 ± 0.25	38.69 ± 0.32	38.52* ± 0.40	38.31* ± 0.28	38.33* ± 0.31	38.14** ± 0.27	37.10** ± 0.25	37.07** ± 0.27
	ISO	38.83 ± 0.28	38.60 ± 0.30	38.58 ± 0.24	38.44 ± 0.24	38.30* ± 0.23	38.28* ± 0.21	38.20* ± 0.29	38.23* ± 0.06	38.06* ± 0.03
SBP (mm Hg)	HAL	146.25 ± 3.47	142.11 ± 3.24	138.25 ± 3.56	127.50** ± 4.70	122.50** ± 6.89	128.25** ± 8.67	130.50 ± 5.96	136.75 ± 4.41	138.00 ± 5.80
	ISO	148.50 ± 1.75	146.72 ± 2.93	140.50 ± 2.61	131.50 ± 3.30	130.75 ± 1.67	132.25 ± 4.57	137.25 ± 2.45	139.25 ± 2.52	142.75 ± 2.97
DBP (mm Hg)	HAL	122.75 ± 4.95	121.25 ± 4.69	117.75 ± 5.28	110.50** ± 3.52	113.50* ± 6.56	106.50** ± 8.53	111.25** ± 8.02	116.50 ± 7.07	118.50 ± 4.27
	ISO	128.25 ± 4.70	126.50 ± 4.48	121.75 ± 4.69	119.75 ± 4.83	115.75 ± 2.92	112.50 ± 3.24	118.50 ± 4.50	121.75 ± 5.35	124.50 ± 5.50
MAP (mm Hg)	HAL	132.50 ± 4.57	129.00 ± 4.69	125.00 ± 5.19	116.50^a** ± 6.14	115.75^bc** ± 7.02	112.20^a** ± 5.74	118.10^b** ± 5.61	122.25 ± 3.91	125.50 ± 3.88
	ISO	135.75 ± 4.24	132.25 ± 5.10	129.25 ± 3.54	124.50^ac ± 2.58	120.25^ac ± 4.06	126.20^c ± 4.04	121.15^ab ± 4.08	127.25 ± 5.44	131.50 ± 7.19

Note: Values with different alphabets differ significantly (p < 0.05) between the two groups at corresponding intervals.

P5, 5 min after premedication; I5, 5 min after induction.

* Differ significantly (p < 0.05) from base value.

** Differ significantly (p < 0.01) from base value.

Respiratory rate decreased significantly (p < 0.01) for a short duration in HAL group. A similar decrease in respiratory rate and depth during halothane anesthesia was reported earlier in bulls (Greene et al. 1998) and cattle (Takase 1976). A statistically significant (p < 0.01) difference in respiratory rate from 15 to 30 min interval was observed between the two groups (Table 2). Greater respiratory depression observed during halothane maintenance anesthesia might be the result of simultaneous administration of halothane with oxygen and hypotension during anesthesia that might have led to the increased physiological dead space (Hull & Reilly 1968). It has been found that halothane depresses ventilation by abolishing peripheral drive from the chemoreceptors at the carotid bodies, by general depression of respiratory centers in the CNS, and by suppression of the function of motor neurons, intercostal muscles, and the diaphragm (all involved in the chemical control of breathing) and also by the loss of wakefulness drive (behavioral control) (Knill & Clement 1984, 1985). The non-significant (p > 0.01) respiratory depression observed during isoflurane maintenance anesthesia might be due to some depressant effect of the inhalant anesthetic on cardiopulmonary function (Table 2). Similar decrease in the respiratory rate during isoflurane anesthesia was reported in horses by Grosenbaugh and Muir (1998). Higher values of respiratory rate observed during isoflurane maintenance anesthesia in water buffaloes in the current study were in accordance with the findings in sheep (Genccelep et al. 2004).

Rectal temperature decreased significantly (p < 0.05) during maintenance anesthesia in both the groups (Table 2). Significant (p < 0.01) hypothermia from 90 to 120 min in HAL group might be attributed to severe heat loss from vasodilation (Jones & McLaren 1965). This hypothermia could also be one of the causes for delay in recovery of buffaloes in HAL group. Hypothermia increases the blood/gas solubility of all the inhaled anesthetics as a result of which the anesthetic will be slow to saturate the blood and tissues and induction and recovery times will also be slow (Dohoo 1990) as seen in the current study in HAL group. In a similar study, halothane anesthesia caused a greater decrease in body temperature in humans in the halothane group compared to the isoflurane group (Ramachandra et al. 1989). A significant decrease in rectal temperature during halothane anesthesia was reported in adult water buffaloes (Malik 2008; Ahmad 2009). Decrease in skeletal muscle tone, reduced metabolic rate, muscle relaxation along with the depression of thermoregulatory center (Matsukawa et al. 1995) might have contributed to decrease in rectal temperature in both the groups.

SBP, DBP, and MAP decreased significantly (p < 0.01) in HAL group (Table 2). A similar hypotensive effect of halothane anesthesia was reported in cattle (Hall 1957; Wolf et al. 1968; Singh 1988) and buffaloes (Bose & Kohli 1983; Gahlawat et al. 1986). No statistically significant (p > 0.05) difference in SBP and DBP was observed between the two groups. However, MAP differed significantly (p < 0.05) between the two groups at 45 min interval (Table 2). The decrease in arterial blood pressure in HAL group might be due to depression of myocardial contractility and a marked decrease in cardiac output during halothane anesthesia (Stoelting 1991). Halothane induces a dose-dependent decrease in arterial blood pressure and cardiac output, but it has a little effect on total peripheral resistance in dogs (Steffey & Howland 1978) and goats (Hikasa et al. 1998). In contrast, isoflurane causes a greater fall in peripheral vascular resistance with a decrease in arterial blood pressure and a smaller decrease in cardiac output in cats and dogs (Steffey et al. 1987; Bernard et al. 1990) and goats (Hikasa et al. 1998). A non-significant decrease in arterial blood pressure observed during isoflurane maintenance anesthesia in the present study might be due to better coronary perfusion (Short 1987) and protection of myocardium provided by modifying intracellular calcium transport during isoflurane anesthesia (Freedman et al. 1983).

A significant (p < 0.01) decrease in pH_a and a significant (p < 0.01) increase in PaCO₂ from 15 to 45 min interval in HAL group suggested respiratory acidosis (Table 3). In ISO group, there was no significant (p > 0.05) change in pH_a and PaCO₂ values. Similar observations were previously reported in buffaloes (Bose & Kohli 1980) and in cattle (Bouda & Pavlica 1970; Gates et al. 1971). Halothane has a powerful respiratory depressant effect which might have contributed to the respiratory acidosis observed in HAL group (Fisher 1961; Donawick et al. 1969). Wolf et al. (1968) reported that an increase in PaO₂ interferes with the transmission of carbon dioxide during halothane narcosis and thus contributes to respiratory acidosis. A time-related increase in PaCO₂ during halothane anesthesia could be due to an increase in the anesthetic depth. No statistically significant (p > 0.05) difference in PaO₂ and HCO₃ values was observed between the two groups (Table 3). Respiratory acidosis in presence of adequate supply of oxygen indicates that carbon dioxide production and accumulation in venous blood is more rapid than its removal from body (Gates et al. 1971).

Table 3. Acid base and electrolyte changes during halothane and isoflurane maintenance anesthesia in water buffaloes (n = 12).

Parameters	Group	Base	P5	I5	15	30	45	60	90	120
pH	HAL	7.41 ± 0.01	7.40 ± 0.03	7.38 ± 0.22	7.28** ± 0.13	7.22** ± 0.16	7.25** ± 0.15	7.32 ± 0.16	7.35 ± 0.16	7.39 ± 0.15
	ISO	7.42 ± 0.02	7.41 ± 0.09	7.40 ± 0.06	7.38 ± 0.08	7.35 ± 0.07	7.39 ± 0.13	7.36 ± 0.26	7.40 ± 0.12	7.41 ± 0.11
PaCO2 (mm Hg)	HAL	36.50 ± 1.36	34.75 ± 1.25	38.75 ± 1.47	47.50** ± 1.07	51.75** ± 2.80	50.30** ± 2.13	44.53 ± 2.68	41.12 ± 2.54	38.75 ± 1.73
	ISO	38.20 ± 1.29	37.50 ± 1.14	39.60 ± 1.34	42.50 ± 2.78	41.25 ± 1.21	44.36 ± 1.24	42.25 ± 2.35	41.15 ± 2.88	40.50 ± 1.87
PaO2 (mm Hg)	HAL	83.25 ± 3.23	82.15 ± 2.88	80.10 ± 3.70	84.25 ± 3.46	89.50 ± 2.75	87.75 ± 2.96	85.75 ± 3.47	88.25 ± 1.67	84.25 ± 1.77
	ISO	84.14 ± 2.45	82.25 ± 2.83	80.50 ± 1.27	87.13 ± 3.17	89.20 ± 3.68	86.75 ± 3.45	88.75 ± 2.23	84.50 ± 2.74	82.25 ± 1.25
HCO3 (mEq/L)	HAL	24.17 ± 2.35	24.10 ± 2.28	22.80 ± 2.38	25.50 ± 1.74	26.30 ± 1.59	28.30 ± 1.49	25.72 ± 1.26	24.85 ± 1.19	22.32 ± 2.00
	ISO	24.77 ± 2.70	22.22 ± 2.32	21.70 ± 2.78	26.22 ± 1.00	22.82 ± 1.18	25.62 ± 2.04	23.22 ± 2.53	21.10 ± 2.19	22.25 ± 1.03
SpO2 (%)	HAL	96.50 ± 1.64	95.50 ± 1.95	93.10 ± 2.57	84.75* ± 1.47	80.50* ± 0.57	89.75 ± 2.83	90.35 ± 2.25	94.25 ± 2.42	92.25 ± 1.10
	ISO	98.75 ± 0.34	97.80 ± 1.40	90.60 ± 2.63	93.15 ± 1.36	91.80 ± 5.38	94.27 ± 2.26	95.07 ± 2.67	93.30 ± 3.98	96.27 ± 3.18
Sodium (Na^+) (mmol/L)	HAL	146.2 ± 3.56	143.7 ± 3.12	145.7 ± 3.32	142.2 ± 4.09	139.0 ± 4.20	140.7 ± 4.10	142.5 ± 4.32	145.3 ± 4.20	144.2 ± 4.15
	ISO	139.5 ± 2.53	141.0 ± 2.88	135.5 ± 4.55	141.7 ± 3.56	141.7 ± 3.56	142.5 ± 4.73	142.5 ± 3.77	143.6 ± 3.18	140.7 ± 3.27
Potassium (K^+) (mmol/L)	HAL	4.40 ± 0.29	4.27 ± 0.25	4.45 ± 0.31	4.27 ± 0.35	4.42 ± 0.39	4.37 ± 0.35	4.25 ± 0.41	4.30 ± 0.41	4.20 ± 0.43
	ISO	4.60 ± 0.16	4.27 ± 0.21	4.15 ± 0.20	4.40 ± 0.16	4.50 ± 0.23	4.60 ± 0.17	4.40 ± 0.18	4.47 ± 0.09	4.75 ± 0.30
Calcium (Ca^++) (mmol/L)	HAL	1.94 ± 0.60	2.53 ± 0.21	2.46 ± 0.22	2.43 ± 0.23	2.47 ± 0.25	2.49 ± 0.25	2.43 ± 0.27	2.56 ± 0.22	2.55 ± 0.21
	ISO	2.27 ± 0.17	2.29 ± 0.17	2.32 ± 0.18	2.34 ± 0.16	2.35 ± 0.16	2.37 ± 0.17	2.43 ± 0.11	2.13 ± 0.32	2.33 ± 0.15
Chloride (Cl^−) (mmol/L)	HAL	102.50 ± 1.35	102.25^ab ± 0.94	101.25 ± 1.25	100.50 ± 0.64	101.25 ± 0.10	100.25 ± 1.25	102.75 ± 0.47	101.50 ± 0.28	100.25 ± 0.62
	ISO	101.75 ± 0.63	103.75^b ± 1.53	102.00 ± 1.98	101.75 ± 1.71	103.50 ± 1.12	105.15 ± 2.09	102.25 ± 2.19	101.15 ± 1.46	101.25 ± 1.42

Note: values with different alphabets differ significantly (p < 0.05) between the two groups at corresponding intervals.

P5, 5 min after premedication; I5, 5 min after induction.

* Differ significantly (p < 0.05) from base value.

** Differ significantly (p < 0.01) from base value.

Significant (p < 0.05) decrease in SpO₂ values from 15 to 30 min in HAL group (Table 3) might be due to the anesthetic-induced depression of central respiratory center and the development of ventilation/perfusion mismatching in the lungs associated with the dorsal recumbency during general anesthesia in HAL group. However, administration of 100% oxygen along with either halothane or isoflurane in both the groups might have led to higher values of SpO₂ observed in the present study. Plasma sodium, potassium, calcium, and chloride concentration did not show any statistically significant (p > 0.05) change in either of the two groups indicating that electrolyte balance is similar during halothane and isoflurane maintenance anesthesia in water buffaloes (Table 3). A similar observation in electrolyte concentration was observed during thiopental–halothane anesthesia in cattle (Singh 1988) and calves (Steffey & Howland 1979). On contrary, a progressive increase in plasma potassium level in cows and cattle and a significant decrease in plasma sodium and potassium concentration in camels were reported during halothane anesthesia (Bouda & Pavlica 1970; Nemecek et al. 1970; Singh et al. 1994).

Recovery from anesthesia was faster in ISO group compared to HAL group. The regaining of reflexes was estimated once the administration of inhalant anesthetic agent was discontinued. No significant (p > 0.05) difference in the TE and time to first head movement was observed between the two groups. However, time to regain sternal recumbency and TS up were significantly (p < 0.01) lesser in ISO group compared to HAL group (Table 4). The relative insolubility, lower blood gas partition coefficient, and faster changes in alveolar concentration during isoflurane anesthesia might have led to a more rapid recovery from anesthesia in ISO group than HAL group. Similar findings were reported in horses by Matthews et al. (1992) and Whitehair et al. (1993). The early recovery associated with isoflurane could also be attributed to its better cardiovascular function, less stress response, less alteration in hepatic blood flow, and quicker elimination (Riazuddin et al. 2004). On the other hand, slower recovery in HAL group might be due to increased stress leading to catabolic pathways and depletion of energy sources (Wagner et al. 1991).

Table 4. Scores for recovery quality and recovery times along with side effects observed during halothane and isoflurane maintenance anesthesia in water buffaloes (n = 12).

Parameters	HAL (n = 6)	ISO (n = 6)
Recovery quality	2.75 ± 1.50	3.75* ± 0.15
TE (min)	15.00 ± 0.00	11.50 ± 0.80
Time to first head movement (min)	21.25 ± 2.39	16.75 ± 0.25
TSR (min)	30.10 ± 4.54	21.15** ± 3.10
TS up (min)	48.02 ± 3.50	29.25** ± 1.18
Side effects
Regurgitation	Mild (2); no regurgitation (4)	No regurgitation (6)
Hypersalivation	Not seen, mild salivation (6)	Not seen, mild salivation (6)
Lacrimation	Mild lacrimation (6)	Mild lacrimation (6)
Post-anesthetic shivering	Shivering (3)	Not observed
Post-recovery limb in coordination	Observed (3)	Not observed

* Differ significantly at 5% level of significance (p < 0.05).

** Differ significantly at 1% level of significance (p < 0.01).

A significant (p < 0.05) difference in quality of recovery was observed between the two groups (Table 4). Isoflurane maintenance anesthesia was associated with a smoother recovery without any excitatory symptoms and an excellent coordination upon standing. Although a transient shivering and limb in coordination was observed in few buffaloes during recovery period following halothane maintenance anesthesia, post-anesthetic shivering observed in three buffaloes in HAL group (Table 4) might either be due to residual excitatory effect produced by central nervous stimulation during anesthesia (Auer et al. 1978) or due to the effect of hypothermia which was observed toward recovery in HAL group. It has been observed that core temperature at the end of surgery is the most important determinant for the occurrence of post-operative shivering (Lienhart et al. 1992), thus thermoregulatory shivering occurs as a result of post-operative hypothermia which was also observed in three buffaloes in HAL group in the current study.

Regurgitation a frequent complication during general anesthesia in ruminants (Reid et al. 1993) was observed in two buffaloes in HAL group (Table 4). Regurgitation during light planes of anesthesia occurs as a result of active but uncontrolled reflex mechanism which causes the ingesta to pass up the esophagus into pharynx while regurgitation during deep levels of anesthesia is a passive process governed by increases in intraruminal pressure unopposed by poor esophageal or eso-pharyngeal sphincter tone. Inadequate fasting and improper positioning of animal increases the risk of regurgitation during dorsal and lateral recumbency, where esophageal opening gets submerged in the ruminal content, preventing normal eructation of gases from rumen, leading to gross distention of rumen and regurgitation of ruminal contents. Despite of proper positioning of animals in both groups, mild regurgitation observed in two buffaloes which might possibly be due to the fact that majority of operations in HAL group were performed in dorsal recumbency where it was difficult to adequately elevate the neck of animal compared to lateral recumbency. Mild to moderate regurgitation was observed after midazolam–thiopentone sodium administration in bovine (Kaur & Singh 2004). Evacuation of ruminal contents and subsequent fasting for 2 days for major surgery of longer duration decreases the risk of regurgitation in bovine (Peshin et al. 1987). Continuous production of copious saliva even during deep plane of anesthesia in recumbent cattle can lead to airway obstruction. To avoid such complication, the buffaloes were restrained in head down position in lateral recumbency so as to facilitate easy drainage of saliva and ruminal contents from the mouth. Mild salivation observed during maintenance anesthesia in both the groups might either be due to decreased swallowing reflex (Kokkonen & Eriksson 1987) or due to partially opened jaws for the endotracheal tube placement. Parasympathetic stimulation or lighter plane of surgical anesthesia could have attributed to the mild degree of lacrimation observed during maintenance anesthesia in both the groups in the current study.

4. Conclusion

Recovery from anesthesia with isoflurane is more rapid than with halothane in premedicated water buffaloes. General anesthesia maintained with isoflurane is associated with a lesser degree of cardiovascular, respiratory, and hemodynamic depression than halothane. Acid base parameters and plasma electrolytes are minimally affected during isoflurane maintenance anesthesia. Isoflurane maintenance anesthesia provides better quality of recovery and is associated with minimum side effects. To conclude, isoflurane maintenance anesthesia is better than halothane maintenance anesthesia in water buffaloes.