Materials and Methods

Animals—Eight client-owned alpacas (5 sexually intact males and 3 females; mean ± SD age, 4.1 ± 2.7 years) with a mean ± SD weight of 61.2 ± 19.3 kg were included in this study. A physical examination, CBC, and serum biochemical analyses were performed 24 hours prior to inclusion in the study to confirm the health of the animals. All procedures were approved by the Institutional Clinical Sciences Research Committee of the Cummings School of Veterinary Medicine at Tufts University.

Anesthesia and instrumentation—Food was with-held for 12 hours with complete water restriction for 6 hours, and alpacas were acclimated for a minimum of 2 hours to the hospital environment prior to induction of anesthesia. A 14-gauge polyurethane catheter^a was aseptically placed in the left jugular vein under light restraint in a commercially available llama chute.^b No premedication was administered. Anesthesia was induced via IV administration of ketamine^c (5 mg/kg) and midazolam^d (0.25 mg/kg). All alpacas were positioned in sternal recumbency for placement of a 10- to 12-mm cuffed orotracheal tube, immediately followed by cuff inflation. Alpacas were subsequently positioned in left lateral recumbency with the head positioned just above the level of the heart. Isoflurane vapor was administered in O₂ at a flow rate of 30 mL/kg/min by use of a small animal anesthesia circuit^e with a 3-L reservoir bag. All alpacas were allowed to spontaneously ventilate and were maintained in a medium plane of anesthesia as defined by the absence of a brisk palpebral reflex, spontaneous blinking, swallowing, neck tone, and purposeful movement. The ETISO was maintained within a range of 1.4% to 1.6%,²⁵ and isotonic fluids^f were administered at a rate of 5 mL/kg/h for the duration of anesthesia. The respiration rate, HR, SAP, DAP, MAP, and ETCO₂ were continually monitored and recorded every 5 minutes throughout the anesthetic period. Rectal temperature, oxygen saturation (measured by means of pulse oximetry), ETISO, and inspired isoflurane concentration were also recorded at 15-minute intervals. Measurements of ETCO₂, ETISO, and inspired isoflurane concentration were obtained through an 8-F polypropylene catheter advanced to the distal end of the endotracheal tube by use of a calibrated infrared gas analyzer.^g Furthermore, a 3-lead ECG was used to monitor HR and potential cardiac arrhythmias. A forced-air warming blanket^h was placed over the trunk of the alpaca to maintain normothermia as needed.

While anesthetized, the alpacas were instrumented with an 18-gauge arterial catheterⁱ in the left saphenous artery at the level just above the stifle joint. Furthermore, a Swan-Ganz catheter^j was placed into the pulmonary artery via the right jugular vein with the assistance of a percutaneous introducer sheath.^k Proper placement in the pulmonary artery was confirmed with either direct viewing under fluoroscopic guidance or by analysis of characteristic pressure waveform changes.

The cardiovascular variables of SAP, DAP, MAP, and pulse rate were obtained from the saphenous arterial catheter. Pulmonary artery pressure and CVP were measured following attachment of a pressure transducer to the distal and central venous ports of the pulmonary artery catheter. Pulmonary capillary wedge pressure tracings were similarly obtained after temporary inflation of the pulmonary artery balloon with 1.5 mL of air. All pressures were displayed on a multiparameter monitor.^l Cardiac output was determined through a standard thermodilution method by use of 10 mL of 0.9% NaCl^f solution at 22°C rapidly injected into the proximal port of the pulmonary artery catheter.²⁶ Temperature changes were recorded by use of a CO module, with a supplied calculation constant of 0.586, and displayed through the same multiparameter monitor.^l All measurements were repeated in triplicate, and mean values were calculated, with an acceptable variability of ≤ 10%.

Blood gas analysis^m was performed on arterial and mixed venous blood samples taken from the saphenous artery and right atrium (distal port of pulmonary artery catheter), respectively. Packed cell volume, total protein concentration, and Hb concentration were determined for mixed venous blood samples only.

Experimental design—Initial baseline data acquisition was performed at 30 minutes following induction to allow for anesthetic equilibration to occur. Subsequently, 4 treatments (DOBL of 4 μg/kg/min, DOBH of 8 μg/kg/min, NORL of 0.3 μg/kg/min, and NORH of 1 μg/kg/min) were randomly assigned and administered in a masked fashion via 15-minute constant rate infusion. Posttreatment cardiopulmonary measurements were obtained and followed by a 20-minute washout period. Successive baseline and posttreatment measurements were repeated until all drugs and dosages had been administered. Awake measurements of all cardiopulmonary variables were obtained 24 hours after initial instrumentation in standing, unsedated alpacas.

The following cardiovascular variables were recorded at each time point: HR, SAP, DAP, MAP, CO, pulmonary capillary wedge pressure, PAP, and CVP. At the same time, mixed venous samples and arterial blood samples were collected for blood gas analysis and measurement of PCV, total protein concentration, and lactate activity. Respiratory rate, rectal temperature, and end-tidal partial pressures (ETCO₂ and ETISO) were also measured. From these variables, the following variables were calculated: CI, SVI, SVR, DO₂, ERO₂, oxygen consumption, arterial oxygen content, and mixed venous oxygen content (Appendix).

Recovery from anesthesia and next-day evaluation—All catheters were secured in place via suture and light elastic wrap prior to recovery. The orotracheal tube was kept in place until swallowing, neck tone, eructation, and mastication movements were present. Alpacas were assisted until standing and remained hospitalized under close supervision overnight. The following morning, each alpaca was lightly restrained in the standing position. Nonsedated cardiovascular, respiratory, and metabolic measurements were obtained as described for comparison with values obtained at anesthetic baseline. All instrumentation was removed upon the completion of the experiment, and the alpacas were returned to their farm environment.

Statistical analysis—All cardiovascular, respiratory, and metabolic values were expressed as mean ± SD at 8 time points (baseline values obtained before each treatment [n = 4] and values obtained after each treatment [4]). Each alpaca served as its own control. The change in cardiopulmonary values between baseline and treatment for each drug at each dosage was documented via the difference in means and 95% confidence intervals. Significant differences between drug types and dosages were evaluated by use of a 2-tailed t test. A value of P < 0.05 was considered significant.

Results

All 8 alpacas completed the study successfully. Baseline MAPs were significantly (P < 0.001) lower during isoflurane anesthesia (range, 62.2 to 74.7 mm Hg), compared with awake measurements (range, 112 to 135 mm Hg), indicating anesthetic-induced hypotension.

Peak and sustained drug effects were reached within 5 minutes following drug administration, as determined on the basis of continuous HR and blood pressure monitoring. The effects of drug type (dobutamine and norepinephrine) and dosage (low and high) on cardiovascular, respiratory, and metabolic variables were determined (Tables 1 and 2). Significant increases in HR were only achieved by use of dobutamine and were unrelated to dose rate. In contrast, values for CI, SAP, DAP, and MAP were significantly increased from baseline after treatment at each dosage of both drugs. Furthermore, NORH significantly increased SAP, DAP, MAP, PAP, and SVI, compared with NORL. The SVR was only affected by norepinephrine infusion, with increases from baseline observed in both NORL and NORH groups. The mean CVP was −0.5 mm Hg (95% confidence interval, −1.4 to 0.5 mm Hg) throughout anesthesia and was unaffected by drug type or dosage. Because of substantial variability in pulmonary capillary wedge pressure measurements obtained during anesthesia, these values were excluded from the analysis.

Table 2—

Mean metabolic and respiratory variables in the same alpacas as in Table 1.

Variable	Low					High
	Drug	Base	Tx	Δ Mean	P value	Base	Tx	Δ Mean	P value
SmvO2 (%)	DOB	84.1a	90.3b	6.2 (3.1 to 9.3)	0.002	81.8a	90.1b	8.30 (4.67 to 11.9)	0.001
	NOR	85.7a	84.8a	−0.863 (−6.51 to 4.78)	0.728	85.5a	86.3a	0.788 (−4.39 to 5.96)	0.730
PmvO2 (mm Hg)	DOB	53.5a	66.0b	12.5 (5.58 to 19.3)	0.004	50.2a	63.5b	13.3 (9.15 to 17.5)	< 0.002
	NOR	51.5a	53.1a	1.81 (−5.05 to 8.67)	0.549	53.1a	55.2a	2.09 (−6.52 to 10.7)	0.584
Hb (g/dL)	DOB	11.3a	13.0b	1.75 (1.04 to 2.46)	0.001	11.1a	13.3b	2.11 (0.484 to 3.74)	0.018
	NOR	11.4a	12.3b	0.850 (0.165 to 1.54)	0.022	11.0a	13.5b	2.55 (1.44 to 3.66)	0.001
CmvO2 (mL/L)	DOB	127a	159b	31.4 (18.8 to 44.0)	0.001	123a	161b	38.4 (14.8 to 62.0)	0.006
	NOR	129a	140b	14.3 (5.9 to 22.7)	0.006	124a	157b	32.2 (15.6 to 48.9)	0.003
CaO2 (mL/L)	DOB	152a	175b	23.5 (14.0 to 33.0)	0.001	150a	179b	28.5 (6.5 to 50.5)	0.018
	NOR	154a	165b	11.5 (2.3 to 20.7)	0.021	148a	182c	34.6 (19.7 to 49.4)	0.001
O2 (mL O2/kg/min)	DOB	2.19a	2.42a	0.232 (−0.329 to 0.794)	0.361	2.64a	2.69a	0.054 (−0.473 to 0.581)	0.815
	NOR	2.36a	3.21a	0.776 (−0.042 to 1.59)	0.059	2.24a	3.54b	1.30 (0.090 to 2.51)	0.039
DO2 (mL O2/kg/min)	DOB	14.8a	27.4b	12.6 (8.1 to 17.1)	< 0.001	15.0a	28.8b	13.8 (6.5 to 21.1)	0.003
	NOR	15.3a	21.3b	6.00 (3.5 to 8.5)	0.001	14.5a	25.9c	11.4 (7.1 to 15.7)	0.012
ERO2 (%)	DOB	15.8a	9.33b	− 6.43 (−9.69 to −3.17)	0.002	18.3b	9.66b	− 8.68 (−12.5 to −4.85)	0.001
	NOR	16.2a	15.6a	−1.08 (−5.47 to 3.31)	0.570	15.6a	14.1a	−1.47 (−7.61 to 4.67)	0.588

C_aO₂ = Arterial oxygen content. C_mvO₂ = Mixed venous oxygen content. P_mvO₂ = Mixed venous partial pressure of O₂. S_mvO₂ = Mixed venous oxygen saturation. O₂ = Oxygen consumption.

See Table 1 for remainder of key.

Compared with baseline values, mixed venous oxygen saturation and mixed venous oxygen concentration were significantly higher following dobutamine infusion but unrelated to dose rate. Hemoglobin, PCV, arterial oxygen content, mixed venous oxygen content, and DO₂ also increased significantly from baseline after DOBL, DOBH, NORL, and NORH. Dose-dependent increases in arterial oxygen content, oxygen consumption, and DO₂ were limited to norepinephrine infusions. In contrast, drug type or dosage had no effect on mixed venous pH, mixed venous partial pressure of CO₂, P_aCO₂, and mixed venous lactate concentration. Values for inspired isoflurane concentration (mean, 1.82% [95% confidence interval, 1.76% to 1.88%]), ETISO (mean, 1.52% [95% confidence interval, 1.47% to 1.57%]), and ETCO₂ (mean, 35.7% [95% confidence interval, 34.1% to 37.3%]) were maintained within target range throughout the study.

Cardiovascular, respiratory, and metabolic variables were collected the day following experimentation (Tables 3 and 4). Cardiopulmonary measurements from blood gas samples could not be accurately obtained in 2 alpacas because of partial occlusion of the proximal port of the pulmonary arterial catheter or the saphenous arterial catheter at the 24-hour time point.

Mild, transient fluid accumulation during anesthesia was observed in the trachea of 2 of the 8 alpacas. Of these, 1 alpaca received short-term nasal oxygen therapy during recovery from anesthesia because of a transient increase in respiratory rate and effort. None of the study alpacas had evidence of upper airway obstruction following extubation. Atrial premature contractions were noted in 1 alpaca after DOBH, whereas ventricular premature contractions and trigeminy were observed in a different alpaca after administration of NORH. Arrhythmias were self-limiting in both alpacas.

Discussion

This study characterized the pharmacologic effects of 2 catecholamines in adult alpacas, a species that historically evolved under the hypobaric, hypoxic conditions of high altitude. New World camelids thus have genetic cardiovascular adaptations, including high blood oxygen affinity and pronounced peripheral vasoconstrictor responses, which have been predominantly studied in llama feti.²⁷ The lack of controlled studies on catecholamine responses in llamas and alpacas as well as the necessity to effectively manage hypotension in critical care and anesthetic management triggered the present evidence-based investigation in healthy camelids.

Cardiac output and CI and their derived variables (DO₂, oxygen consumption, ERO₂, stroke volume, and SVR) provide an assessment of overall cardiovascular function.¹⁰ The present study revealed that CI significantly increased following administration of high and low doses of dobutamine and norepinephrine. These results are in agreement with a previous study¹⁰ evaluating the administration of comparable dosages of dobutamine to anesthetized foals. In contrast, CO in sheep only increased significantly at a DOBH (10 μg/kg/min).²¹ Cardiac index or CO are most likely increased as a product of HR and stroke volume and the direct result of the positive chronotropic effect of dobutamine. The present study in alpacas did not reveal a dose-dependent increase in CI as was documented in foals¹⁰; however, the latter study also detected a dose-dependent increase in HR. The reason for a lack of a dose-dependent chronotropy in alpacas, as seen in foals, is unknown, but this finding could be explained by a species-specific difference in dobutamine's affinity for β-receptors or an increased responsiveness of the β-receptors to activation in this species.

As expected, both DOBH and DOBL significantly increased the HRs of the alpacas in the present study. The β₁-adrenergic receptor stimulation caused by dobutamine similarly increases the HRs of foals,^10,13 ponies,¹⁶ horses,^15,17,18 and lambs.¹⁹ The ability of dobutamine to induce a clinically important HR increase at low and high infusion rates suggests that the lowest possible dose should be administered to avoid potential arrhythmogenesis. Administration of 8 μg of dobutamine/kg/min resulted in transient atrial premature contractions in 1 of 8 alpacas, which resolved upon discontinuation of the infusion. Cardiac arrhythmias have also been observed at the same administration rate of dobutamine in ponies¹⁶ and at 5 μg/kg/min in horses.¹⁷ Additionally, premature atrial contractions were reported in 2 horses treated with 1.7 μg of dobutamine/kg/min while anesthetized with halothane.¹⁸ The dose of dobutamine should therefore be carefully titrated in the clinical setting. A starting point of 0.5 to 1 μg of dobutamine/kg/min in adult horses and 1 to 3 μg/kg/min in foals has been recommended.⁸

In the present study, isoflurane was considered a potent vasodilator that caused substantial hypotension and cardiovascular depression while maintaining a medium plane of anesthesia. These findings were supported by a significant (P = 0.005) reduction in MAP, CVP, SVR, and SVI in anesthetized alpacas, compared with awake alpacas. Awake cardiovascular variables in the alpacas of the present study (HR, CI, SVI, MAP, mean PAP, mean CVP, and SVR) were similar to those in standing llamas.²⁸ These similarities, specifically for PAP, CVP, and SVR, further support the contention that the alpacas were normovolemic and appropriately instrumented. The awake blood gas variables (mixed venous partial pressure of oxygen, mixed venous oxygen saturation, mixed venous oxygen content, and Hb concentration) were also comparable to those in a previous report²⁹ in awake alpacas.

The SAP, DAP, and MAP significantly increased from baseline following DOBH and DOBL in alpacas. These findings were similar to documented results in halothane-anesthetized horses (1, 5, and 10 μg of dobutamine/kg/min)¹⁷ and ponies (1, 2.5, and 5 μg/kg/min and not at 10 μg/kg/min).¹⁶ Comparable data were also reported in isoflurane-anesthetized foals (2.5, 5, and 10 μg/kg/min)¹⁴ with dose-dependent effects observed in a separate equine neonatal study¹⁰ (4 vs 8 μg/kg/min). The documented increase in arterial blood pressures may partly be attributed to the slight α-adrenergic effects of dobutamine but is most likely associated with direct β₁-adrenergic receptor stimulation and the subsequent increase in CO. A significant increase in MAP, SAP, and DAP has also been associated with norepinephrine infusion in normotensive sheep,^22,23 normotensive foals,¹³ and hypotensive neonatal foals.¹⁰ In the present study, all measurements of SAP, DAP, MAP, PAP, and SVI revealed dose-dependent increases with infusion of norepinephrine, as was previously seen for all variables in foals¹⁰ and SVI in sheep.²⁰ In the clinical setting, the dose of norepinephrine should be carefully titrated. A starting infusion of 0.1 μg/kg/min has been recommended for neonatal foals.⁸ However, effects may be seen in some foals at doses as low as 0.01 μg/kg/min.

In the present study, norepinephrine increased CI from baseline at low and high doses, as reported in normotensive and hypotensive foals^10,13 and normotensive sheep.^22,23 Furthermore, norepinephrine increased SVI at the high dose (1.0 μg/kg/min) but not the low dose (0.3 μg/kg/min; P = 0.082). Increased CI and SVI were also observed in another study¹³ that used a much smaller dose of norepinephrine (0.1 μg/kg/min) in normotensive, diazepam-sedated foals. However, isoflurane-anesthetized sheep given doses of 0.4 μg of norepinephrine/kg/min had a significant enhancement of CO but no change in SVI.^22,23 The increase in CI was most likely the result of norepinephrine-mediated direct α- and β-adrenergic stimulation resulting in an increase of MAP and SVR. It is interesting that in sheep with septic peritonitis, only a combination of dobutamine (5 μg/kg/min) and norepinephrine (0.5 μg/kg/min) was sufficient to maintain increased CI.²⁰ This same combination has also been investigated in foals and found to maintain CI and MAP throughout the time of sedation.¹³ In the present study, mean CVP measurements at baseline were comparable to those of llamas receiving propofol anesthesia²⁸ yet considerably lower than those reported in isoflurane-anesthetized alpacas receiving butorphanol.³⁰ This discrepancy in measured CVP should be taken into account when comparing SVR between the present and previous reports because this is a calculated variable partially determined by CVP (Appendix).

All treatments increased Hb concentrations. The range of Hb concentrations was dose dependent and thought to be the result of varying degrees of splenic contraction in response to the administration of norepinephrine and dobutamine. This agreed with previous findings indicating greater increases in Hb concentration of excited camelids.^31,32 The increase in arterial oxygen content, mixed venous oxygen content, and DO₂ from baseline at both doses of norepinephrine and dobutamine most likely related to increasing concentrations of Hb, which is considered the most important contributor to these calculated variables (Appendix). Although a significant increase in mixed venous oxygen saturation and mixed venous partial pressure of oxygen from baseline was detected at high and low doses of dobutamine, their effect on calculated arterial oxygen content was comparatively small. Similar to our findings, a slight but not significant increase in mixed venous partial pressure of oxygen was observed in halothane-anesthetized ponies exposed to increasing dosages of dobutamine.¹⁶

The measured mean PAP (8 mm Hg at baseline) in the present study remained consistently lower than values reported by Garcia-Pereira et al³⁰ in isoflurane-anesthetized alpacas (16 to 18 mm Hg), but this was most likely attributable to our use of spontaneous ventilation as opposed to intermittent positive-pressure ventilation. Values for mean PAP (16.4 mm Hg) in standing, unanesthetized llamas,²⁸ however, were similar to those obtained in awake alpacas in the present study (14.4 mm Hg). In contrast to the study reported here, dobutamine infusions have been associated with increases in mean PAP in horses anesthetized with isoflurane¹⁵ and halothane.³³ Similarly, norepinephrine was responsible for an increase in PAP in halothane-anesthetized sheep³⁴ and in isoflurane-anesthetized foals,¹⁴ complementing results following administration of NORH in the present study. Mild pulmonary fluid accumulation during anesthesia was observed in the airways of 2 of the 8 alpacas following NORH infusion, 1 of which required temporary oxygen administration during recovery. Because mean PAP values following NORH infusion remained less than values obtained in awake alpacas, it appeared unlikely that increased PAP substantially contributed to extravasation of fluid into the alveoli.

Dobutamine and norepinephrine are appropriate choices to improve CI and MAP and thus overall DO₂ in alpacas with isoflurane-induced hypotension. Norepinephrine is a potent vasoconstrictor in alpacas that specifically increased SVR and oxygen consumption at both dosages. Only dobutamine reduced relative oxygen consumption while improving DO₂ and overall tissue oxygenation. On the basis of these results, we recommend that these catecholamines be carefully titrated in the clinical setting to avoid potential arrhythmogenic effects of dobutamine and excessive increases in MAP following norepinephrine infusion. Results of the present study provide guidelines for the use of dobutamine and norepinephrine in systemically healthy alpacas in the clinical setting. Further investigation of the use of these catecholamines in critically ill patients and their possible therapeutic value as a combined infusion is warranted.