Aconsensus panel of the American College of Veterinary Internal Medicine has indicated that long-term pharmacological management of dogs with CHF attributable to valvular degeneration should include furosemide, pimobendan, and an ACE inhibitor.1 This approach is also widely advocated for disorders of myocardial failure (eg, dilated cardiomyopathy).2–8 Administration of the loop diuretic furosemide can result in activation of the RAAS in dogs, which calls into question its use as a lone therapeutic agent.9–11 Activation of the RAAS is also associated with the administration of the afterload-reducing (vasodilatory) agents amlodipine12 and hydralazine.11 Pimobendan, a benzimidazole-pyridazinone, has positive inotropic and vasodilatory actions. Pimobendan was approved by the US FDA Center for Veterinary Medicine for the treatment of CHF on the basis of a clinical trial that indicated the hemodynamic benefits and survival time for dogs receiving pimobendan were equivalent to those for dogs receiving enalapril, an ACE inhibitor.13 Other studies2–4,14 have found that pimobendan improves clinical signs and survival time in dogs with dilated cardiomyopathy and mitral valve insufficiency In another study15 conducted by our laboratory group, we found that although a high dosage of pimobendan (0.5 mg/kg, PO, q 12 h) alone did not activate the RAAS, the coadministration of pimobendan and furosemide appeared to have an additive effect on furosemide-induced RAAS activation.
Although RAAS activation can have acute hemodynamic benefits, chronic RAAS activation harms cardiac, renal, and vascular tissues.16,17 Angiotensin II and aldosterone contribute to myocardial and vascular remodeling. Specifically, in addition to being a potent vasoconstrictor, angiotensin II stimulates thirst and induces the release of antidiuretic hormone and aldosterone, which increases ventricular after-load and causes cardiac and vascular remodeling.16 Aldosterone in turn causes fluid retention and is independently associated with renal and cardiac remodeling, endothelial cell and baroreceptor dysfunction, inhibition of myocardial norepinephrine uptake, and reduced heart rate variability.17–20 In light of the extensive adverse effects of chronic RAAS activation, blunting activation of the RAAS has become an integral part of the long-term management of heart disease, hypertension, and renal disease in dogs.
The purpose of the study reported here was to evaluate the effect of administration of the labeled dosage of pimobendan to dogs with furosemide-induced RAAS activation. We hypothesized that the labeled dosage of pimobendan would not have an additive effect on furosemide-induced RAAS activation. Furthermore, we investigated whether the effect of furosemide on the RAAS would plateau or continue to increase with long-term administration.
Materials and Methods
Animals—Twelve mature (> 1 year old) hound-type dogs (6 females and 6 males) were enrolled in the study. Mean ± SD body weight of the dogs was 27.5 ± 8.8 kg (range, 20.8 to 33.4 kg). Each dog was assessed as healthy on the basis of history, results of a physical examination, and analysis of a minimum data set that consisted of systolic blood pressure and results of a CBC, serum biochemical analysis, and urinalysis. The dogs were housed in an Association for Assessment and Accreditation of Laboratory Animal Care International–approved facility with controlled light cycles. All dogs were fed a standard commercial diet.a The North Carolina State University College of Veterinary Medicine Institutional Animal Care and Use Committee approved the study.
Experimental design—Dogs were allowed to acclimate for 7 days before the administration of the medications. Dogs were allocated to 2 groups. Six dogs (3 males and 3 females) received furosemideb (2 mg/kg, PO, q 12 h) for 10 days. The other 6 dogs (3 males and 3 females) received a combination of furosemide (2 mg/kg, PO, q 12 h) and pimobendanc (0.25 mg/kg, PO, q 12 h) for 10 days. First day of administration was designated as day 1. Medications were administered at 7:00 am and 7:00 pm. To measure the time course and magnitude of the response of the RAAS to the medications, body weight, heart rate, SAP, and serum electrolyte concentrations were monitored on days 0 (baseline), 5, and 10. Urine samples for determination of urinary A:C were obtained on those same days (2 urine samples/d).
SAP evaluation—The SAPd was measured via the coccygeal artery. The mean of 3 consecutive measurements with values within 10% of each other was calculated.
Urinary A:C—Approximately 5 mL of urine was obtained from each dog by means of cystocentesis or midstream catch in the morning (2 to 3 hours after administration of morning medications) and evening (8 to 10 hours after administration of morning medications) for determination of urinary A:C. Each urine sample was frozen at −70°C within 1 hour after collection. Samples were subsequently thawed, and equal aliquots of the 2 daily samples (morning and evening) for each dog were mixed, refrozen, and submitted to a laboratorye for determination of urinary A:C, as described elsewhere.12,21
Statistical analysis—The SAP, body weight, urine creatinine concentration, urine aldosterone concentration, urinary A:C, serum electrolyte (phosphorus, magnesium, sodium, potassium, and chloride) concentrations, and serum bicarbonate concentration were measured at 3 time points. Data had an approximate Gaussian distribution. Accordingly, these outcomes were analyzed by use of a mixed-model ANOVA to assess the effect of time with treatment and the effect of treatment group at baseline. Values of P < 0.05 were considered significant. All analyses were performed by use of statistical software.f
Results
Mean ± SD dosage for dogs administered furose-mide alone was 2.00 ± 0.13 mg/kg. For dogs administered the combination of medications, the mean dosage of furosemide was 2.00 ± 0.11 mg/kg and the mean dosage of pimobendan was 0.25 ± 0.03 mg/kg.
No abnormalities were detected on the basis of results of the physical examination, CBC, serum biochemical analysis, urinalysis, or blood pressure evaluation conducted on day 0. Treatment did not significantly affect heart rate (Table 1). The baseline SAP was not significantly different between groups. Furosemide administration was associated with a modest but significant initial decrease in SAP at day 5 (Figure 1), but SAP returned to baseline values by day 10. The SAP on days 5 and 10 for the combination of furosemide and pimobendan was not significantly different from the value obtained at baseline.
Mean ± SD SAP on days 0, 5, and 10 in dogs (6 dogs/group) receiving furosemide (2 mg/kg, PO, q 12 h) or the combination of furosemide (2 mg/kg, PO, q 12) and pimobendan (0.25 mg/kg, PO, q 12 h) for 10 days. First day of administration was designated as day 1; values for day 0 (baseline values) were obtained before medication administration. *Within a treatment group, the value differs significantly (P < 0.05) from the value for day 0.
Citation: American Journal of Veterinary Research 72, 12; 10.2460/ajvr.72.12.1646
Mean ± SD values for body weight, heart rate, and serum electrolyte concentrations for dogs (6 dogs/group) receiving furosemide (2 mg/kg, PO, q 12 h) or the combination of furosemide (2 mg/kg, PO, q 12 h) and pimobendan (0.25 mg/kg, PO, q 12 h) for 10 days.
| Treatment | Body weight (kg) | Heart rate (beats/min) | Phosphorus (mg/dL) | Sodium (mmol/L) | Chloride (mmol/L) | Potassium (mmol/L) | Bicarbonate (mmol/L) |
|---|---|---|---|---|---|---|---|
| Furosemide | |||||||
| Day 0 | 30.1 ± 3.67 | 101.3 ± 12.30 | 3.5 ± 0.47 | 145.2 ± 1.90 | 112.3 ± 1.50 | 4.7 ± 0.57 | 20.3 ± 2.40 |
| Day 5 | 28.9 ± 4.90* | 101.3 ± 17.10 | 4.2 ± 0.61* | 145.0 ± 1.22 | 105.2 ± 1.90* | 4.2 ± 0.30* | 24.5 ± 1.87* |
| Day 10 | 29.6 ± 3.90 | 88.7 ± 9.85 | 3.6 ± 0.45 | 144.0 ± 1.86 | 106.8 ± 1.94* | 4.4 ± 0.27 | 22.3 ± 2.34 |
| Furosemide and pimobendan | |||||||
| Day 0 | 24.8 ± 4.10 | 102.3 ± 15.50 | 3.8 ± 0.74 | 145.3 ± 1.97 | 112.2 ± 1.70 | 4.7 ± 0.44 | 20.7 ± 2.80 |
| Day 5 | 24.4 ± 3.80 | 111.3 ± 14.78 | 3.9 ± 0.49 | 145.0 ± 1.09 | 106.0 ± 2.19* | 4.3 ± 0.28* | 24.7 ± 2.80* |
| Day 10 | 24.4 ± 4.00 | 93.0 ± 5.00 | 4.0 ± 0.28 | 145.3 ± 0.82 | 107.0 ± 2.19* | 4.6 ± 0.37 | 23.5 ± 2.50* |
| Reference range | NA | 70.0–160.0 | 2.0–6.7 | 147.0–154.0 | 104.0–117.0 | 3.9–5.2 | 18.0–25.8 |
First day of administration was designated as day 1; values for day 0 (baseline values) were obtained before medication administration.
Within a treatment group, the value differs significantly (P, 0.05) from the value for day 0.
NA= Not applicable.
The administration of furosemide resulted in a significant decrease in body weight at day 5 (Table 1). Furosemide alone and the combination of furosemide and pimobendan resulted in a significant decrease in the serum potassium concentration at day 5 and a sustained significant decrease in the serum chloride concentration at days 5 and 10; however, it did not cause a significant change in the serum sodium concentration, compared with the baseline concentration. The administration of furosemide alone was associated with a significant increase in the serum bicarbonate concentration at day 5, which subsequently decreased and was similar to the baseline value at day 10. Administration of the combination of furosemide and pimobendan resulted in a sustained significant increase in the serum bicarbonate concentration at days 5 and 10, compared with the baseline value. Furosemide alone and the combination of furosemide and pimobendan were associated with a significant decrease in the magnesium concentration at day 5, which subsequently increased and was similar to the baseline value at day 10. Administration of furosemide alone resulted in a significant increase in the phosphorus concentration at day 5, which subsequently decreased and on day 10 was not significantly (P = 0.70) different from the baseline value. In contrast, administration of the combination of furosemide and pimobendan did not cause a significant change in the serum phosphorus concentration.
Furosemide administered alone resulted in a significant increase in urinary A:C from day 0 (mean ± SD urinary A:C, 0.37 ± 0.14 μg/g) to day 5 (mean urinary A:C, 0.89 ± 0.23 μg/g; Figure 2). The effect appeared to plateau by day 5 because there was not a significant (P = 0.06) increase in urinary A:C from day 0 to day 10 (mean urinary A:C on day 10 was 0.95 ± 0.63 μg/g). The combination of furosemide and pimobendan resulted in a similar significant increase in urinary A:C from day 0 (mean ± SD urinary A:C, 0.36 ± 0.22 μg/g) to day 5 (mean urinary A:C, 0.88 ± 0.55 μg/g), which also appeared to plateau at or before day 5 because there was not a significant (P = 0.10) increase in urinary A:C from days 0 to 10 (mean urinary A:C on day 10 was 0.85 ± 0.21 μg/g).
Box-and-whisker plots of the urinary A:C on days 0, 5, and 10 in dogs receiving furosemide (2 mg/kg, PO, q 12 h; A) or the combination of furosemide (2 mg/kg, PO, q 12 h) and pimobendan (0.25 mg/kg, PO, q 12 h; B) for 10 days. First day of administration was designated as day 1; values for day 0 (baseline values) were obtained before medication administration. Each box represents the 25th to 75th confidence interval, the horizontal line in each box represents the median, and the bars represent the range for urinary A:C values. Compared with the value on day 0, there is activation of the RAAS with furosemide administration alone, and this activation appears to plateau by day 5 (A). There is also evidence of activation of the RAAS with administration of the combination of furosemide and pimobendan (B). See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 72, 12; 10.2460/ajvr.72.12.1646
Discussion
In the study in healthy dogs reported here, furosemide-induced RAAS activation plateaued at or before day 5 of treatment and was associated with the administration of furosemide alone as well as administration of the combination of furosemide and pimobendan. Thus, furosemide-induced RAAS activation in healthy dogs was not attenuated or enhanced with coadministration of pimobendan at the manufacturer's recommended dosage. In another study15 conducted by our laboratory group, it appeared that furosemide-induced RAAS activity was exacerbated by the addition of a high dose of pimobendan or that this effect was the result of progressive furosemide-induced activation of the RAAS over time. In the present study, there was no exacerbation of RAAS activity with the addition of pimobendan at 0.25 mg/kg every 12 hours, which is the labeled dosage and was half the dosage used in the other study.15 We chose to evaluate the dosage of 0.25 mg/kg, PO, every 12 hours because it is the labeled dosage for use in dogs with CHF.1 This raises the question as to whether the vasodilatory properties of a high dose of pimobendan may have resulted in potentiation of furosemide-induced RAAS activation in that other study15 because furosemide administered alone resulted in an apparent plateau in RAAS activation by day 5 in the present study.
The initial (day 5) mild decrease (compared with the baseline value) in SAP in dogs receiving furosemide alone was not evident in dogs receiving a combination of furosemide and pimobendan (Figure 1). The blood pressure–lowering properties of loop diuretics are typically attributed to loss of fluid and sodium, although these effects have been determined in rats and human patients parenterally administered loop diuretics regardless of volume status, which suggests the possibility of a direct vasodilatory mechanism.22–24 This decrease in blood pressure is typically modest and is mediated and modified by the degree of sodium depletion as well as renin, angiotensin II, and prostaglandin concentrations.22–24 The initial mild decrease in blood pressure in dogs that received furosemide alone may have been attributable to a direct vascular effect of furosemide.22 However, blood pressure did not change significantly in the dogs that received the combination of furosemide and pimobendan, despite suspected but unproven volume depletion and a possible furosemide-induced decrease in systemic vascular resistance. This is thought to be a result of the positive inotropic effect of pimobendan offsetting the effects of furosemide or an effect of pimobendan on an unmeasured component of the RAAS.
Furosemide-induced RAAS activation, in the absence of baroreceptor stimulation, may be attributable to diminished delivery of NaCl to the macula densa,25,26 adrenergic stimulation of the β-receptors of the juxta-glomerular apparatus,27,28 or the direct effect of furosemide on renin release.25 A decrease in glomerular filtration rate results in a diminished delivery of NaCl to the macula densa. The serum phosphorus concentration, which is considered a systemic marker of glomerular filtration rate, initially (day 5) had a significant increase (compared with the baseline value) after the administration of furosemide alone. However, the phosphorus concentration subsequently decreased and was similar to the baseline value at day 10. The serum phosphorus concentration can increase in dogs receiving furosemide, with a subsequent return to baseline values when the circulating volume is corrected.29 It is possible that dogs in the present study had not fully replaced their urinary fluid loss until after day 5. However, in another study15 conducted by our laboratory group, the serum phosphorus concentration in clinically normal dogs did not change after administration of furosemide alone. The small number of dogs, the variation in serum phosphorus concentrations, and the limited number of samples likely contributed to the variation in results between the 2 studies. Serum phosphorus concentration did not change substantially over the course of furosemide and pimobendan administration. Therefore, it is less likely that a decrease in glomular filtration rate resulted in renin release and the observed RAAS activation.
Serum norepinephrine concentrations were not evaluated. Clinically imperceptible enhancement of adrenergic activity may have resulted in β-adrenergic stimulation of the juxtaglomerular apparatus with subsequent release of renin.28 Studies in dogs,27 rats,26 and mice25 have revealed that furosemide stimulates renin release by inhibiting Na-K-Cl cotransporter–dependent NaCl transport across the macula densa cells. These studies revealed a direct effect of furosemide on the macula densa, which results in renin secretion independent of β-adrenergic stimulation, sodium concentration within the tubular lumen, and diminished renal perfusion pressure. Therefore, furosemide-induced RAAS activation in either group was not unexpected given the direct effect of furosemide on the macula densa, regardless of volume status, renal perfusion pressure, and β-adrenergic activity.
Studies30–34 in dogs have revealed variable results with respect to RAAS activity and clinical stage of heart disease. This variability may be related to the underlying disease process, stage of the disease, or concomitant cardiac treatment. It has been speculated that dogs with dilated cardiomyopathy may have earlier RAAS activation, compared with activation in dogs with chronic degenerative mitral valve disease.34 Therefore, it is likely that there is RAAS activation prior to the first dose of furosemide in some dogs with decompensated heart disease. As stated previously, standard treatment for CHF includes administration of furosemide,1 which results in rapid activation of the circulating RAAS, as indicated by a rapid increase in plasma and urine concentrations of aldosterone.10,15,35
Chronic RAAS activation has a multitude of deleterious circulatory and tissue effects. Although acute activation of the circulating RAAS is beneficial, resultant chronic elevations in preload and afterload can lead to clinical deterioration and progression of cardiac disease and clinical signs. Activation of the tissue RAAS may follow a time course that differs from that of the circulating RAAS.36 It has been proposed that tissue RAAS may be active prior to the activation of circulating RAAS or for conditions in which pharmacological intervention has suppressed circulating RAAS.37 Formation of tissue angiotensin II leads to pathological myocardial hypertrophy, cardiac fibroblast hyperplasia and collagen biosynthesis, and induction of myocyte apoptosis and promotes vascular remodeling and inflammation.36–39 Multiple double-blinded, randomized, placebo-controlled clinical trials in dogs have indicated the benefits of ACE inhibitors in CHF.5–8
Studies40,41 conducted to evaluate the effect of pimobendan on neurohormonal compensatory mechanisms have revealed a reduction in the activity of atrial natriuretic peptide and brain natriuretic peptide and concentrations of norepinephrine, renin, and angiotensin II in human patients with CHF. Although pimobendan improves the clinical status in dogs with CHF caused by mitral valve disease and dilated cardiomyopathy,2–4,14 diminishes plasma norepinephrine concentrations in dogs with experimentally induced mild mitral valve re-gurgitation,42 and reduces inflammatory cytokine activity,43,44 the effect of pimobendan on the RAAS in dogs with naturally occurring disease remains unknown. Data of the present study do not support such an effect.
Furosemide administration was associated with an initial decrease in serum potassium concentrations in both groups. The kaliuretic effect of furosemide,45 which is induced by increased renal tubular flow and thereby secretion of aldosterone, led to an expected decrease in the serum potassium concentration. Hypokalemia is clinically relevant because it predisposes an animal to digitalis intoxication and muscular weakness and it may induce or worsen cardiac arrhythmias. Investigators in another study46 found low serum potassium concentrations in dogs with clinical CHF. In that study,46 diuretic-treated dogs with ventricular ectopy had significantly lower plasma potassium concentrations than did other dogs undergoing diuretic treatment. Hypokalemia has been associated with poor outcomes in people with CHF, and most hypokalemia-associated deaths are sudden and probably the result of arrhythmias.47 Provision of supplemental potassium in human patients is generally not tolerated well and is only used for a short-term period, with long-term maintenance of normokalemia being maintained with an ACE inhibitor and an aldosterone receptor antagonist.47 Dogs with CHF receiving captopril, furosemide, and a sodium-restricted diet did not have significant changes in the serum potassium concentration.48
Mild hypochloremic alkalemia is a commonly detected diuretic-induced electrolyte disturbance. This is most likely attributable to the inhibitory effect of furosemide on chloride transport, contraction alkalosis, and renal loss of chloride and potassium.49 The modest effects of furosemide on serum chloride and bicarbonate concentrations in the present study were largely unchanged by the addition of pimobendan. Suppression of the RAAS may ameliorate contraction alkalosis in addition to preventing the urinary loss of potassium, which thus mitigates the furosemide-induced acid-base imbalance.49,50
The study reported here involved a small number of dogs, with measurements obtained only on days 0, 5, and 10; daily measurements would have provided more exact information regarding the timing of the plateau of RAAS activation. The duration of the study was relatively brief, compared with the duration of treatment (ie, months) for most veterinary patients with CHF. We cannot assume RAAS activation remains constant with prolonged treatment. We did not evaluate the RAAS by using a control group that received a placebo. We evaluated the effects of furosemide and the combination of furosemide and pimobendan in healthy dogs; thus, extrapolations to dogs with CHF may not be accurate. Several pieces of information that would have made interpretation of the data more complete include results of assessment of the tissue RAAS, corresponding pharmacokinetic data, and SUN, creatinine, angiotensin II, renin, and norepinephrine concentrations. However, investigators in another study10 found no clinically relevant changes in SUN and serum creatinine concentrations when treatment with furosemide alone was compared with treatment with the combination of furosemide and pimobendan.
For the study reported here, we concluded that furosemide-induced RAAS activation appears to plateau prior to or at day 5 of furosemide administration. The addition of the recommended dosage of pimobendan to furosemide administration does not exacerbate or mitigate furosemide-induced RAAS activation. It remains unclear whether pimobendan at higher dosages potentiates furosemide-induced RAAS activation, and this should be investigated in future studies. Although results of the present study must be confirmed in clinical patients, on the basis of the present study, it appears that treatment to suppress the RAAS should accompany furosemide treatment, alone or in combination with pimobendan, in dogs.
Abbreviations
| A:C | Aldosterone-to-creatinine ratio |
| ACE | Angiotensin-converting enzyme |
| CHF | Congestive heart failure |
| RAAS | Renin-angiotensin-aldosterone system |
| SAP | Systolic arterial blood pressure |
Iams ProActive Health MiniChunks, Dayton, Ohio.
Salix, Intervet Canada, Whitby, ON, Canada.
Vetmedin, Boehringer Ingelheim, Bracknell, Berkshire, England.
Model 811-B, Parks Medical Electronics Inc, Aloha, Ore.
Esoterix Laboratories, Calabasas, Calif.
SAS, version 9.2, SAS Institute Inc, Cary, NC.
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