Beyond Furosemide: The Role of Diuretics in Congestive Heart Failure Part 1 – Torsemide

Diuretics are a critical component of the pharmacotherapy of congestive heart failure (CHF) (typically pulmonary edema or ascites).

In humans with heart failure, 90% receive at least one type of diuretic.¹ Of these, loop diuretics—furosemide, bumetanide, and torsemide—are the most potent and commonly used. When a single drug is administered in humans, furosemide is given 87% of the time.¹

Furosemide has been, and remains, the diuretic of choice for acute and chronic management of CHF in both humans and animals since its release in 1966. However, interesting alternatives and adjunctive therapies are now available, including:

• Torsemide, a loop diuretic that can be used as an adjunct or alternative to furosemide
• Spironolactone, a weak, potassium-sparing diuretic and mineralocorticoid receptor (MR)blocker that is used primarily for additional blockade of the renin–angiotensin–aldosterone system (RAAS). In heart failure, it typically accompanies a more potent diuretic, such as furosemide.

This article, the first in a 2-part series, discusses torsemide, while the second article will address spironolactone.

Loop Diuretics

Loop diuretics, in general, exemplify the phrase “double-edged sword”: They are typically necessary—often lifesaving—yet are invariably harmful in some respects, particularly with long-term use (Table 1).² Although adverse effects differ among the loop diuretics, complications are common to all diuretics.

Loop diuretics are pyridine-3-sulfonurea drugs that act on the thick portion of the nephron’s ascending loop of Henle, where they inhibit the sodium–potassium–chloride (Na+–K+–2Cl–) cotransporter, leaving sodium (and other ions) to be lost, with water, in the urine.

Furosemide

For nearly 50 years, furosemide has been a workhorse for cardiologists and internists. It has undoubtedly saved countless lives when used to manage acute/emergent heart failure and has increased the longevity of patients when used long term.

Furosemide is typically not used as a monotherapy but rather given concurrently with:

• Inotropes: Pimobendan, digoxin, or dobutamine
• Other diuretics: Thiazides and/or potassium-sparing diuretics
• Vasodilators: Angiotensin-converting enzyme (ACE) inhibitors, amlodipine, hydralazine, or nitroprusside
• Antiarrhythmic agents: Lidocaine, sotalol, or amiodarone.

Adverse Effects

Furosemide’s popularity is deserved, and most experienced clinicians are comfortable administering this drug. However, negative effects are associated with any potent diuretic (Table 1), which include:

• Production of electrolyte disturbances (notably hypokalemia and hypomagnesemia), which may contribute to muscular weakness and arrhythmias
• Activation of the RAAS due to reduced renal blood flow and sodium loss (Figure 1), which produces adverse effects in the heart, vessels, and kidneys.^3-5

FIGURE 1. Mean urine aldosterone-to-creatinine ratio (A:Cr) in 12 normal dogs challenged with furosemide at 2 mg/kg Q 12 H. At day 5 and 10, the RAAS is activated and, with continuous treatment, stays activated for at least 10 days. The urine A:Cr ratio indicates the amount of aldosterone found in the urine over 24 hours and reflects RAAS activation.

Aldosterone and angiotensin II contribute to congestion and hypertension via sodium retention and vasoconstriction, respectively. Excessive, chronic production of these 2 hormones, therefore, further increases the workload on an already failing heart. Additionally, the activated RAAS stimulates the sympathetic nervous system, which is toxic to cardiac myocytes and increases heart rate, produces vasoconstriction, and predisposes to cardiac arrhythmias.⁶

Pharmacokinetics

Furosemide’s rapid onset of action makes it indispensable in the emergent treatment of cardiogenic pulmonary edema. It does, however, have a relatively short duration of action, which often necessitates repeated bolus dosing in the emergency setting and, at home, up to 3 to 4 times per day dosing in patients with advanced CHF. This characteristic—multiple peaks and valleys in serum drug concentrations—may enhance furosemide’s stimulation of the sympathetic nervous system and RAAS, contributing to diuretic resistance.7 Increasing dosages are required over time and, in many cases, other diuretics are added to maintain patient comfort.

Resistance

Diuretic resistance is multifactorial, resulting from neurohormonal activation as well as:

1. Enhanced sodium and solute reabsorption at the proximal tubule in response to diuretic-induced contraction of the extracellular fluid volume
2. Nephron hypertrophy in response to increased solute delivery to distal nephron segments
3. Decreased renal responsiveness to natriuretic peptides.⁸

Torsemide appears to be less affected by resistance, but a mechanism for this characteristic has not been identified.

Torsemide

Torsemide, like furosemide, is a very potent diuretic.

In the Literature

Although not studied as thoroughly as would be desirable in animals, torsemide’s profile in humans suggests that quality of life and survival are enhanced when compared with furosemide.^1,9,10

There are no large clinical trials published to date in veterinary medicine, however:

• A study of 7 dogs with clinically stable CHF demonstrated that replacement of furosemide with torsemide was both safe and effective.¹¹
• Another small case series suggested that duration and quality of life in 3 dogs with advanced refractory CHF were prolonged and improved, respectively, with torsemide.¹²
• One feline and several canine laboratory studies^13-15 demonstrated drug traits that could potentially make use of torsemide advantageous in treating CHF in veterinary patients (Table 2).

Pharmacokinetics & Pharmacodynamics

Diuretics exert their diuretic action at the nephron, and duration of diuretic action is related to urinary excretion rate.

The pharmacokinetic and pharmacodynamic characteristics of torsemide (when compared with furosemide, Table 2) include, slower release from the plasma to the renal tubular fluid and resultant slower urinary excretion rate. The slower transfer rate of torsemide is hypothesized to be due to inherent properties of torsemide (it is less acidic and less of it exists in an anionic form, as compared with furosemide). The result is greater, smoother, and longer-acting diuresis (Figure 2).^13-15

FIGURE 2. Urine production in control animals, those receiving standard-dose furosemide, and those receiving torsemide orally. Note the similar peak effect between agents and the brisk and longer-acting diuresis with torsemide. These studies were performed in normal dogs.

Two veterinary studies have addressed torsemide pharmacodynamics, each showing superior diuresis when compared with furosemide (Figures 2 and 3), at substantially lower dosage.^13,15 In addition, furosemide begins to lose efficacy (diuretic resistance) at 14 days of oral therapy in healthy dogs (Figure 3), while torsemide is less affected.⁷ The relevance of the latter finding in clinical cases remains to be seen.

FIGURE 3. Crossover study performed in normal dogs that compared urine volume of dogs receiving oral furosemide (2 mg/kg Q 12 H) and torsemide (0.2 mg/kg Q 12 H) acutely and after 14 days of treatment. In the dogs receiving torsemide, note the higher peak at days 1 and 14, slightly later peak, and greater duration of enhanced urine flow compared with dogs that received furosemide. Most important, in the dogs receiving furosemide, note the decline in diuresis on day 14 compared with the lack of resistance on day 14 in dogs receiving torsemide.

Diuretic efficacy occurs when diuretic delivery to the kidney is in the appropriate range. As shown in Figure 4, diuresis persists for 3 to 5 hours following once-daily administration. Using this dose of furosemide once daily, there is no risk for toxicity and the time in the therapeutic range is maximized.

FIGURE 4. Diuresis occurs when urine levels of furosemide are within the therapeutic range. Urinary furosemide levels, of course, depend on the plasma concentration delivered to, and filtered by, the glomerulus, which, in turn, is affected by furosemide bioavailability. In this hypothetical example, diuresis begins by 1 hour and persists for 3 to 5 hours. This represents the ideal dosage for once-daily treatment, as corresponding plasma levels (reflected in urinary concentration) are high enough to maximize time in the therapeutic range but not create a risk for ototoxicity.

When standard doses of diuretics become inadequate to maintain the patient’s quality of life, several strategies can be used, such as increasing dosage, increasing frequency of administration, adding new drugs (ie, sequential nephron blockade with such agents as hydrochlorothiazide and spironolactone), or changing to another drug (eg, torsemide or bumetanide) to boost diuresis (Figures 5 and 6).

FIGURE 5. If resistance to furosemide results or other factors (eg, a high salt meal) dictate that more diuresis is needed, 1 of 2 strategies may be used: First is to administer a higher dosage, as shown with the red pharmacodynamic curve. The increase in dosage increases the urine level of the diuretic but provides only a small increase in time in the effective range, enhancing diuresis (red arrow) but increasing risk for toxicity. A better approach, although problematic in terms of owner compliance, is to administer multiple doses throughout the day. As shown in this figure, administering the same dosage 3 times triples the time in the effective range and avoids the risk for toxicity.

FIGURE 6. If resistance to furosemide results or other factors (eg, a high salt meal) dictate that more diuresis is needed, another approach (versus the 2 strategies demonstrated in Figure 5) would be to change to a longer-acting diuretic, such as torsemide (yellow curve). Note the higher peak urine level compared with furosemide at the original dosage, slight delay in peak diuresis, and prolonged period of time in the therapeutic range (dashed arrow; 10–12 H in this model).

RAAS Activation

Whereas furosemide is known to activate the RAAS (Figure 1 and Table 1), the reason for high plasma aldosterone concentrations associated with use of torsemide is less clear. Although torsemide may activate the RAAS, evidence suggests that serum aldosterone levels may be elevated because of antialdosterone effects via MR blockade, whereby high dosages of torsemide led to in vivo binding of the MR in rat kidney cells.¹⁶

A subsequent study,¹⁷ however, demonstrated that torsemide does not bind the MR in rat cardiomyocytes. This suggests that the increase in serum aldosterone seen in the study by Hori and colleagues¹⁵ is not entirely the result of MR blockade and likely represents RAAS activation from sodium depletion and diuresis.

Other antialdosterone and antifibrotic effects of torsemide are postulated, however, and warrant further study.^18-20 Regardless of whether there is an MR-blocking effect, it seems prudent to administer an ACE inhibitor or spironolactone with torsemide due to the very high plasma aldosterone concentrations associated with its use (Figure 7).

FIGURE 7. Serum aldosterone concentrations in normal dogs receiving placebo, furosemide, or torsemide on days 1 (white bar) and 14 (green bar). Note the 6-fold increase with torsemide. This finding is potentially related to RAAS activation or blockade of MR. If the former, this effect would be a disadvantage of using torsemide as opposed to furosemide.
* = value significantly (P < 0.05) different from short-term administration value; † = value significantly (P = 0.01) different from placebo treatment value; ‡ = value significantly (P = 0.01) different from value of the corresponding furosemide administration

Electrolyte Effects

One study indicated that in experimental mitral insufficiency, there was less potassium loss with torsemide than with furosemide.¹² Another study that compared normal dogs receiving an ACE inhibitor alone or an ACE inhibitor with torsemide (0.2 mg/kg for 28 days) showed no significant decrease in serum magnesium (Mg++) and only a slight decrease in serum potassium (K+), despite an increase in urine fractional excretion of both electrolytes.²¹

These small studies could not prove whether torsemide is actually less “wasteful” of potassium and magnesium than furosemide.

Monitoring of serum electrolytes is still advisable (Table 3).

Indications & Dosage

Torsemide is available for both intravenous and oral use, with tablet sizes of 5, 10, 20, and 100 mg. The accepted oral torsemide dosage is one tenth that of the expected, or current, furosemide dosage, which works out to 0.2 mg/kg Q 12 H if a patient is started on torsemide initially (ie, 10% of the 2 mg/kg furosemide dosage).

To our knowledge, intravenous administration of torsemide has not been studied in veterinary medicine. We are reluctant to speculate from the human dosage because of the apparent differences between species with regard to administration of oral torsemide.

Currently, torsemide is used most often as a rescue agent when furosemide (usually with standard CHF drugs, including hydrochlorothiazide and spironolactone) no longer adequately controls signs of congestion (Table 4).

One author considers using torsemide when the furosemide response is inadequate at a furosemide dosage of 4 mg/kg Q 24 H or greater.¹² In our practices, the set point for change is somewhat higher, at a furosemide dosage greater than 6 to 8 mg/kg Q 24 H. Torsemide can be initiated in this circumstance as one dose per day, replacing one dose of furosemide (at one tenth the mg dosage) and leaving other treatments (including later daily doses of furosemide) intact.

Alternatively, with twice daily furosemide therapy, torsemide could be added as the third diuretic dose of the day. Lastly, it might be used to replace all diuretic therapy, given orally Q 12 H at one tenth the furosemide dosage, and titrated upward as needed.

Renal function should be carefully monitored during the addition of, or transition to, torsemide, and use of additional diuretics, such as hydrochlorothiazide, should be limited. Table 3 suggests monitoring practices for patients receiving potent loop diuretics.

In Summary

Unfortunately, the data available on diuretics in veterinary patients are still limited. More studies are needed to establish comparative aspects of the undesirable adverse effects and the efficacy of various loop diuretics.

Read Part 2 of this article series—a discussion on spironolactone—in an upcoming issue of Today’s Veterinary Practice.

ACE = angiotensin-converting enzyme; BUN = blood urea nitrogen; CHF = congestive heart failure; MR = mineralocorticoid receptor; RAAS = renin–angiotensin–aldosterone system

FIGURE CREDITS

Figure 1 reprinted with permission from Lantis AC, Atkins CE, DeFrancesco TC, Keene BW. The effect of furosemide and pimobendan on the circulating renin angiotensin aldosterone system (RAAS) in dogs. Am J Vet Res 2011; 72:1646-1651.

Figure 2 reprinted with permission from Uechi M, Matsuoka M, Kuwajima E, et al. The effects of the loop diuretics furosemide and torasemide on diuresis in dogs and cats. J Vet Med Sci 2003; 65(10):1057-1061.

Figures 3 and 7 reprinted with permission from Hori Y, Takusagawa F, Ikadai H, et al. Effects of oral administration of furosemide and torsemide in healthy dogs. Am J Vet Res 2007; 68(10):1058-1063.

Figures 4 to 6 adapted from Vargo DL, Kramer WG, Black PK, et al. Bioavailability, pharmacokinetics, and pharmacodynamics of torsemide and furosemide in patients with congestive heart failure. Clin Pharm Therap 1995; 57(60):601-609.

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This article reviews information from the session, Cardiology Without Intimidation, presented at the NAVC Institute 2014. The NAVC Institute 2016 takes place in Orlando, Florida, May 15 to 20; visit navc.com/institute for further information.