Background: The effects of furosemide on left atrial pressure (LAP) in dogs with mitral regurgitation (MR) have not been documented in a quantitative manner and between different routes of administration.
Objective: To document LAP and echocardiographic parameters in MR dogs administered furosemide IV or PO, in order to document changes in LAP after furosemide treatment.
Animals: Five healthy Beagle dogs (3 males and 2 females; aged 2 years) were used.
Methods: Experimental, cross-over, and interventional study. LAP was measured before the administration of furosemide, and 30 minutes, 1, 1.5, 2, 3, 4, 5, 6, 8, 12, and 24 hours after administration. Furosemide 1, 2, or 4 mg/kg IV, PO or placebo was administered.
Results: LAP was significantly decreased with all administrations of furosemide but not after placebo (P < .05, respectively). The max reduction was observed 1 hour (1 mg/kg IV, 15.04 ± 7.02 mmHg), 3 hours (2, 4 mg/kg IV, 13.28 ± 8.01, 9.23 ± 4.92 mmHg), 4 hours (1 mg/kg PO, 14.68 ± 11.51 mmHg), and 5 hours (2, 4 mg/kg PO, 13.19 ± 10.52, 10.70 ± 7.69 mmHg). E wave and E/Ea were significantly decreased corresponding to the reduction of LAP after administration of 2 and 4 mg/kg (P < .05, respectively).
Conclusions and Clinical Importance: LAP was decreased in proportion to the dosage of furosemide, which did not significantly differ between IV and PO of the same dosages. E wave and E/Ea might be useful for the treatment evaluation of furosemide.
Mitral valve disease is the most common cardiac disease in dogs. As many as three quarters of all dogs with signs of congestive heart failure suffer from mitral regurgitation (MR) caused by myxomatous degeneration of the valve leaflets or the chordae tendineae.1,2 MR increases the left atrial pressure (LAP) which potentially results in left atrial dilation. Elevated LAP causes pulmonary edema that can lead to cough, dyspnea, and even death.3 Therefore, LAP needs to be reduced to avoid these clinical signs.
Furosemide, a loop diuretic, has a strong diuretic effect and is commonly used for the treatment of pulmonary edema in patients with MR.4 Furosemide reduces the total circulating blood volume, which in turn reduces the LAP or the left ventricular filling pressure, and leads to the clinical improvement of the patients.5–9 There are several administration routes for furosemide, including IV, PO, SC, and IM. Pharmacokinetics of furosemide has been compared between IV and PO administration.9 However, the effects of furosemide on LAP in dogs with MR have not been well documented in a quantitative manner because there are some difficulties in measuring LAP directly. We have previously reported the 24-hour LAP profiles of and the effect of angiotensin-converting enzyme (ACE) inhibitors on experimentally induced MR dogs with a radio telemetry system.10,11 The previous study has demonstrated that this system is useful for the evaluation of hemodynamic changes that occur with the administration of furosemide.
In the present study, we monitored the LAP of dogs with experimentary induced MR with a radio telemetry system and evaluated the effect of furosemide in different administration routes. Echocardiographic evaluations and blood pressure measurements were also conducted, and the effect of furosemide on MR was evaluated by analyzing of pathophysiological changes.
Material and Methods
Five 2-year-old Beagle dogs, which consisted of 3 males and 2 females, weighing 13.2 ± 1.5 kg (range: 11.7–14.7 kg) were used. Before starting the experiment, the dogs were evaluated by general clinical examination, blood and serum biochemical examinations, electrocardiography, thoracic radiography, and echocardiography examinations. All dogs were acclimatized to the experimental environment and human handlings. During all phases of the present study, the dogs were managed and cared for in accordance with the standards established by the Tokyo University of Agriculture and Technology (TUAT), which is described in its “Guide for the care and use of laboratory animals.” This study was approved by the animal experimental committee of TUAT (acceptance no. 21–19).
Preparing for MR Model Dogs and Transmitter Implantation
All dogs underwent surgical procedures to induce MR, and a telemetry transmitter cathetera was implanted in the left atrium. The surgical procedure and postoperative care were conducted according to the procedure of our previous report.10 Thoracic radiograph and echocardiograph examinations were performed for the presence of pulmonary edema and cardiac dilation. After the radio telemetry transmitter implantation, the dogs were rested for at least 5 weeks, until no major variations were identified on echocardiographic evaluation and LAP.
LAP Measurement Method and Protocol for the LAP Measurement
Details of all radio telemetry systemb and recording procedures are as described in our previous report.11 The maximum, mean, and minimum LAP were obtained as averages of 10-second recorded segments from continuous waveform recordings. LAP was measured at each measurement points; before the administration of furosemide,c and at 30 minutes, 1, 1.5, 2, 3, 4, 5, 6, 8, 12, and 24 hours postadministration. Furosemide of dosages at 1, 2, or 4 mg/kg was administered to each dog by either IV or PO routes or placebo. The selection of dosages and routes were followed in a cross over study. The interval between each administration was at least 1 week. To evaluate the effect of drinking, drinking water was restricted for 12 hours postadministration and it was freely accessible after 12 hours. The administrations of drugs were initiated after several hours of fasting.
Urinary Volume and Specific Gravity
A balloon catheterd was placed in the urinary bladder of each dog. Urine samples were obtained from the catheter and urinary specific gravity was measured by a urinometere at the same time as the LAP measurements.
Echocardiography measurements were performed before and after the administration of furosemide, along with the blood pressure and LAP measurements. A single investigator performed the transthoracic conventional echocardiography as well as the 2-dimensional spectral Doppler and the tissue Doppler echocardiography. Each dog was positioned in left and right recumbencies and echocardiographic examinations were performed with a disital ultrasonographic systemf with a 5.0 MHz sector transducer. Sweep speed during the Doppler and M-mode recordings were set at 150–200 mm/s. Optimized right parasternal projections were used to measure heart dimentions. LA/Ao was assessed in a short-axis M-mode at heart base level for the evaluation of the scale of LA enlargement. Based on the scale of LV enlargement by the use of a short axis M-mode projection, left ventricular end diastolic diameter (LVEDD) was measured at mitral valve chordal level. Optimized left apical parasternal projections of the left ventricular inflow and outflow tracts were used to assess the mitral inflow and the MR flow by a 2-chamber view, entailing the use of 6 mm of sample volume. Forward stroke volume (SV) and cardiac output (CO) were calculated with a left ventricular outflow projection. The same mitral inflow tract view on the 2-chamber view was used to evaluate the lateral mitral annulus velocity (Ea)12 with pulsed tissue Doppler imaging, entailing the use of the same sample volume. Using the Doppler signals of the mitral inflow, peak transmitral early diastolic wave (E wave) velocity was measured, and E/Ea was calculated. MR flow was recorded with a high-intensity continuous wave spectral Doppler signal, and the MR pressure gradient (MRPG) was calculated based on a modified Bernoulli's equation. These echocardiographic profiles were obtained from 10 consective beats, and the averages were used. All the echocardiographic recordings were stored on the internal hard drive of the echocardiography and trasmitted to the DICOM server online.g
Blood Pressure Measurements
All indirect arterial BP recording were obtained by the oscillometric method.h Cuff size appropriate for tail circumference was selected for each dog and the BP measured. BP measurements were performed simultaneously with the MR velocity measurements on the echocardiography, and 5 consecutive measurements were averaged for each dog. Estimated LAP was calculated as systolic blood pressure (SBP)—MRPG. Systemic vascular resistance (SVR) was calculated as SVR = 79.9* (Mean BP-Right atrial pressure)/CO, and right atrial pressure was tentatively defined as 5 mmHg because no signs of right-sided heart failure were observed.
All data are represented as mean plus or minus the standard deviation (SD). A 1-way analysis of variance (ANOVA) in conjunction with a Bonferroni's multiple comparison test was used for comparing the total urinary volume. A 2-way repeated measures ANOVA in conjunction with a Bonferroni's multiple comparison test was used to compare the LAP and the echocardiographic variables before and after each furosemide administrations. Statistical significance was defined as P < .05. GraphPad Prism version 5.0ai and EXCEL 2008j were used to perform these statistical analyses.
The operation performed to rupture the mitral valvular chordae tendineae and to implant the transmitter was successful in all dogs. The severity of the regurgitation jet varied among dogs, and these dogs were numbered and arranged in the order of MR severity on the basis of their echocardiographic profiles. LVEDD were 4.2 ± 0.5 cm (range: 3.7–4.9 cm, normal range is 3.1–3.4 cm), LA/Ao values by use of short-axis projection were 1.79 ± 0.53 (range: 1.48–2.77, normal range is < 1.3), E wave velocities were 1.19 ± 0.21 m/s (range: 1.05–1.56 m/s, normal range is 0.86 ± 0.10 m/s). Three dogs did not have any clinical signs associated with MR; 2 dogs had decreased their activity and appetite, although these parameters were subjectively observed rather than objectively.
Maximum LAP was significantly decreased half an hour after administration of furosemide of 1, 2, and 4 mg/kg IV (Fig 1), and maximum reduction was seen 1 hour (1 mg/kg) and 3 hours (2 and 4 mg/kg) after administration (18.38 ± 11.18 –15.04 ± 7.02 mmHg, P<.05; 18.16 ± 10.02–13.28 ± 8.01 mmHg, P<.05; and 17.96 ± 13.74–9.23 ± 4.92 mmHg, P<.05, respectively). The significant reduction of maximum LAP lasted for 12 hours (15.30 ± 7.50, 14.33 ± 9.18, and 11.03 ± 8.50 mmHg, respectively). Maximum values of LAP reduction after the administration were 3.34 ± 4.76, 4.88 ± 2.46, and 8.74 ± 10.3 mmHg, respectively. There were significant reductions in maximum LAP with 1, 2, and 4 mg/kg IV compared with placebo. In contrast, maximum LAP was significantly decreased 1.5 hours after administration with furosemide 1, 2, and 4 mg/kg PO (Fig 1) and maximum reduction was seen up to 4 hours (1 mg/kg) and 5 hours (2 and 4 mg/kg) after administration (17.61 ± 13.90–14.68 ± 11.51 mmHg, P < .05; 17.46 ± 14.13–13.19 ± 10.52 mmHg, P < .05; and 15.93 ± 10.74–10.70 ± 7.69 mmHg, P < .05, respectively). Significant reduction of maximum LAP lasted for 12 hours (15.10 ± 8.93, 13.24 ± 11.04, and 9.34 ± 6.74 mmHg, respectively). Maximum values of LAP reduction after administration were 2.96 ± 3.35, 4.56 ± 4.52, and 6.59 ± 4.98 mmHg. There was a significant decrease in LAP with 1, 2, and 4 mg/kg PO compared with placebo. LAP values returned close to the each base line 24 hours after the administration. The difference of LAP value was not significant with 1 mg/kg IV and PO compared with placebo. However, The difference was significant with 2 and 4 mg/kg IV and PO compared with placebo.
Maximum reduction of maximum LAP was proportional to the dosage of furosemide in both administration routes. Interestingly, there was no significant maximum reduction of LAP for IV administration compared with PO with the same dosages. Results were similar to the mean LAP and the minimum LAP. There were significant differences of LAP with 4 mg/kg IV at half an hour and 1 and 2 hours compared with 4 mg/kg PO (Fig 2). The result was similar to 2 mg/kg. There was no significant difference of LAP with 1 mg/kg IV compared with PO.
Urinary Volume and Specific Gravity
Furosemide 1, 2, and 4 mg/kg IV increased the urinary volume soon after administration. A significant diuretic effect peaked half an hour after administration and disappeared within 2 hours (Fig 3). In contrast, 1, 2, and 4 mg/kg PO increased the urinary volume half an hour (4 mg/kg) to 1 hour (1 and 2 mg/kg) after the administration. The significant diuretic effect peaked at 1.5 hours (4 mg/kg) or 2 hours (1 and 2 mg/kg) after administration and disappeared within 5 hours (Fig 3). An increase in urinary volume was dose-proportional in each administration route. Although a greater increase in the total urinary volume was seen in PO when compared with IV with the same dosage, there was no significant increase in urinary volume for PO compared with IV (Fig 4). Urinary specific gravity was decreased as urinary volume was increased. After the loss of the diuretic effect of furosemide, the gravity was increased beyond the baseline value.
Echocardiography and Circulatory Parameters
There was no significant reduction in LVEDD compared with each baseline (Fig 5). Peak E with 2 and 4 mg/kg IV was significantly decreased compared with the placebo. Significant reductions were observed with 2 and 4 mg/kg IV compared with each baseline, 4 and 5 hours postadministration (Fig 6). Peak E with 2 and 4 mg/kg PO was significantly decreased compared with the placebo. Significant reductions were observed with 2 and 4 mg/kg PO compared with each baseline, 4 and 2 hours postadministration. Peak E/Ea with 2 and 4 mg/kg IV was significantly decreased compared with the placebo. Significant reduction was observed with 2 and 4 mg/kg IV compared with each baseline, 12 and 5 hours after administration (Fig 7). Peak E/Ea with 2 and 4 mg/kg PO was significantly decreased compared with the placebo. Significant decrease was observed with 2 and 4 mg/kg PO compared with each baseline, 12 and 2 hours after administration. LA/Ao, SYS-MRPG did not change significantly with decreasing LAP (supporting information Table S1).
SV was significantly decreased 12 hours after the administration of furosemide (Table 1). In addition, SVR was significantly increased; however, little change was seen with MBP after the administration of furosemide.
Table 1. Comparison of SV, SBP, and SVR at baseline and 12 hours after the furosemide administration.
This study has 5 important findings. Firstly, LAP was greatly reduced with furosemide in dogs with MR. Secondly, LAP was decreased in proportion to the dosage of furosemide, and the administration route did not significantly affect the peak pharmacological action when given at the same dosage. Thirdly, the time of which the LAP begins to decline with IV was an hour earlier compared with PO. Fourthly, E wave and E/Ea were significantly decreased corresponding to the reduction of LAP. Finally, SV was decreased and SVR was increased after the administration of furosemide.
Most studies so far on the evaluation of LAP in dog are acute experiments conducted in few hours, and performed under general anesthesia.12–14 Few studies have evaluated the LAP after the administration of furosemide with comparison between IV and PO routes. In the present study, our findings indicate that LAP is significantly lowered by furosemide in experimental cases, and the 2 administration routes have almost the same pharmacological actions in LAP-lowering effect. From these results, we can select the administration routes depending on the urgency of the disease. In our previous report, an ACE inhibitor lowered LAP, although the difference was not significant compared with preadministration.11 The present study and the previous study confirmed the potency of furosemide in reducing the preload and LAP compared with ACE inhibitor. The effects of inodilators (eg, pimobendan), vasodilators (eg, nitroglycerin), and other cardiovascular drugs on the LAP are unclear; however, they warrant further examination.
Estimation of reduction of LAP by echocardiography is most useful information because LAP cannot be measured directly in clinical situation. LVEDD and LA/Ao indicate structural changes, and are used in the diagnosis of chronic heart failure.15 In the present study, however, these parameters did not change significantly with decreasing LAP when compared with the baseline (supporting information Table S1). The present study indicates that the reduction of LVEDD and LA/Ao requires continuous reduction in LAP at least for over 24 hours. For this reason, LVEDD and LA/Ao are not applicable parameters for the evaluation of short-term treatment evaluation of furosemide. Although SBP-MRPG is most widely used in the estimation of LAP in human medicine,16,17 SYS-MRPG did not change significantly in response to reduction in LAP in the present study. In veterinary medicine, the estimation of LAP using the noninvasive blood pressure in MR is not reliable because the estimation is heavily affected by slight error. For example, if an error associated with MR velocity measurement is 0.25 m/s, SYS-MRPG will differ approximately 10 mmHg from the true value. Therefore, the present study suggests that SBP-MRPG is not suitable for the evaluation of short-term furosemide treatment. Transmitral blood flow is most widely used to evaluate left ventricular diastolic function in patients with cardiac disease in human medicine.17–20 However, transmitral blood flow cannot be used to evaluate diastolic function in MR patients in veterinary medicine, with exception of the mild cases. This is because in severe MR cases the E wave increases in relation to the increase in LAP, irrespectively to the diastolic dysfunction. A few reports have revealed that parameters which use E wave are well correlated with the degree of MR severity in both human and veterinary medicine.21–25 E/Ea is an index that evaluates only the preload because it divides the Ea to exclude the effect of diastolic function. Many reports have shown that E/Ea is a good index for the estimation of LAP and left ventricular filling pressure.12–14,26–28 In the present study, E wave and E/Ea were significantly decreased after the administration of furosemide (Figs 6 and 7). This result suggests that E wave and E/Ea can be used for the evaluation of furosemide treatment at a high dosage, and monitoring of the reduction of LAP in the short term.
Adverse effects of furosemide include reduction of SV and CO.8 In the present study, SV was significantly decreased after the administration of furosemide (Table 1). This result shows that echocardiography can be used for the evaluation of the SV after the administration of furosemide. Also in the present study, SVR was significantly increased, whereas little change was seen with SBP after the administration of furosemide (Table 1). This result suggests that increased SVR reflects vasoconstriction in response to decreased blood volume. Therefore, concurrent additional use of a vasodilator, including an ACE inhibitor, should be considered when furosemide is used in MR dogs. Further studies are needed to clarify the exact combination effects of hemodynamic change after the administration of furosemide, ACE inhibitors, and other cardiovascular drugs.
In the present study, five 2-year-old Beagle dogs were used and a 5-week period was defined as subchronic period for experimentally induced MR. In clinical situations, most dogs with MR and their cardiac function and myocardial tissue damage might differ from model dogs in this study. Therefore, our model might be closer to an acute MR and differ from clinically occurring MR dogs. Because the severity of MR differed substantially among the model dogs used in this study, each measurement has extremely large SD. Therefore, if the severity of MR had equaled, the results of statistical analysis might differ. In the present study, model dogs were fasted and restricted drinking water 12 hours postadministration. However, diet may have possibly reduced drug absorption. Also the amount of drinking water affects total circulating blood volume. Therefore, results of this study cannot be extrapolated to the clinical cases without modification. In the present study, we did not demonstrate additional characteristics of MR, such as differing renal function and hormonal environments from normal dogs. Therefore, the diuretic effects might be closer to normal dogs and differ from clinically occurring MR dogs.
LAP was significantly reduced with furosemide in dogs with surgically induced MR. In addition, LAP was decreased in proportion to the dosage of furosemide; however, it did not significantly differ between IV and PO at the same dosage. Time of onset of the reduction of LAP with IV was an hour earlier than with PO. E wave and E/Ea were significantly decreased after the administration of furosemide at the high dosage and simultaneously with the reduction of LAP. E wave and E/Ea may be useful for treatment evaluation of furosemide at high dosage in short-time treatment.
a TA11PA-D70, Data Sciences International, St Paul, MN
b DSI Dataquest A.R.T. 4.1, Data Sciences International
c Lasix, Sanofi-Aventis K.K., Tokyo, Japan
d EL-Balloon Catheter, TERUMO CO, Tokyo, Japan
e URC-JE, ATAGO Co Ltd, Tokyo, Japan
f SSD-5,000, Aloka Co Ltd, Tokyo, Japan
g DICOM Server, ImageONE Co Ltd, Tokyo, Japan
h BP-100D, Fukuda ME, Tokyo, Japan
i GraphPad Prism version 5.0a, GraphPad, San Diego, CA
j EXCEL 2008 for Macintosh, Microsoft, Redmond, WA
We express our sincere gratitude to Dainippon Sumitomo Pharma Co Ltd (Osaka Japan), for the financial support, which enabled us to carry out the study on LAP in dogs with MR.
Disclaimer: Supporting information is published as submitted and not corrected or checked for scientific content, typographical errors or functionality. The responsibility for scientific accuracy remains entirely with the authors.