Name: Author Spotlight: Investigating HR-Dependent Cardiac Function in Mouse Models Through a Novel Atrial-Pacing Approach
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&#65279;This work was supported by research grants from the Fukuda Foundation for Medical Technology (to E.T. and G. N.) and the JSPS KAKENHI Scientific Research Grant-in-Aid 21K08047 (to E.T.).

This animal protocol was approved by the Institutional Animal Care and Use Committee and followed the regulations for animal experiments and related activities at the University of Tokyo. For the present study, 8-12 week old male C57/Bl6J mice were used. The animals were obtained from a commercial source (see the Table of Materials). A model of HFpEF was established by administering a high-fat diet for 15 weeks in conjunction with NG-nitro-L-arginine methyl ester, as described previously<sup class="xref"...

Exploring HR-Dependent Cardiac Performance Using an Atrial-Pacing Protocol

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We present a methodology to assess pressure-volume relationships with the application of transesophageal pacing. Exercise intolerance is one of the key characteristics of HFpEF, yet there are no techniques available for the evaluation of cardiac function in mice during exercise. Our pacing protocol offers a valuable tool for detecting diastolic dysfunction, which may not be apparent under resting conditions.
To achieve a PV loop of accurate and consistent quality, the following steps must be m...

Heart failure represents a leading cause of hospitalization and death across the globe, and heart failure with preserved ejection fraction (HFpEF) accounts for around 50% of all heart failure diagnoses. HFpEF is characterized by diastolic dysfunction and impaired exercise tolerance, and the associated hemodynamic abnormalities, such as diastolic dysfunction, can be clearly detected through exercise-stressed hemodynamic testing or MRI scans1,2.
In experimental models, however, available modalities for assessing the physiological abnormalities related to HFpEF are limited3,4. Treadmill exercise testing (TMT) is used to determine running time and distance, which might reflect exercise-stress cardiac hemodynamics; however, this method is susceptible to interference from extraneous variables such as the body weight, skeletal muscle strength, and mental status.
To circumvent these limitations, we have devised an atrial-pacing protocol that detects subtle but crucial changes in diastolic performance based on the heart rate (HR) and have validated its usefulness in a mouse model of HFpEF5. Several physiological factors contribute to exercise-related cardiac function, including the sympathetic nerve and catecholamine response, peripheral vasodilation, the endothelial response, and the heart rate6. Among these, however, the HR-pressure relationship (also called the Bowditch effect) is known as a critical determinant of cardiac physiological features7,8,9.
The protocol involves performing a conventional pressure-volume analysis at baseline to assess the systolic and diastolic function, including parameters such as the rate of pressure development (dp/dt), the end-systolic pressure-volume relationship (ESPVR), and the end-diastolic pressure-volume relationship (EDPVR). However, it should be noted that these parameters are influenced by the HR, which can vary between animals due to differences in their intrinsic heart rate. Additionally, the effects of anesthesia on the HR should also be considered. To address this, the HR was standardized by administering atrial pacing concomitantly with ivabradine, and cardiac parameter measurements were performed at incremental heart rates. Notably, the HR-dependent cardiac response distinguished HFpEF mice from the control group mice, while no significant differences were observed in the baseline PV loop measurements (using the intrinsic heart rate)5.
While this pacing protocol may seem relatively complicated, its success rate exceeds 90% when it is well understood. This protocol would provide a useful way to study the underlying mechanisms of diastolic dysfunction in HFpEF mouse models and help in the development of new treatments for this condition.

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	<tbody>
		<tr>
			<td>2-0 silk suture, sterlie</td>
			<td>Alfresa Pharma Corporation, Osaka, Japan</td>
			<td>62-9965-57</td>
			<td>Surgical Supplies</td>
		</tr>
		<tr>
			<td>2-Fr tetrapolar electrode catheter</td>
			<td>Fukuda Denshi, Japan and UNIQUE MEDICAL, Japan</td>
			<td>custom-made</td>
			<td>Surgical Supplies</td>
		</tr>
		<tr>
			<td>Albumin Bovine Serum</td>
			<td>Nacalai Tesque, Inc., Kyoto, Japan</td>
			<td>01859-47</td>
			<td>Miscellaneous</td>
		</tr>
		<tr>
			<td>C57/BI6J mouse</td>
			<td>Jackson Laboratory</td>
			<td></td>
			<td>animals</td>
		</tr>
		<tr>
			<td>Conductance catheter</td>
			<td>Millar Instruments, Houston, TX</td>
			<td>PVR 1035</td>
			<td></td>
		</tr>
		<tr>
			<td>Electrical cautery, Electrocautery Knife Kit</td>
			<td>ellman-Japan,Osaka, Japan</td>
			<td>1-1861-21</td>
			<td>Surgical Supplies</td>
		</tr>
		<tr>
			<td>Etomidate</td>
			<td>Tokyo Chemical Industory Co., Ltd., Tokyo Japan</td>
			<td>E0897</td>
			<td>Anesthetic</td>
		</tr>
		<tr>
			<td>Grass Instrument S44G Square Pulse Stimulator</td>
			<td>Astro-Med, West Warwick, RI</td>
			<td></td>
			<td>Pacing equipment</td>
		</tr>
		<tr>
			<td>Isoflurane</td>
			<td>Viatris Inc., Tokyo, Japan</td>
			<td>8803998</td>
			<td>Anesthetic</td>
		</tr>
		<tr>
			<td>Ivabradine</td>
			<td>Tokyo Chemical Industory Co., Ltd., Tokyo Japan</td>
			<td>I0847</td>
			<td>Miscellaneous</td>
		</tr>
		<tr>
			<td>LabChart software</td>
			<td>ADInstruments, Sydney, Australia</td>
			<td>LabChart 7</td>
			<td>Hemodynamic equipment</td>
		</tr>
		<tr>
			<td>MPVS Ultra</td>
			<td>Millar Instruments, Houston, TX</td>
			<td>PL3516B49</td>
			<td>Hemodynamic equipment</td>
		</tr>
		<tr>
			<td>Pancronium bromide</td>
			<td>Sigma Aldrich Co., St. Louis, MO</td>
			<td>15500-66-0</td>
			<td>Anesthetic</td>
		</tr>
		<tr>
			<td>PE10 polyethylene tube</td>
			<td>Bio Research Center&#160; Co. Ltd., Tokyo, Japan</td>
			<td>62101010</td>
			<td>Surgical Supplies</td>
		</tr>
		<tr>
			<td>PowerLab 8/35</td>
			<td>ADInstruments, Sydney, Australia</td>
			<td>PL3508/P</td>
			<td>Hemodynamic equipment</td>
		</tr>
		<tr>
			<td>PVR 1035</td>
			<td>Millar Instruments, Houston, TX</td>
			<td>842-0002</td>
			<td>Hemodynamic equipment</td>
		</tr>
		<tr>
			<td>Urethane (Ethyl Carbamate)</td>
			<td>Wako Pure Chemical Industries, Ltd., Osaka, Japan</td>
			<td>050-05821</td>
			<td>Anesthetic</td>
		</tr>
		<tr>
			<td>Vascular Flow Probe</td>
			<td>Transonic, Ithaca, NY</td>
			<td>MA1PRB</td>
			<td>Surgical Supplies</td>
		</tr>
	</tbody>
</table>

a pacing-controlled procedure for the assessment of heart rate-dependent diastolic functions in murine heart failure models

Heart failure with preserved ejection fraction (HFpEF) is a condition characterized by diastolic dysfunction and exercise intolerance. While exercise-stressed hemodynamic tests or MRI can be used to detect diastolic dysfunction and diagnose HFpEF in humans, such modalities are limited in basic research using mouse models. A treadmill exercise test is commonly used for this purpose in mice, but its results can be influenced by body weight, skeletal muscle strength, and mental state. Here, we describe an atrial-pacing protocol to detect heart rate (HR)-dependent changes in diastolic performance and validate its usefulness in a mouse model of HFpEF. The method involves anesthetizing, intubating, and performing pressure-volume (PV) loop analysis concomitant with atrial pacing. In this work, a conductance catheter was inserted via a left ventricular apical approach, and an atrial pacing catheter was placed in the esophagus. Baseline PV loops were collected before the HR was slowed with ivabradine. PV loops were collected and analyzed at HR increments ranging from 400 bpm to 700 bpm via atrial pacing. Using this protocol, we clearly demonstrated HR-dependent diastolic impairment in a metabolically induced HFpEF model. Both the relaxation time constant (Tau) and the end-diastolic pressure-volume relationship (EDPVR) worsened as the HR increased compared to control mice. In conclusion, this atrial pacing-controlled protocol is useful for detecting HR-dependent cardiac dysfunctions. It provides a new way to study the underlying mechanisms of diastolic dysfunction in HFpEF mouse models and may help develop new treatments for this condition.

Author Spotlight: Investigating HR-Dependent Cardiac Function in Mouse Models Through a Novel Atrial-Pacing Approach 

A Pacing-Controlled Procedure for the Assessment of Heart Rate-Dependent Diastolic Functions in Murine Heart Failure Models

The heart rate-related relationship is important for determining cardiac characteristics, particularly, in HFpEF. This protocol aims to assess heart rate-dependent cardiac function. Any experimental models, available modalities for assessing heart rate-related cardiac function, are limited to treadmill exercise testing.The treadmill test, however, is influenced by variables such as body weight. To overcome these limitations, we have developed around atrial-pacing protocol to detect subtle, yet significant changes in the later performance that are linked to heart rate. Our protocol offers a variable tool for identifying cardiac dysfunction, manifested by heart rate changes.It also presents a number approach to studying diastolic dysfunction in mirroring heart failure models, particularly in HFpEF.

The baseline PV loop data are displayed in Figure 1 and Table 1. At baseline (in the absence of pacing), there were no significant differences in diastolic parameters such as the relaxation time constant (Tau), the minimum rate of pressure change (dP/dt min), and EDPVR between the control and HFpEF mice. However, the HFpEF mice exhibited higher blood pressure and arterial elastance (Ea), as shown in Figure 1, and demonstrated a typical mountain-...

Watch this Scientific Journal Video about Investigating HR-Dependent Cardiac Function in Mouse Models Through a Novel Atrial-Pacing Approach at JoVE.com

The present protocol describes obtaining the pressure-volume relationship through transesophageal pacing, which serves as a valuable tool in evaluating diastolic function&#160;in mouse models of heart failure.

Heart failure with preserved ejection fraction (HFpEF) is a condition characterized by diastolic dysfunction and exercise intolerance. While ...

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Research

JoVE Journal

Biology

Instrument and Animal Preparation for Catheterization in a Murine Heart Failure Model

To begin, place a conductance catheter in normal saline, then connect it to an MPVS module, comprising the PowerLab 8/35 and a pressure volume unit. Electronically calibrate the pressure and volume by recording the predetermined pressure and volume parameters on the MPVS module. For the murine heart failure model, anesthetize the animal, and ensure that the surgical site is adequately shaved and disinfected.Expose the trachea and insert an endotracheal tube into it. Locate the internal jugular vein beneath the sternocleidomastoid muscle. Then insert a central venous catheter consisting of PE10 Silastic connected to a 30 gauge needle into the jugular vein.Administer a bolus infusion of five to six microliters per gram of body weight of 10%Albumin in sodium chloride for about three minutes. Follow this with a constant infusion rate of five to 10 microliters per minute.

Surgical Procedure for Left Ventricular Catheterization (Open Chest Approach) in a Murine Heart Failure Model

To begin, place the anesthetized animal in the surgical area, ensuring that the surgical site is adequately shaved and disinfected. Make a horizontal incision below the xiphoid process, measuring two to three centimeters in length. Then, use blunt scissors to separate the skin from the chest wall.Using electrical cautery, make lateral cuts on both sides through the chest wall. Cut through the diaphragm to expose the heart. Gently remove the pericardium from the heart using forceps.Next, insert a 27 gauge needle into the apex of the left ventricle, then retro greatly insert a conductance catheter into the left ventricle via the puncture hole, and adjust the catheter position until obtaining a square shaped pressure volume loop.

Determining the End-Systolic and End-Diastolic Pressure-Volume Relationships in a Murine Heart Failure Model and Transesophageal Pacing

To begin, place the anesthetized animal in the surgical area, ensuring that the surgical site is adequately shaved and disinfected. Next, properly place the conductance catheter through standard surgical procedures. To record and analyze the baseline pressure volume or PV loop, in the lab chart software, select the baseline parameters, then click the Analyze Now button.Select the baseline option. Open the hemodynamic table and select the average data. Perform inferior vena cava or IVC occlusion by compressing the IVC using forceps.In the software, manually select the point of occlusion of IVC where both left ventricular pressure and volume decrease. Then record the PV loop for at least 20 cardiac cycles while the IVC remains occluded by selecting the occlusion option and then clicking the Analyze Now button. To perform transesophageal pacing, insert a 2-French tetrapolar electrode catheter into the esophagus.Connect the catheter to a pulse stimulator, then proceed to determine the atrial capture threshold. To slow the heart rate below 400 beats per minute, administer 20 milligrams per kilogram of ivabradine intraperitoneally. Following stabilization, acquire 20 continuous cardiac cycles of PV loops while pacing the heart at different rates from 400 to 700 beats per minute.The baseline PV loop data showed no significant differences in diastolic parameters, such as the relaxation time constant, the minimum rate of pressure change, and the end diastolic pressure volume relationship, or EDPVR, between the control and HFpEF mice. However, the HFpEF mice exhibited higher blood pressure and arterial elastins, demonstrating a typical mountain-shaped PV loop during ventricular systole. This PV loop was distinguishable from a spike caused by direct contact of the ventricular muscle on the pressure transducer.Notably, using atrial pacing, the diastolic function could be clearly distinguished between the control and HFpEF mice. In the control group, both the relaxation time constant and EDPVR improved as the pacing rate increased. Whereas in the HFpEF group, both the relaxation time constant and EDPVR worsened as the heart rate increased with atrial pacing.

Saline Calibration and Aortic Flow Calibration in a Murine Heart Failure Model

To begin, place the anesthetized animal in the surgical area, ensuring that the surgical site is adequately shaved and disinfected. Catheterize the animal through standard surgical procedures. Next, hold the ventilator and administer 5 to 10 microliters of hypertonic saline solution intravenously through the central venous catheter.During the saline injection, record the changes in pressure and volume, and utilize power lab to calculate the VP value. To avoid disrupting the volume signal, position the mouse on its left side, then make a lateral thoracotomy towards the spine between the third and fifth thoracic vertebrae. Gently dissect a small part of the descending aorta using forceps, then place a vascular flow probe over the aorta to measure the cardiac output.