Info

dalmatians, mastiffs); cat

Tricuspid valve dysplasia

Dog (Old English sheepdogs, German shepherds, weimaraners, Labrador retrievers); cat

Note: Other than the dog, limited literature is available for all other species.

Note: Other than the dog, limited literature is available for all other species.

Several obvious technical limitations for the use of small animals exist, for example, in the case of implanting mechanical devices such as heart valves. In such situations, the animal species must be chosen to the fit the device. Nevertheless, great strides have been made in both imaging (ultrasound and magnetic resonance imaging, or MRI) and miniaturized electronic equipment (e.g., Transoma Medical, Arden Hills, MN) that is used for the monitoring of physiological parameters, allowing for more use of smaller animal models.

In choosing the animal species, the researcher should attempt to match the physiological parameter to be investigated in the experimental animal to the corresponding human value to obtain relevant results. Note that tables of known physiological values have been published for many commonly used research animal species, and these should be used to assist in choosing the appropriate model (2).

3. SPONTANEOUSLY OCCURRING ANIMAL MODELS OF CONGENITAL CARDIAC DISEASE

Naturally occurring animal models of cardiac disease arise infrequently and are usually associated with other congenital abnormalities that hinder breeding efforts. Furthermore, genetic manipulation of breeding stock for specific mutations has economical, ethical, and moral issues that preclude the development of such breeding lines. As a result, the commercial availability of animals for such specific research purposes is quite limited, necessitating the development of iatrogenic models in most cases. We have included a brief tabulation of some known cardiac defects, though the use of such models may be impractical because of the lack of availability and uniformity of lesions (Table 1) (3).

4. ALTERNATIVES TO WHOLE ANIMAL MODELS

Isolated cardiac cell lines in culture, segments of myocardium, or isolated hearts may provide an effective alternative to whole animal models in some cases. Isolated preparations have been particularly useful for the study of metabolic pathways because the perfusate can be modified, and the effluent can be easily collected for analysis. In addition, functional measurements can be easily completed in this in vitro environment.

The allure of alternatives to whole animal models is strong, driven by the need both to reduce cost and to limit the number of animals used in research. However, caution must be taken in extrapolating the experimental findings from isolated in vitro models to actual clinical use. For example, studies involving pharmaceutical agents often demonstrate usefulness in vitro, but the concentrations used may be either toxic or not well tolerated by the intact animal. Nevertheless, many researchers have used these alternatives for studies pertaining to myocardial ischemia, transplantation, and pharmaceutical development.

4.1. Isolated Cardiomyocytes

The use of isolated cardiomyocytes has allowed researchers to eliminate confounding interactions with surrounding tissue elements; it has also allowed for measurement of intracellular changes on a single-cell level. Yet, care must be taken to match the culture conditions to those of the intact organ to ensure both the quality and the viability of the cells used (4,5). This simple model does provide an important alternative for the use of whole animals in early phase testing of experimental protocols and is of particular interest for use in the testing of new pharmacological agents or gene therapies.

Cardiac myocyte cultures can be obtained from: (1) freshly isolated tissue; (2) the differentiation of embryonic stem cells or multipotent adult progenitor cells; or (3) available immortalized tumor cell lines such as HL-1 from the At-1 mouse (6). Common functions of cardiac myocytes that can be examined include: (1) contractile (using optical or mechanical detectors); (2) protein/ribonucleic acid (RNA) activity with quantitative polymerase chain reaction (Q-PCR); and (3) membrane integrity (lactate dehydrogenase, creatine phosphokinase, or tropo-nin release) (5,7-11).

4.2. Isolated Perfused Hearts

An isolated heart perfusion setup replicates the physiological conditions outside the body, allowing for easy access for measurement of the perfused effluent. Yet, isolated in vitro perfusion studies have been performed using the entire heart or a portion of the heart (e.g., intraventricular septum, papillary muscle). Commercially available setups for such in vitro studies are available for the mouse, rat, and guinea pig hearts (ADInstruments, Colorado Springs, CO) (Fig. 1). Larger setups have also been described to accommodate canine, porcine, ovine, or human hearts. An excellent example of an application of the isolated heart model is provided on the accompanying Visible Heart® CD (also see www.visibleheart.org).

Two different methods of studying the isolated heart are available: Langendorff perfusion or the isolated working heart model (12,13). In the Langendorff method, constant pressure flow through an aortic cannula forces the aortic valve closed, and the perfusate passes through the coronary arteries without flowing through the left ventricle. This perfusion provides the myocardium with energy, allowing the heart to beat without filling the four chambers. This method was named to honor Oscar Langendorff, who in 1895 was the first person to describe an experimental model of an isolated mammalian heart as a technique to assess its contractile activity.

The advantage of the Langendorff perfusion method is that measurement of electrocardiogram changes can be easily assessed, as well as measurement of metabolites that drain from the coronary sinus. Yet, the lack of flow in the ventricle may limit the usefulness of this preparation, for instance, minimal blood entering into the ventricle may promote clot formation, in turn affecting the viability of the preparation. In addition, the lack of flow in the ventricle may result in abnormal 3D confor-mational changes in the heart, which may cause coronary vascular compression. However, placement of a fluid balloon connected to a pressure transducer may allow control of this phenomenon and may be useful experimentally to assess changes in left ventricular function.

The major disadvantage of the Langendorff preparation is that it does not eject the perfusate from the left ventricle and is therefore a nonwork-producing model. This problem was overcome by Neely, who used an isolated working heart that simulates physiological flow through the heart's four chambers (14). In this model, the perfusate is supplied by a cannula inserted into the left atrium; outflow through the left ventricle is monitored, and left atrial pressure or aortic pressure is controlled. This setup is considered ideal for the study of pressure and flow in the aorta as well as the left and right ventricles.

4.3. Problems With Isolated Perfused Heart Models

Interestingly, both types of isolated heart preparations have problems in common that must be considered when attempting to extrapolate results from the in vivo condition. First, the isolation process used for these models requires global myocardial ischemia (a period of no perfusion). Typically, once the organ is reperfused, baseline data (heart rate, left ventricular pressure, coronary blood flow) must be collected after a stabilization period to ensure viability of the preparation. Clearly, both the ischemic time and stabilization time may influence research outcomes. Therefore, any results obtained must be carefully analyzed with reference to the preparation's baseline state rather than normal in vivo values to avoid falsely attributing changes in cardiac function to the experimental protocol.

The composition of the perfusate can have a great impact on the function and viability of the preparation in both of the aforementioned models. Early studies utilizing isolated heart models have employed the obvious choice of whole blood as a perfusate (15). However, significant problems with clotting and hemolysis may limit the time that the preparation remains viable. Saline compounds, which lack the potential for clotting and hemoly-sis, are considered useful alternatives to whole blood. However, such buffers have a lower colloid osmotic pressure and, coupled with the lower coronary vascular resistance, will typically result in severe edema; this results in interstitial edema formation and nonuniform perfusion. To extend the usefulness

Fig. 2. Comparison of different valves with their flow characteristics. The evolution of the valve from the Starr-Edwards ball, the current standard bileaflet (printed with permission from St. Jude Medical) and a novel trileaflet design (printed with permission from Triflow Medical Inc.) currently in development. Pictured below each valve is a stylized representation of the flow patterns across each valve reflecting the improvement in nonobstructive valve design.

Fig. 2. Comparison of different valves with their flow characteristics. The evolution of the valve from the Starr-Edwards ball, the current standard bileaflet (printed with permission from St. Jude Medical) and a novel trileaflet design (printed with permission from Triflow Medical Inc.) currently in development. Pictured below each valve is a stylized representation of the flow patterns across each valve reflecting the improvement in nonobstructive valve design.

Fig. 3. The Medtronic Mosaic® stented tissue valve. Printed with permission from Medtronic, Inc.
Fig. 4. A typical bileaflet heart valve.

Table 2

Qualities of Ideal Heart Valve Replacement

• Biologically inert

• Nonthrombogenic

• Facilitates laminar flow

• Easily implanted by the surgeon

• Quiet of the preparation, osmotically active substances can be added to the medium used for bathing and perfusing the preparation in an attempt to limit edema (16,17). Despite the technical difficulties associated with these models, isolated hearts have been used in research ranging from ischemia to transplant studies.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment