Pulmonary Hypertension

Chapter 167


Pulmonary Hypertension




Definition and Classification


Pulmonary hypertension (PH) refers to elevations in pulmonary artery (PA) pressure. The PA pressure depends on flow across the pulmonary circulation (cardiac output) as well as resistance to flow with pressure. Resistance to PA flow can occur at the level of the pulmonary arteries and arterioles, within the alveolar capillary units, and at the levels of the pulmonary veins and left atrium. Thus disorders of the left side of the heart, diffuse diseases of the lung, and pulmonary arterial diseases can lead to PH.


The diagnosis of PH is confirmed by cardiac catheterization in human patients, but in veterinary medicine it is usually defined as an elevated systolic PA pressure as estimated noninvasively by Doppler echocardiography (Figure 167-1). Veterinary publications have used estimated systolic PA pressures of more than 31 mm Hg (up to 45 mm Hg) as indicators of PH. Despite publication of a number of clinical studies and reports, no consensus statements or prospective studies are available to definitively guide diagnosis or management of PH in dogs or cats. It should be noted that healthy dogs, as well as humans and other animal species, can reach much higher values of PA pressure while exercising, and this point certainly is relevant in animals with tachycardia that are undergoing Doppler echocardiographic examination.



PH is a well-recognized condition of dogs and has been described sporadically in cats. A variety of conditions and a spectrum of histopathologic lesions within the pulmonary vasculature lead to PH. Historically PH was classified as primary (idiopathic) or secondary depending on the presence or absence of identifiable causes; however, more recently PH has been classified in relation to pathophysiologic mechanisms, histologic lesions, and therapeutic options. In dogs primary PH is a relatively rare condition. Although specific diseases commonly associated with PH in animals differ from those in humans, similar classifications can be devised (Table 167-1). Analysis of data from different case series encompassing a total of 197 dogs with PH of varying causes showed that the most common identified reason for PH was left-sided heart failure (category 2, 50% of cases). Primary lung disease (category 3) represented the second most common reason for PH (24% of cases). Diseases affecting the pulmonary arteries (pulmonary arterial hypertension, or category 1) were responsible for 10% of cases. Chronic thrombotic or embolic disease (category 4) was the cause of 9% of cases, whereas category 5—a compilation of miscellaneous conditions—was last, accounting for 7% of cases.



The most commonly identified reasons for PH in each category were as follows: heartworm disease and congenital systemic-to-pulmonary shunts for category 1; myxomatous mitral valve disease (MMVD) and dilated cardiomyopathy for category 2; pulmonary interstitial fibrosis, chronic bronchial disease, and tracheal collapse for category 3; heartworm disease and pulmonary thromboembolism for category 4; and neoplasia for category 5 (Bach et al, 2006; Guglielmini et al, 2010; Johnson et al, 1999; Kellum and Stepien, 2007; Pyle et al, 2004).



Diagnosis and Clinical Presentation


Right-sided heart catheterization is considered the gold standard for diagnosis of PH. This technique provides accurate measurements of systolic, diastolic, and mean PA pressures and also can provide information about cardiac output and right ventricular function and filling pressures. However, in veterinary medicine right-sided heart catheterization rarely is performed in routine practice, and PH generally is diagnosed based on the presence of clinical and echocardiographic findings.



Clinical Signs


The presentation of dogs with PH is varied, and in general the severity of clinical signs relates to the severity of PH. Owners most commonly report cough, exercise intolerance, tachypnea or respiratory distress, and syncope. In dogs PH is one of the more common causes of sudden collapse or syncope and should always be considered in such cases, especially when these signs are associated with exertion. Physical examination findings may include tachypnea, abnormal respiratory sounds, ascites from right-sided heart failure, and cyanosis. A prominent jugular pulsation may be evident during careful examination.


Dogs with PH frequently have a systolic murmur; however, in most cases this is caused by mitral or tricuspid regurgitation due to MMVD. One possible exception to this generalization is cases in which a systolic murmur is detected with greatest intensity over the tricuspid valve area; some of these dogs will be found to have PH related to cor pulmonale. Dogs with PH might show other auscultatory findings, including increased intensity or abnormal splitting of the second heart sound, a soft ejection murmur into the PA, and rarely a soft diastolic murmur caused by pulmonic insufficiency.



Diagnostic Tests


Commonly performed diagnostic tests include thoracic imaging, echocardiography (see later), and diagnostic tests for pulmonary parenchymal diseases or vascular diseases. The latter may include arterial blood gas analysis, cytologic analysis and culture of respiratory secretions, fine-needle aspiration or biopsy of the lung, and routine hematologic testing. Tests for heartworm disease are usually indicated in cases of suspected PH. Diagnostic tests for coagulopathy, such a platelet count, D-dimer level, and thromboelastography (see Chapter 15) may be useful to exclude or support a diagnosis of pulmonary thromboembolism. Echocardiography is usually the diagnostic test of choice for PH and is discussed later.



Thoracic Imaging


The lungs, pulmonary vasculature, mediastinum, and pleural space can be evaluated by combinations of radiography, noncardiac ultrasonography, and computerized tomography. Thoracic radiographs are of great importance for assessing the type and severity of underlying thoracic disease but lack sensitivity for the diagnosis of PH. Enlarged and tortuous pulmonary arteries can be observed in more severe cases, particularly in dogs with heartworm disease, but these changes often are subtle and difficult to interpret. Right ventricular enlargement and dilatation of the main pulmonary artery is common in severe PH. Thoracic computerized tomography is probably more sensitive for diagnosis of some pulmonary parenchymal diseases and also can be useful for recognition of pulmonary vascular changes and pulmonary thromboembolism.



Electrocardiography


The electrocardiograph (ECG) is important for diagnosing underlying arrhythmias, but findings are not specific. The most commonly observed arrhythmias are pronounced sinus arrhythmia, isolated premature atrial or ventricular contractions, atrial fibrillation, and various degrees of atrioventricular block. Aside from pronounced respiratory sinus arrhythmia, most of these arrhythmias are identified in dogs with concomitant left-sided heart failure or are a consequence of the disease responsible for PH. ECG findings of P pulmonale or right ventricular hypertrophy occasionally can be seen in dogs with moderate to severe PH, but these are found inconsistently. Echocardiography is more sensitive for detection of mild to moderate right-sided heart enlargement and is discussed in the next section.



Doppler Echocardiography


The noninvasive gold standard for the diagnosis of PH in veterinary medicine is Doppler echocardiography (Figures 167-2 and 167-3; see Figure 167-1). This method also can establish the diagnosis of PH due to primary cardiac disease. It should be stressed that the diagnosis of PH in veterinary medicine is based on a combination of clinical and echocardiographic signs, and a single observation or variable cannot be relied upon.


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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Pulmonary Hypertension

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