Introduction

Pulmonary hypertension (PH) is defined as a pulmonary arterial systolic pressure >30 mm Hg. Right-heart catheterization is considered the “gold standard” for diagnosis of PH. PH is a complex syndrome that has historically resulted in a poor prognosis. Pulmonary arterial pressure (PAP) is influenced by pulmonary blood flow, pulmonary vascular resistance (PVR), and pulmonary venous pressure. The elevated PAP of the syndrome of PH may be caused by pulmonary vascular abnormalities associated with increased blood flow (i.e., “hyperkinetic” PH secondary to a patent ductus arteriosus), changes affecting resistance to flow (precapillary pulmonary arterial hypertension, PAH) or caused by increased “downstream” resistance (postcapillary pulmonary venous hypertension, PVH). Diagnosis of PH requires diagnostic testing that quantifies the degree of elevation of PAP, determines the underlying disease process if possible, and identifies the degree of hemodynamic impairment. Significant advances in therapy that target the derangements of the PH pathophysiology have been made in animals and people, providing an improved prognosis for survival and better quality of life in dogs. This presentation will review the most common diagnoses associated with PH and how PH may be managed.

Classification of PH

Pulmonary hypertension can be classified as pre- or postcapillary PH, or can be classified based on the disease process causing PH. The categories include pulmonary arterial hypertension, pulmonary venous hypertension, hypoxic PH, PH secondary to respiratory disease, PH secondary to thromboembolic disease, and PH secondary to miscellaneous etiologies (Table 1). Geriatric dogs are likely to develop pulmonary fibrosis, but the most common cause for PH in dogs, by far, is left sided valvular heart disease; i.e., myxomatous mitral valve disease (MMVD) accounting for 40–85% of the cases.

Table 1. Pulmonary Hypertension World Health Organisation (WHO) clinical classification system based on etiology

PH in MMVD

The mitral regurgitation (MR) leads to increased pulmonary venous pressure. The processes that govern the development of PH in MR are multifactorial and are not yet fully understood. Elevation of left atrial pressure, which occurs not only in acute MR, but also in chronic MR, is believed to be the initiating factor in the pathogenesis of PH. Sustained elevation of left atrial pressure, which is passively transmitted backward into the pulmonary veins, can lead to disruption of the delicate alveolar-capillary complex in a process known as alveolar capillary stress failure, with resulting capillary leakage and pulmonary edema. In the initial phases, this lesion may be reversible. However, with long-standing pulmonary venous hypertension, the alveolar-capillary unit may be irreversibly altered by a remodeling process characterized by excessive type IV collagen deposition, leading to a reduction in alveolar diffusion capacity. The result is an increased resistance of the pulmonary arteries, and increased pressure in the pulmonary artery. The right ventricle has limited capacity of compensating for an increased pulmonary arterial pressure and usually dilate in response. Dogs with MMVD should always be thoroughly examined by echocardiography for the presence of tricuspid regurgitation.

Diagnosis of PH

In veterinary medicine, right-heart catheterization is rarely performed in routine practice, and echocardiography is the standard noninvasive technique for the diagnosis of PH. The maximal flow of this regurgitation is useful in assessing the severity of PH, because the right ventricular pressure may be estimated using the modified Bernoulli equation (which gives an estimate of the tricuspid pressure gradient). Adding estimated right atrial pressure to this pressure gradient may provide an estimate of the right ventricular systolic pressure. A right ventricular systolic pressure of 30–45 mm Hg has been suggested to be the cutoff for diagnosing PH in dogs, and a study showed that a tricuspid pressure gradient of >55 mm Hg confers a worse outcome. There are also other indicators of PH in dogs with MMVD. A study suggested that a tricuspid pressure gradient was associated with a combination of either an increased or decreased left ventricular diastolic dimension, decreased pulmonary arterial acceleration/deceleration time, increased right ventricular diastolic diameter and left atrial to aortic root (LA/Ao) ratio.

Treatment of PH

Because most cases of PH are secondary to an underlying disease process, treatment aimed at eliminating or improving the underlying disease status is the basis for therapy. If the PH is not controlled by primary disease therapy or if the etiology of the PH appears to be idiopathic, then direct pulmonary arterial pressure modulation through the use of pulmonary vasodilators should be implemented. For example, it is not uncommon that MMVD dogs with a recent onset of signs of CHF present with mild PH that may reverse once effective CHF therapy has started. Dogs with PH despite having their signs of CHF controlled by medical therapy probably benefit from medical therapy aimed at reducing pulmonary arterial pressure, but there are few veterinary studies available supporting this. Dogs with more pronounced PH are more likely to benefit from this type of therapy. In human medicine, there are several different medical treatments available, such as endothelin-receptor blockers, prostacyclin analogues, and L-arginine, but these are not available in veterinary patients because of costs and lack of efficacy in this specie. The current most commonly used drug for controlling PH in dogs is, therefore, Sildenafil. Pimobendan is another drug that lowers the pulmonary arterial pressure (as well as systemic arterial resistance).

References

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