pulmonary hypertension

• The pulmonary circulation is usually a low pressure, low resistance, high capacitance system to minimise right ventricular workload (the RV is weak and often doesn't need much strength as it pumps to the lungs rather than the entire body). Increases in pulmonary blood flow are usually accommodated with minimal increase in pulmonary artery pressure due to…

  • Highly distensible thin walls of the pulmonary vasculature 
  • Recruitment of under perfused vessels
  • Large pulmonary capillary surface area.
• Pulmonary hypertension can be defined as a systolic pulmonary artery pressure (PA) greater than 35 mmHg and/or a diastolic PA pressure greater than 10 mmHg
• The causes of pulmonary hypertension are…

  • Alveolar hypoxia with pulmonary vasoconstriction / remodelling
  • Pulmonary vascular obstructive disease
  • Pulmonary over circulation
  • High pulmonary venous pressure
  • Idiopathic
• Alveolar hypoxia with pulmonary vasoconstriction / remodelling occurs in severe respiratory diseases e.g. IPF, neoplasia etc.
• In interstitial fibrosis, there are permanently unoxygenated areas causing permanent vasoconstriction (increased PVR). This eventually leads to thickened walls so they resemble systemic arteries causing a reduction in vascular compliance causing permanently elevated (mild) pulmonary arterial pressure. These arteries lose their ability to vasodilate. Consequently, the right ventricle enlarges to combat the higher pressures.
• The pulmonary vascular obstructive disease occurs with pulmonary thromboembolisms and heart worm disease dogs are not hypercoagulable usually and hence are unlikely to throw a clot. Protein losing diseases can sometimes cause dogs to form clots that clog the pulmonary artery (can present like cats with ATE and get saddle thrombi too)
• Pulmonary overcirculation commonly occurs due to large congenital shunts.

  • A large VSD or ASD will shunt blood from the left to the right causing an increase in blood volume through the pulmonary arteries. These arteries then remodel (the increase blood flow increases perfusion pressure and damages them causing them to thicken via an increase in muscle) and become more like systemic arteries leading to an increase in pressure.
• High pulmonary venous pressure occurs in left sided heart failure (of various causes)

  • When a dog has had left congestive heart failure for a while, pressure build up in the right atrium and pulmonary vein leading to an increased pressure in the lungs. Consequently, the right ventricle has to adapt (dilated and hypertrophies) to eject blood into a system under higher pressure than normal leading to RHS heart failure. In short, Secondary hypertension occurs due to the left sided failure contributing to the development of RHS CHF.
• The signs of pulmonary hypertension can be subtle and varies but tends to occur in middle aged to older dogs with no sex predilection.

  • You are more likely to be able to identify the signs of the primary disease, especially respiratory ones so be sure to consider the breeds that are predisposed to airway disease or valvular disease (CKCS)

  That said, suspect PH when you see the following without an obvious cause…

  • Persistent respiratory difficulty
  • Fatigue
  • Exercise intolerance
  • Syncope
• The clinical signs of a pulmonary thromboembolism can also be very variable and include: dyspnoea, tachycardia, lethargy, altered mentation, vomiting and diarrhoea.
• On physical exam there is likely to be a right sided murmur but severe pulmonary hypertension can have split S2

  • A split s2 occurs when the closure of aortic and pulmonic valves are out of sync

  Other physical findings will relate to the primary disease for example with chronic lung disease you would expect to also find crackles, cyanosis or a murmur associated with left sided disease if LCHF is the cause

  • The right sided murmur will likely be louder than the left side
• There are three conditions that allow for a thrombus formation…

  • A hypercoagulable state
  • Endothelial damage
  • Static blood
• Cor pulmonale (pulmonary heart) is when a lung issue causes the right ventricle to get so big that your heart starts to fail.

  • Other complications include abnormal heart rhythms, hypoxia, syncope, kidney and liver disease. Death can also occur
  • Causes include COPD, pulmonary embolism, trauma or surgery that removes lung tissue, mechanical ventilation injuries, pulmonary fibrosis, sleep apnoea, neuromuscular disorders that affect breathing and lung issues that block veins
• Signs suggestive of pulmonary hypertension on radiography include cardiomegaly (especially right sided enlargement) and enlarged pulmonary arteries (lobar veins and arteries). There may also be right sided heart failure signs (hepatomegaly, large caudal vena cava, pleural effusion) and pulmonary parenchymal infiltrates (reflecting primary lung disease)
• The changes seen on echocardiography are often proportional to the severity of pulmonary hypertension and often include right ventricular hypertrophy (greater than left ventricular wall thickness) and dilation, pulmonary artery dilation, right atrial enlargement and a flattened interventricular septum.
• there is no cure but there are a few treatment options but also treat the underlying disease. Treatment options include…

  • Sildenafil (Viagra)
  • Effective in some cases but there is usually limited capacity for pulmonary vasodilation
  • Pimobendan
  • Oxygen therapy
  • Enothelin antagonists (Bosentan)