Pulmonary Hypertension
Pulmonary Circulation
Anatomy – pulmonary circulation: gets entire output of RV; primary function is gas exchange, but also provides nutrition to lung parenchyma; large surface area is provided by capillaries in intra-alveolar septa; drainage is to LV
- bronchial circulation
: 1-2% of CO; arises from aorta or first intercostal vessels; supplies all intrapulmonary structures except the parenchyma (bronchial tree, nerves, walls of veins, arteries, lymphatics); can supply the parenchyma if the pulmonary circulation is compromised
- true bronchial veins drain to the azygous, hemiazygous, intercostal veins, and then to the RA
- bronchopulmonary veins (from communication of bronchial capillaries with pulmonary veins) drain into the LA
- Þ
bronchial circulation provides for a small right to left shunt
Physiology – pulmonary circulation is a high flow, low pressure system with thin-walled arteries that are distensible, rather than actively dilating or constricting; mean pressure is 10mmHg, and CO can Ý up to 2.5 times without an Ý in mean pressure–CO can Ý 3-4 times before more than a small Ý in pressure so pulmonary pressure Ý little, even with heavy exercise
- Ohm’s
: pressure (PAP - PVP) = flow x resistance
Þ if flow increases, resistance must decrease to maintain pressure
Þ resistance decreases in high CO due to distension of vessels, and recruitment of more vessels
both of these increase the effective radius of the pulmonary vasculature according to Poiseuille’s R=8m L/r4
distribution of bloodflow
zone 1: Palv > Pv > Pa Þ collapsed blood vessels - no flow (only in the most apical portions of the healthy lung)
zone 2: Pa > Palv > Pv Þ flow is independent of Pa
zone 3: Pa > Pv > Palv Þ flow is dependant on Pa - Pv
Pulmonary Hypertension
Pathophysiology: Ý pulmonary artery pressure can come from Ý PVP, Ý resistance, Ý viscosity, Ý flow
- increased flow
: must be extreme to promote HTN (5-6x); this can occur in congenital heart diseases with left-right shunt; such changes in flow promote changes in lung that
Ý resistance and can reverse the shunt (Eisenmenger Syndrome)
Ý pulmonary venous pressure: LVF and mitral disease are most common
- rarely atrial myxoma, pulmonary veno-occlusive disease
- long term
Ý PVP leads to remodeling of pulmonary vessels and irreversible increases in resistance
Ý blood viscosity: erythrocytosis usually due to chronic hypoxia; also leukemia with extreme leukocytosis
Ý pulmonary vascular resistance: decreasing the cross-sectional area of the vascular bed
- vasoconstriction
: caused by hypoxia, acidemia (also potentiates hypoxic response); vasoconstriction is protective by diverting flow from under-ventilated areas, but when it involves the whole lung, it is pathologic; acute hypoxic vasoconstriction can be reversed, chronic hypoxia leads to irreversible remodeling
- alteration in vascular morphology
: primary pulmonary hypertension; collagen vascular diseases; toxins; secondary to increased flow or hypoxemia; vascular destruction in emphysema or pulmonary fibrosis
- once pulmonary HTN exists, vasculature is always altered
Þ vicious cycle
obstruction: embolism; extrinsic compression; congenital defects
- without cardiopulmonary disease, at least 50% of the lung must be occluded to cause HTN
- thromboembolism also leads to vasoconstriction which worsens the situation
Clinical Features – many signs are only present once cor pulmonale has developed
- dyspnea is the predominant symptom; also fatigue, substernal chest pain, syncope indicates poor CO – severe disease; peripheral edema indicates right failure and severe disease
- physical exam: lungs usually normal; CV may be more indicative; hepatomegaly and peripheral edema are late signs
- labs: CXR may show enlarged pulmonary arteries and RV; echocardiogram may show ventricular hypertrophy/dilation
Diagnosis
echo is the first choice test because it is non-invasive
right heart cath directly measures pulmonary artery pressures and is the gold standard; an elevated wedge pressure indicates left-sided failure or mitral stenosis; normal wedge pressure indicates pulmonary artery disease
V/Q scan can demonstrate pulmonary embolism; pulmonary angiogram is diagnostic
Treatment – treat the primary cause
- supplemental oxygen is very effective in hypoxic patients
- vasodilators are 20-30% effective in primary pulmonary HTN, but long-term benefits are sketchy and risks are high
- lung transplant is the current best therapy in primary pulmonary HTN
Cor Pulmonale
Definition (controversial): hypertrophy of right ventricle resulting from diseases affecting the function and/or structure of the lungs, except when these pulmonary alterations are the result of diseases that primarily affect the left side of the heart as in congenital heart disease
Pathophysiology: most common cause is COPD (40-50% of COPD patients develop cor pulmonale)
- most important etiologic factor is hypoxemia; the hypertrophied RV is very susceptible to ischemia
Clinical Features: same as for pulmonary hypertension; also fluid retention due to the effects of hypoxia and hypercapnia on CO and renal blood flow; syncope during exercise can result due to the insufficiency of the RV
Treatment: treat the underlying cause Þ bronchodilators in COPD; supplemental oxygen in hypoxemia