IUHS Student-2-Student USMLE Step 1 Recall
Respiratory
Diseases of Vascular Origin
Pulmonary Hypertension
pulmonary circulation is normally one of low resistance, and pulmonary blood pressure is only about 1/8 of systemic blood pressure...
pulmonary hypertension (when mean pulmonary pressure reaches one fourth of systemic levels) is most frequently secondary to structural cardiopulmonary conditions that increase pulmonary blood flow or pressure (or both), pulmonary vascular resistance, or left heart resistance to blood flow...
elevation of the mean pulmonary arterial pressure is the result of endothelial dysfunction and vascular changes...
a distinctive arteriolar change, a plexiform lesion, consists of intraluminal angiomatous tufts that form webs...this pattern is thought to be diagnostic of primary hypertension...
plexogenic pulmonary vasculopathy...
most common in women who are 20-40 years of age...
dyspnea and fatigue, but some patients have chest pain of the anginal type...
severe respiratory distress, cyanosis, and right ventricular hypertrophy occur, and death from decompensated cor pulmonale, often with superimposed thromboembolism and pneumonia, usually ensues within 2 to 5 years in 80% of patients...
continuous therapy with vasodilators (calcium channel blockers or inhaled nitric oxide) and anti-thrombotic medications (warfarin, prostacyclin, and thromboxane receptor blockers)...appears to improve outcome in patients...
Pulmonary Embolism
large emboli may impact in the main pulmonary artery or its major branches or lodge at the bifurcation as a saddle embolus...
sudden death often ensues, owing largely to the blockage of blood flow through the lungs...
death may also be caused by acute dilation of the right side of the heart (acute cor pulmonale)...smaller emboli can travel out into the more peripheral vessels, where they may or may not cause infarction...
in patients with adequate cardiovascular function, the bronchial artery supply can often sustain the lung parenchyma despite obstruction of the pulmonary arterial system...
pulmonary embolism usually causes infarction only when the circulation is already inadequate, as in patients with heart or lung disease...about 3/4 of all infarcts affect the lower lobes, and in more than half, multiple lesions occur...
pulmonary infarct is classicly hemorrhagic and appears as a raised, red-blue area in the early stages...often the apposed pleural surface is covered by a fibrinous exudate...the red cells begin to lyse within 48 hours, and the infarct becomes paler and eventually red-brown as hemosiderin is produced...
usually a complication of previous underlying disease such as cardiac disease or cancer, or who are immobilized for long periods...
primarily, hypercoagulable states, antithrombin II, protien C deficiencies, defective fibrinolysis, and, paradoxically, the presence of lupus anticoagulant...
secondarily,
obesity, recent surgery, cancer, oral contraceptive use, pregnancy...
pulmonary emboli may be caused by thrombi, air (after surgery), amniotic fluid (complications of labor), fat (associated with trauma causing fractures of long bones), or tumors (renal cell carcinomas invading the ena cava)...
pulmonary emboli are common and are found in about 10-20% of hospital autopsies...
occlusions of the pulmonary arteries by blood clots are almost always embolic, arising from thrombi in the deep veins of the leg (DVTs)...femoral vein...
calf tenderness, associated with DVTs is a useful clincial sign that points toward pulmonary emboli as the cause of breathing problems after surgery...
ventilation/perfusion scan is the screening test of choice...there is ventilation but no perfusion...pulmonary angiogram is confirmation...
Adult
Respiratory Distress Syndrome (ARDS)
syndrome caused by diffuse alveolar capillary damage...it is characterized clinically by the rapid onset of severe life-threatening respiratory insufficiency, cyanosis, and severe arterial hypoxemia that is refratory to oxygen therapy and that may progress to extrapulmonary multisystem organ failure...
in most patients, there is evidence of severe pulmonary edema, with chest radiographs showing a diffuse alveolar infiltration...
direct lung injuries causing ARDS include diffuse pulmonary and other viral infections, oxygen toxicity, inhalation of toxins and other irritants, and aspiration of gastric contents...
systemic conditions causing ARDS include septic shock, shock associated with trauma, hemorrhagic pancreatitis, burns, complicated abdominal surgery, narcotic overdose and other drug reactions, hypersensitivity reactions to organic solvents, hemodialysis, and cardiac surgery involving extracorporeal pumps...
in the acute edematous stage, the lungs are heavy, firm, red, and boggy...
they exhibit congestion, interstitial and intra-alveolar edema, and inflammation...
in addition to the congestion and edema, there is fibrin deposition...
the alveolar walls become lined with waxy hyaline membranes that are similar to those seen in hyaline membrane disease of neonates...
alveolar hyaline membranes consist of fibrin-rich edema fluid mixed with the cytoplasmic and lipid remnants of necrotic epithelial cells...subsequently, type II epithelial cells undergo proliferation in an attempt to regenerate the alveolar lining...there is organization of fibrin exudate, with resultant intra-alveolar fibrosis...marked thickening of the alveolar septa ensues, caused by proliferation of interstitial cells and deposition of collagen...
loss of type I pneumocytes...
ARDS is the end result of acute alveolar injury caused by a variety of insults and probably initiated by different mechanisms...
the final common pathway is diffuse damage to the alveolar capillary walls, this is followed by a relatively nonspecific, often predictable series of morphologic and physiologic alterations leading to respiratory failure...
the initial injury is to either capillary endothelium (most frequently) or alveolar epithelium (occasionally), but eventually both are clearly affected...damage to these cells leads to increased capillary permeability, interstitial and then intra-alveolar edema, fibrin exudation, and formation of hyaline membranes...
importantly the exudate and diffuse tissue destruction that occur with ARDS cannot be easily resolved, and the result is generally organization with scarring, producing severe chronic changes, in contrast to the transudate of cardiogenic pulmonary edema...
neutrophils aggregate in the lung microvessels and have the capacity to injure pulmonary endothelial cells through release of toxic oxygen metabolites and destructive enzymes...
macrophages are an alternative source of injury in patients with ARDS...macrophages can generate toxic oxygen products, proteases, arachidonic acid metabolites, platelet activating factor (PAF), and cytokines that regulate inflammation...
to some extent, neutrophilic inflammation in ARDS may be driven by macrophage derived chemokines, for example, IL-8...additional physiologic effects of these mediators include vasoconstriction and platelet aggregation, both of which may decrease blood flow to aerated regions of lungs...
Hemodynamic Mechanism of Pulmonary Edema
attributable to increased hydrostatic pressure, as occurs in left-sided congestive heart failure...
accumulation of fluid in this setting can be accounted for by Starling's Law of capillary interstitial fluid exchange...
pulmonary congestion and edema are characterized by heavy, wet lungs...fluid accumulates initially in the basal regions of the lower lobes b/c hydrostatic pressure is greater in these sites...
histologically the alvolar capillaries are engorged, and an intraalveolar granular pink precipitate is seen...alveolar microhemorrhages and hemosiderin-laden macrophages ("heart failure cells") may be present...
in long standing cases of pulmonary congestion, such as those seen in mitral stenosis, hemosiderin-laden macrophages are abundant, and fibrosis and thickening of the alveolar walls cause the soggy lungs to become firm and brown (brown induration)...