Rales, crackles, crepitations: Usually inspiratory. Early to mid are due to secretions in large to medium airways (e.g., bronchitis) and clear with coughing. Late are due to opening of small airways and do not clear with coughing. Causes: pneumonia, pulmonary edema, interstitial fibrosis (e.g., sarcoidosis). Fine crackles (like rubbing strands of hair together) are more indicative of interstitial pathology, such as CHF or fibrosis; coarse, airway bronchiectasis.
Wheezing: Usually expiratory, high-pitched musical sound. Caused by narrowing of small airways and segmental bronchi by inflammation (e.g., asthma, bronchitis); peribronchiolar pulmonary edema (e.g., cardiac asthma), or pulmonary embolism/infarct (platelets release TXA2, a bronchoconstrictor).
Rhonchi: Both inspiratory and expiratory, low-pitched snoring sound. Causing by secretions in large airways (e.g., bronchitis); clear with coughing.
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These lung findings are less reliable in vascular diseases like pulmonary embolism.
Pulmonary embolism is often caused by deep vein thrombosis (main risk factor: stasis, hypercoagulable states -- e.g., Factor V Leiden, which is not degraded by Protein C).
In PE you have V/Q mismatch because the blood goes preferentially to upper lobes, but there is less ventilation there. You also get pulmonary hypertension since the same amount of blood has to go through less parallel circuits.
The main problem in PE is not losing part of the lung, or hypoxemia, but hemodynamic instability, since the RV is a poor pump and can't deal with pulmonary hypertension. As RV pressures increase, coronary artery driving pressure (BP - P in muscle of RV) decreases, as this leads to right ventricular ischemia. Hence, giving a systemic pressor like phenylephrine can paradoxically increase cardiac output by improving RV oxygenation.
Pulmonary hypertension: compensation, then decompensation (dyspnea, dizziness, syncope). Can be due to large vessel obstruction (Chronic Thromboembolic Pulmonary Hypertension -- basically scarring from PE); small vessel obstruction (idiopathic, collagen vascular disease); congenital heart disease giving left to right shunts
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