HY lecture notes:
Pulmonary embolism –> most common thing you see on an ECG = sinus tachycardia (really HY) –> they might say patient has HR of 92 and that ECG shows no abnormality (same thing; that’s sinus tachy) –> first Tx is heparin, then do spiral CT scan of chest.
In pregnant women, do V/Q scan (ventilation/perfusion scan), not spiral CT, because of the radiation.
Now this is where it gets weird: if they tell you a pregnant woman has a V/Q scan that shows segmental defects (positive for PE), and then they ask you for the next best step in diagnosis, the answer is spiral CT. This is where you say, “Wtf? You just said we don’t do spiral CT in pregnant women cuz of the radiation.” Correct, we don’t. We definitely do V/Q scan instead. But if the question asks you what the next best step in diagnosis is after a V/Q scan has already been performed, the answer is still spiral CT. Weird, I know. But this is actually in UWorld for 2CK, and everyone gets it wrong for obvious reasons.
Dead space vs shunt
Dead space = ventilation without perfusion
Shut = perfusion without ventilation
Physiologic (total) dead space = anatomic dead space + alveolar dead space.
Anatomic dead space = ventilation within the conducting zone of the airways that is unable to participate in gas exchange because there is simoply an anatomical lack of respiratory epithelium; this includes the trachea, bronchus, bronchioles, and terminal bronchioles; in contrast, the respiratory bronchioles and alveoli participate in gas exchange.
Alveolar dead space is ventilation within underperfused alveoli (usually negligible in healthy patients).
USMLE wants you to know that pulmonary embolism = dead space because this presents a scenario of ventilation without perfusion.
A shunt refers to pretty much any other cause of lung pathology, where ventilation is reduced for whatever reason.
Shunt = foreign body aspiration; obstructive conditions, e.g., asthma, atelectasis, bronchitis; certain restrictive lung conditions, e.g., pulmonary edema, fibrosis. These conditions can all result in areas of lung that receive less ventilation.
Aspiration of a foreign object is an easy example –> area of lung gets closed off and becomes underventilated –> this means we can say there’s “zero” for this part of the lung –> so even if we were to give a patient oxygen and all of the areas of healthy lung are maximally saturated with O2, the zero still averages in, making the sum of the oxygenation for the entire lungs to be less than it should be –> result is patient still has low arterial oxygen.
This process is called a shunt because the mixing of deoxygenated blood with oxygenated blood is considered to be a “right to left” process. This is distinct from a cardiac R –> L shunt (i.e., Eisenmenger syndrome with VSD, where deoxygenated blood from the RV enters the LV, causing the systemic arterial oxygen to be low); a pulmonary shunt means the R –> L mixing of an underventilated, deoxygenated lung segment with other oxygenated ones, with the net result being the patient’s arterial O2 is still low.
