It’s 4 in the morning; I am somewhere between Riga and Stockholm. The moon is full and bright and rippling across the black, Baltic Sea. This warm, June darkness is cut like onyx by deep vibrations of a cruise ship and its collections of giggling Swedes; they karaoke ‘Spaceman’ by The Killers in perfect English. I’m perched portside, caught in a yawning ocean breeze, surrounded by a symphony of Slavic slangs and cigarette smoke; my mind turns to the innards of the ship and I imagine a physiological analogy.
The Hull as the Thorax
Consider sitting deep within the hull of this cruise ship, ignorant to the outside. A leak is sprung and ocean begins to rush in. Thinking quickly you activate the bilge pump which, appropriately, ejects the ocean outside again. You note that the bilge pump has a number of settings from ‘low’ to ‘high’ corresponding to the rate at which it evacuates ocean from inside the hull. When the pump’s setting is ‘low,’ ocean water rises inside the hull – you feel the water’s pressure around your ankles. When you increase pump activity to ‘high,’ the pressure and volume of ocean water around your feet abate. Knowing only this, can you infer the size of the body of water beyond the hull? Are you in the relatively small Baltic or the immense Pacific? Lake Ontario? The Hudson River? Wreck Beach?
The answer is that you cannot know; the amount of ocean water within the hull of the ship is a function of the size of the body of water outside of the hull and its inflow, but also on the efficacy of the bilge pump. Thus, the analogy unfolds – we have approximated volume status [the vastness of the ocean], venous return [the ocean rushing into the boat], the thorax [the hull of the boat], the heart [the bilge pump] and the central venous pressure, great vein and cardiac chamber volume [the pressure and volume of the ocean water accumulating within the hull of the boat].
Yet, I continue to see clinicians use right atrial pressure, IVC volume/collapse and echocardiographically-measured left ventricular volume as markers of a patient’s volume status. Looking only into the thorax for patient’s volume status is as preposterous as estimating the size of an ocean based on the amount sea water accumulating within the hull of a leaky ship.
Read full article at: http://pulmccm.org/main/2017/uncategorized/icu-physiology-1000-words-mean-systemic-filling-pressure-part-1/
Related article at: Anatomy and Physiology Help
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