To the Editor.—
The ex cathedra reply of Cane and Shapiro in the LETTERS section1 that true-negative pressure ventilators do not diminish venous return should not go unchallenged, despite decline in the use of such machines. The truth lies somewhere between Johnson's view that both negative- and positive-pressure ventilators are identical and Cane and Shapiro's view that they are different.Venous return, and hence cardiac output, is very nearly an Ohm's law relationship. The electrical law E = IR translates hydrodynamically to driving pressure = venous return × resistance. Driving pressure for venous return is the difference between venular pressure in tissues and right atrial pressure. Venular pressure in turn is partly dependent on tissue ambient pressure. Right atrial pressure is similarly dependent on alveolar pressure and lung compliance. In a full tank respirator, lowering the tank pressure to create a breath will lower driving pressure quite like an equivalentsized breath
Smith TC. Venous Return and Mechanical Ventilators. JAMA. 1986;256(1):38. doi:10.1001/jama.1986.03380010042016
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