OK. Here goes. Fluctuations in the a-line trace CAN be used as a surrogate for estimating fluid status when the patient is ventilated. The explanation is as follows.
PPV causes incr. intrathoracic pressure which will decr venous return leading to a drop in Cardiac output during the positive pressure portion of the ventilator cycle. In addition, alveolar expansion causes compression of the peri-alveolar blood vessels in West's Zones 1 and 2, which results in decreased Left atrial filling and consequently decr. LV preload and resultant decreased apmlitude of your a-line trace. These are all normal physiological responses to PPV. When the fluid status is borderline, however, they are exaggerated, particularly the decreased alveolar blood flow. This results in the characteristic swing, which may be exacerbated by the use of PEEP.
In terms of spontaneously breathing patients, inspiration will lead to incr. venous return, leading to increased cardiac output. In terms of the peri-alveolar blood vessels, they are opened further by inspiration due to radial traction applied across the trans-pulmonary unit, which should also serve to increase preload to the LV. I don't believe that expiration is essentially different in vented or spontaneously breathing patients, so LV preloading in both groups should be essentially the same.
The upshot of this long argument is that I agree with you toughlife. I don't buy it either in the spontaneously breathing patient. WIll check today with our resident genius physiologist attending. anyone disagree?