New thoughts on Acute Volume Therapy

ABSTRACT

Modern fluid therapy requires an understanding of the underlying physiological abnormalities induced by acute illness; the nature of the fluids to be administered; the differences between the various intravenous fluid preparations and concepts regarding appropriate amounts of volume to be given. Crystalloid solutions expand the extracellular fluid (ECF) space and are redistributed between the intravascular and extracellular compartments in a ratio of 14 in proportion to the normal distribution of fluid between these two spaces. The most widely used crystalloids; 0.9saline and balanced salt solutions such as Ringer's lactate; fall well short of the desired composition. 'Normal' saline is significantly hypertonic (osmolality 308 mOsm/l) and has a very high chloride content (154 mmol/l; normal plasma range 95 - 105 mmol/l). Infusions of as little as 2 litre 0.9saline during surgical procedures will produce a significant; metabolic acidosis owing to the chloride load. Ringer's lactate (or acetate); like 0.9saline; is not an ideal solution. The Cl- content is substantially higher than plasma chloride (111mmol/l); the Na+ content lower (131 mmol/l) and the osmolarity of the solution is 274 mOsm/l. Colloids are suspensions of particles of various sizes that aim to maintain plasma volume by maintaining the colloid osmotic pressure in plasma; thus retaining the administered volume within the circulation. Recently the 'third space' concept has been questioned and attention has focused on the potential adverse effects of excess administration of crystalloid solutions. Intravenous fluid loading is often used as first-line therapy for patients with hypotension or circulatory failure; but in only half of patients does cardiac output respond positively after fluid challenge. For the remainder of patients fluid loading may be associated with adverse consequences. Mechanically ventilated patients have a higher mean CVP; more indicative of mean intrathoracic pressure than of cardiac filling or intravascular fluid status. The patient who demonstrates a positive response to passive leg raising has been reliably shown to benefit from the administration of intravenous fluid. It has recently been established that not only systolic pressure variation but also several analogous derivatives provide valuable clinical information on a continuous basis. The predictive value of systolic pressure variation is well attested to in ventilated patients. Patients breathing spontaneously demonstrate the same phenomenon; but critical assessment is more difficult