Purpose

The purpose of optimizing preload it’s to improve, when needed, the cardiac output. However a preload assessment measure will not tell when a further increase on preload will improve it, and more fluid could be at best be ineffective and at worst lead to fluid overload and pulmonary edema.

Some indexes of dynamic preload assessment have been created, however they require conditions that could lead them to be hard or impossible to use.

Fortunately raising the patient’s legs will easily increase preload in a reversible manner, returning to the heart about 150 to 300 ml of blood, leading to improvement of cardiac output if the patient its preload responsive. If the patient it’s not longer preload responsive the fluid can be safely “removed” by lowering the legs.

Measurement

1. Note the cardiac output or its chosen proxy (pulse pressure, descending aorta blood flow, etc)
2. Place the patient on a bed in a 45º semi recumbent position.
3. Pivot the patient until its body is at a 0º angle and the legs are elevated to 45º.
4. After 30 to 90 seconds note the change on cardiac output or it’s chosen proxy.

Conditions and caveats

• Try to have a continuos recording of a right heart preload assessment measure (eg: CVP), to ensure that the preload is really increasing with the passive leg raising
• The cardiac output (or its proxy) should be measured with a fast-response device
• Avoid in head trauma
• Could lead to false negatives in patients with abdominal hypertension[1] (http://www.ncbi.nlm.nih.gov/pubmed/20639753)
• Minimize pain as it can lead to sympathetic stimulation and an artificial increase of cardiac output

Reference Values

Bibliography