Standing up translocates a large fraction of thoracic blood volume into the dependent body parts thereby reducing venous return. When upright systemic venous return is sufficiently impaired, orthostatic intolerance occurs and may be related to decreased blood volume, enhanced gravitational pooling of blood within the dependent veins in the lower body, or loss of vascular volume through microvascular filtration.  A principal defense against orthostatic intolerance in man is the “skeletal muscle pump” in which contractions of leg and gluteal muscles propel venous blood back to the heart . Many investigations of orthostatic intolerance deliberately subvert the muscle pump by use of tilt tables or lower body negative pressure. However, recent work has reinforced the importance of muscle pump activity in relieving orthostatic intolerance .

Skeletal muscle integrity is dependent on adequate blood flow supine and upright; muscle ischemia produces reduced muscle mass . Impairment of blood flow may cause skeletal muscle wasting, which may further compromise muscle flow through its effects on the muscle pump .

Skeletal muscle pump testing follows initial supine blood flow measurements. The method was adapted from the work of Nicolaides . A strain gauge is secured at the maximum circumference of the calf which is measured to the nearest mm with a tape measure. The leg is lifted by the ankle to an angle of 15-35o to empty venous blood from the calf and obtain the estimated minimum calf volume. The patient then swivels off the examining table to a standing position taking care not to dislodge the strain gauge. Weight bearing is initially maintained on the leg contralateral to the strain gauge. Balance is maintained by use of a stand-up walker. A schematic recording of volume changes is shown in the figure.  Single tiptoe maneuvers are performed by going up on the toes using both legs to bear weight for 1 second. This generally produces calf emptying. Once the singleton tiptoe is complete, the contralateral leg again carried the subject’s weight while the calf volume recovers. Repeat single tiptoes are performed. Later, the subject performs 10 tiptoes in a row taking approximately one second per tiptoe. The sequence of multiple tiptoes is repeated. Sequential tiptoes give the most consistent emptying of calf veins. The complete venous volume is designated VV, the ejection volume of a tiptoe is designated EV, and the residual volume is RV. The ratio of EV/VV or ejection fraction is used as a normalized index of skeletal muscle pump adequacy. A decrease in ejection fraction therefore corresponds to a decrease in pump function independent of muscle mass. The ratio of RV to VV is also calculated. We used data from multiple tiptoe sequences for these indices.

The time to 90% recovery of calf volume (VFT90) from a singleton tiptoe is used as an index of the venous filling. Multiple sequential tiptoes are less useful for this purpose because of time dependent effects. The velocity of recovery to 90% volume (=0.9*VV/VFT90) represents an index of average venous filling rate. The force necessary for the maneuver varies from patient to patient according to weight. However, this form of muscular stress is thought to most physiologically represent the ability of the skeletal muscle pump to enhance venous return when upright .




Peripheral Vascular Properties Measured by Strain Gauge Plethysmography (SPG)
Laser Doppler Flowmetry (LDF)
Indicator Techniques to Measure Blood Volume and Cardiac Output
Heart Rate and Blood Pressure
Near Infrared Spectroscopy
Microneurography and Muscle Sympathetic Nerve Activity (MSNA)
Transcranial Doppler Ultrasound (TCD)
Impedance Plethysmography (IPG)
Skeletal Muscle Pump