Wescor's Colloid Osmometer

Model 4420

Wescor's Colloid Osmometer combines simplicity, reliability and precision in the vital measurement of colloid osmotic pressure (COP) in hospital critical care and surgical centers. Developed with a singular purpose: to provide the physician with a patient's COP status before administering albumin or synthetic colloids and as a precise monitor of this therapy. With many advanced features, Wescor's Colloid Osmometer stands apart as the superior instrument for measuring COP.

Wescor's Colloid Osmometer surpasses the competition by virtue of its ground breaking dual flow-through sample and reference chambers. A small number of lower molecular weight macromolecules are likely to transude any practical membrane. If not corrected, this will compromise the reference solution and give rise to errors in subsequent measurements. In the Colloid Osmometer, reference solution can be flushed and exchanged with the same ease and facility as the sample solution. This advantage makes the Wescor Colloid Osmometer the preferred instrument for measuring colloid osmotic (oncotic) pressure.

The instrument measures the colloid osmotic pressure (COP) of solutions using a membrane-selectively impermeable to protein molecules-and a sensitive pressure transducer in an osmometer cell designed for optimum clinical performance.

The transducer output signal is amplified and converted directly to pressure units displayed in easy to read digital format (LCD). The Wescor Colloid Osmometer is simple to operate and maintain.

At present, the most important specific application of COP measurement is the prediction of the onset and degree of edema, particularly pulmonary. From the previous discussion, it is clear that this information is best provided by knowledge of both COP and capillary hydrostatic pressure. If the latter exceeds the COP, the normal direction of water movement will be reversed, producing an enlarged extravascular space (hemodynamic pulmonary edema). Such a situation could be corrected by reducing excess hemodynamic pressure and/or increasing COP. Intravenous administration of hyperoncotic albumin solution may be used to augment vascular colloid concentration and thus increase COP.

The value of using colloid osmometry to detect edema formation and to monitor colloid therapy has been substantiated. Specifically such monitoring has prognostic significance. Studies have shown that edematous patients with COP values below 12.5 mmHg generally do not survive. On the other hand, patients with values above 20 mmHg have favorable prognoses.

The instrument measures the colloid osmotic pressure (COP) of solutions using a membrane-selectively impermeable to protein molecules-and a sensitive pressure transducer in an osmometer cell designed for optimum clinical performance. The transducer output signal is amplified and converted directly to pressure units displayed in easy to read digital format (LCD). The Wescor Colloid Osmometer is simple to operate and maintain.

Colloid osmotic pressure (COP, or oncotic pressure) is the equilibrium pressure exerted on a semi permeable membrane separating two solutions of differing osmolality, at least one of which contains membrane-permeable (colloid) particles.

Human physiology provides a very important example of such a system. The interstitial fluid, which normally contains electrolytes, diffusible non-electrolytes and relatively small amounts of protein (colloid) is separated from the vascular fluid (blood) by the semi-permeable vascular capillary membrane. The vascular fluid contains considerable amounts of protein solutes in addition to diffusible low molecular weight solutes. In response to differences in the osmolality of these two fluids, water and diffusible solutes will freely transude the vascular membrane until reaching a stable equilibrium.

At this point, the intravascular fluid will have a slightly higher osmolality than the interstitial fluid (up to 2 mmol/kg difference). This osmolal difference, which accounts for the colloid osmotic pressure, is due to two factors: (a) the presence of impermeable protein molecules, per se, and (b) an unequal distribution of the diffusible electrolytes between the two compartments at equilibrium. The electrolyte imbalance, known as the Gibbs-Donnan Effect, is brought about by the overall negative electrical charge that the protein molecules possess at blood pH values. Typically, assuming a normal COP value of 25 mmHg, the osmolality of protein particles contributes approximately 15 mmHg pressure. The electrolyte imbalance caused by these anionic impermeable protein molecules accounts for the remaining 10 mmHg pressure.

Serum colloid osmotic pressure is a vitally important factor influencing the quantity of water flux into the vascular compartment across vascular capillary membranes that display normal semipermeable characteristics. Its influence is reduced by the opposing capillary hydrostatic pressure; the net result, in normal circumstances, being a small pressure differential favoring water movement from the interstitial fluid into the vascular circulation. Starling first described the physiological significance of this system in 1896.

Clinical Applications:

At present, the most important specific application of COP measurement is the prediction of the onset and degree of edema, particularly pulmonary. From the previous discussion, it is clear that this information is best provided by knowledge of both COP and capillary hydrostatic pressure. If the latter exceeds the COP, the normal direction of water movement will be reversed, producing an enlarged extravascular space (hemodynamic pulmonary edema). Such a situation could be corrected by reducing excess hemodynamic pressure and/or increasing COP. Intravenous administration of hyperoncotic albumin solution may be used to augment vascular colloid concentration and thus increase COP.

The value of using colloid osmometry to detect edema formation and to monitor colloid therapy has been substantiated. Specifically such monitoring has prognostic significance. Studies have shown that edematous patients with COP values below 12.5 mmHg generally do not survive. On the other hand, patients with values above 20 mmHg have favorable prognoses.

Careful judgment should be exercised in clinical interpretations of COP measurements on serum samples. For example, in some patients the clinical features are complicated by a significant increase in the permeability of the vascular membrane. This allows serum proteins to enter the interstitial compartment to a marked degree. Since laboratory measurement of COP is routinely made with colloid free isotonic saline solution in the reference chamber, the measured COP in such cases will not truly reflect the 'in vivo' situation. Studies of COP in endotracheal fluid extracted from the lungs lend insight to the presence of vascular membrane lesions and can indicate whether the edema is cardiogenic (hemodynamic) or non-cardiogenic (permeability) in origin.

Note that COP will accurately predict direction and extent of water movement only when a dynamic equilibrium exists (with respect to water and electrolyte movement) between the interstitial and vascular compartments. Gross transient differences in osmolality between these spaces will produce water movements under the influence of forces that temporarily outweigh the relatively subtle effect of vascular colloid. In such circumstances, concomitant serial observations of serum osmolality trends will provide valuable information to draw the attention of medical staff to this possibility.

Recent findings in the field of obstetrics and gynecology have shown that COP measurements can be valuable for monitoring pathologic conditions that arise during pregnancy. These include preeclampsia, pregnancy-induced hypertension, hypovolemic shock and tocolytic drug therapy when there is a danger of pulmonary edema.

Direct measurement of COP is faster and more reliable than indirect inference of COP from chemical assay of total protein. This is understandable, since the total protein concentration provides no information concerning the specific particle weights or anionic charge of the components of the variable spectrum of serum proteins. Such factors must be quantitatively assessed if theoretical COP is to be accurately calculated . In normal individuals, blood pH and serum protein electrophoretic pattern may be assumed to be normal, leading to a reasonable correlation between calculated and measured COP values. However, in critically ill patients, marked departures from normality invalidate calculated values of COP.

Used in conjunction with serum osmolality measurements, colloid osmometry rapidly reveals the probability of abnormal changes in the disposition of water among the various body compartments. Webster¹ provides additional background in his comprehensive review of the theory and clinical application of colloid osmometry.

1.Webster, H.L., Colloid Osmotic Pressure: Theoretic aspects and background. Clinics in Perinatology 9, 505 (1982).