Compliance is change in volume per change in pressure, and is expressed in milliliters/centimeter water. It represents bladder distensibility and is attributed to the vis-coelastic properties and spherical shape of the bladder. Absence of involuntary contractions during filling also contributes to bladder compliance. As a result, the detru-sor pressure remains low throughout bladder filling. Normally during bladder filling, the detrusor pressure does not exceed 5 to 10 cm H2O greater than the starting pressure. Therefore, the kidneys are protected.

The CMG is divided into four phases. The first three phases occur during filling and the fourth occurs during voiding. Phase I reflects the bladder's initial response to filling. Phase II is the tonus limb and reflects the bladder pressure during the majority of the filling phase. Phase III occurs when the upper limit of viscoelastic properties is reached and the pressure begins to increase. Phase IV represents the voluntary contraction. Compliance is calculated at the end of filling as A volume/A Pdet.

According to Ghoniem,10 compliance in the normal individual is ^20mL/cm H2O. Compliance less than 10mL/cm H2O is clinically significant (Figure 3-2.6). Low compliance is found in patients with severe outlet obstruction, meningomyelocele, chronic inflammation (tuberculosis and schistosomiasis), and after radiotherapy. High compliance may be associated with large capacity and overdis-tended bladders, and has little clinical significance as an isolated finding.10

Ghoniem Compliance Nomogram Compliance = 3.91 ml/cmH20

3 9 50 70 90 110 130 150 170 190 210 230 Infused Volume

Normal .. JEquiv.ocai- Isevece,-

Figure 3-2.6. Impaired compliance.

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