These methods are based on measuring IAP indirectly across the walls of an intra-abdominal organ or structure. The indirect method used most commonly is the intravesical method performed via a urinary catheter. When the bladder is drained or has a minimal volume, the IAP is transmitted through the bladder’s compliant wall and is measured by transducing the urinary catheter. A minimum volume of 3 mL or 1 mL/kg up to a maximum of 25 mL saline is instilled into the Foley catheter to establish a continuous fluid column with the pressure transducer, and IAP is recorded (10). IAP may be measured using a fluid column, in which case the reading is in cmH₂O and should be converted to mm Hg by dividing by 1.36. Bladder detrusor muscle spasm may occur during saline instillation; thus, allowing time (30 seconds to 1 minute) for equilibration of pressures permits an accurate steady-state IAP to be measured (13). IAP should be expressed in mm Hg and measured at end-expiration in the supine position after ensuring that abdominal muscle contractions are absent and with the transducer zeroed at the level of the mid-axillary line.

Intragastric, intrauterine, venacaval, and intrarectal sites have also been described for indirect measurement of IAP (14,15,16). The intravesical method is most often used in clinical practice because it is simple, reliable, and easily performed at the bedside. Kimball et al. using a standardized protocol for intravesical measurement demonstrated that intraobserver and interobserver variation in IAP readings is minimal (Pearson’s correlation coefficient 0.934 and 0.950, respectively) (17). Indirect intravesical IAP measurement can be performed using a commercially available kit or assembled low-cost components.

This method measures IAP directly from the peritoneal space using a pressure transducer or a fluid column. Direct measurement of IAP involves the placement of a needle or catheter into the peritoneum, making it more invasive, and generally less preferred than indirect measurement. However, if a preexisting catheter (such as a peritoneal dialysis catheter) is in place, the direct method can be used with equal ease (18).

The agreement between direct IAP measurement and indirect intravesical measurement has been studied in children. Two studies have shown that intravesical IAP measurement was most accurate with 1 mL/kg saline instilled into the bladder as
compared with larger volumes of instillation (18,19). Another study suggests that instilling a standard volume of 3 mL is as reliable as the 1 mL/kg volume for IAP measurements in children weighing less than 50 kg (10).

Many factors commonly encountered in critically ill patients may affect the accuracy of IAP measurements. In children, elevation of the head of the bed significantly increases the IAP. An elevation from 0° to 30° increased the IAP reading by 2 mm Hg (20). If the head of the bed cannot be lowered briefly for IAP measurement, subsequent IAP readings should also be measured at the same elevation for consistency of comparison. In this case, the IAP trend, rather than single readings, become more important. Unlike in adults, body mass index does not appear to affect IAP measurement (20,21). Inappropriately large instillation volume, transducer location, prone positioning, inadequate sedation, coughing, or agitation may all falsely raise measured IAP (13,21,22,23,24). High respiratory pressures and obesity have been shown to result in inaccurately high IAP in adults (23

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