Fig. 1
(a) Abrams-Griffiths nomogram. Plot uses detrusor pressure at maximum urinary flow and maximum urinary flow rate. Reproduced from Lim CS and Abrams P. The Abrams-Griffith nomogram. World J Urol. 1995;13:35, with permission of Springer. (b) Initially equivocal on the Abrams-Griffiths nomogram further analyzed as an unobstructed pattern. Slope ≤ 2 cm H2O/ml/s and minimal voiding detrusor pressure ≤ 40 cm H2O. (c) Initially equivocal on the Abrams-Griffiths nomogram further analyzed as obstructed pattern because slope >2 cm H2O/ml/s. (d) Initially equivocal on the Abrams-Griffiths nomogram further analyzed as obstructed pattern because minimal voiding pressure >40 cm H2O
The Abrams-Griffiths nomogram boundaries were created by a combination of clinical and theoretical observations. Patients had been evaluated clinically prior to the pressure flow study and determined to have obstruction. For those patients who fall in the equivocal region, further analysis based on the traditional criteria can be performed to determine whether the patient is obstructed or not. A pressure-flow curve of the complete micturition is used in this case. If the mean slope of the pressure-flow plot is less than 2 cm H2O/ml/s and the minimal voiding detrusor pressure is less than 40 cm H2O, then the bladder outlet is unobstructed (Fig. 1b). Conversely, either a slope greater than 2 cm H2O/ml/s (Fig. 1c) or a minimal voiding detrusor pressure greater than 40 cm H2O (Fig. 1d) indicates a bladder outlet obstruction . Using this method, all patients can be classified as obstructed or unobstructed.
The nomogram has been used to study outcomes of prostatectomy in 123 patients [2]. In this prospective study, patients were selected for prostatectomy based on clinical symptom score, but the results of the pressure-flow studies were blinded both pre- and post-operatively (at 6 months after surgery). Thirty-six out of 123 patients were found to be “unobstructed” based on the Abrams-Griffiths nomogram pre-operatively; unsurprisingly, their post-operative pressure-flow curve remained in the unobstructed category. In the 87 patients found to be “obstructed” pre-operatively, their pressure-flow parameters after prostatectomy improved significantly and were categorized as unobstructed. The success rate as measured by severity of symptoms was 93.1 % in the obstructed group, but only 77.8 % in the unobstructed group. The Abrams-Griffiths nomogram not only helps determine obstructed from unobstructed patients but also provides information on the outcome of the operation as those who were categorized as obstructed pre-operatively had a more significant positive clinical outcome. The use of the Abrams-Griffiths nomogram has also been validated for transurethral resection of prostate [3]. Similar to Jensen’s group, Rollema and Mastrigt observed that patients who were previously obstructed according to the nomogram had improved pressure-flow parameters after the transurethral resection of the prostate. They also found that patients who were categorized as unobstructed prior to surgery had a less dramatic improvement in voiding symptoms compared those who were categorized as obstructed. It is believed that, in those patients who are symptomatic but have an unobstructed pressure-flow curve, the problem is impaired detrusor contractility and surgery for bladder outlet obstruction will not improve their symptoms. They suggest using the Abrams-Griffiths nomogram as possible method to screen patients prior to surgery.
The Abrams-Griffiths nomogram is therefore useful for the diagnosis of bladder outlet obstruction. Specifically, it is helpful to predict which men with bladder outlet obstruction from enlarged prostates will benefit from surgical intervention.
Schäfer Nomogram
Werner Schäfer initially studied aeronautical and spacecraft engineering from the Aachen University in Germany. His background in physics and biomedical engineering was instrumental in his detailed analysis of voiding function. He developed a nomogram in 1990 based on the concept that flow of urine is initiated when the pressure from the bladder is equal to or slightly exceeds the intrinsic urethral pressure. The collapsed urethra differs from a rigid pipe in that intraluminal pressure is required to open the lumen before flow can occur. The pressure point where the urethra opens to allow micturition is also called the urethral opening pressure . A special feature of flow in a collapsible and distensible tube, like the urethra, is that the pressure-flow relation can be controlled by a single small segment acting as a flow-controlling zone. Under physiologic conditions, this zone is at the pelvic floor level; however in pathological outflow conditions, the obstruction itself takes over the role of the flow-controlling zone. As the bladder pressure increases from the urethral opening pressure, the urethra opens and the rate of flow increases sharply. Therefore, by graphing the detrusor pressure vs. flow rate during a course of micturition, one can obtain the urethral resistance to flow (Fig. 2).
Fig. 2
Detrusor pressure vs. flow rate curves . Reproduced from Blaivas J. Multichannel urodynamic studies. Urol. 1984;23:421–438, with permission of Elsevier. (a) Normal: flow is initiated at a pressure of 50 cm H2O, and flow rate increases to almost 20 ml/s with no further appreciable rise in pressure. (b) Bladder outlet obstruction: flow is not initiated until detrusor pressure of 100 cm H2O and, despite further rise in detrusor pressure, maximum flow attains only 6 ml/s
Changes in lumen size and opening pressure can affect the pressure-flow curve separately, thus creating different forms of obstruction: constrictive or compressive. Constrictive obstruction can be exemplified by urethral stricture disease; compressive obstruction is typically seen in benign prostatic hyperplasia. These can be differentiated using urodynamics studies. With a constrictive obstruction, micturition can be initiated and maintained with a normal low pressure, but the energy balance during the mid-flow is unfavorable. In a compressive obstruction, the increased energy demand is not just limited to the mid-flow but also for the initiation and termination of micturition. The higher urethral opening pressure requires a prolonged isovolumetric contraction phase before flow can start and requires a proportionately higher minimum muscle power to maintain flow. The difference in minimum voiding power explains why large residual urine volumes are common in compressive obstructions but rare in constrictive obstructions. For ease of description and understanding, the curve can be rotated to have the flow rate on the Y-axis and detrusor pressure on the X-axis. The flow-pressure plot can show complex patterns; however it is most important to determine the lowest resistance since this is closely related to bladder outlet morphology. The line generated on the flow-pressure plot is a simple quadratic shape, which is referred to as the passive urethral resistance relation (PURR). The slope and position of the passive urethral resistance relation can provide information about the opening pressure and the effective lumen size of the flow. Obstruction caused by benign prostatic hyperplasia has a compressive curve and shifts the micturition curve further to the right on the flow vs. pressure diagram, whereas urethral strictures have constrictive curves and are “flat” (Fig. 3).
Fig. 3
Passive urethral resistance relation . Specific types of obstruction are demonstrated. Features that establish outflow condition, the opening pressure (pmuo) and the effective size, can be altered separately, leading to distinct forms of obstruction. In normal circumstances, once urethra has assumed its minimal resistance (Pmuo), there is little further change in detrusor pressure despite increasing flow rate. In constrictive obstruction, the opening pressure remains the same but a higher pressure is needed during the mid-voiding. In compressive obstruction, the opening pressure is increased because the obstruction acts as the new flow-controlling zone. The pressure is increased not only during mid-voiding but also during initiation and termination of flow. Reproduced from Schäfer W. Principle and clinical application of advanced urodynamic analysis of voiding function. Urol Clin North Am. 1990;17:553–6, with permission of Elsevier
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