Clinical Case Scenario
A patient in the intensive care unit with septic shock and acute kidney injury is receiving continuous Veno-venous hemofiltration (CVVH). Over the past few hours, the treating team has become concerned about rising filter pressures and possible early filter clotting. The CRRT machine displays the following pressures:
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Wrong Answer: A. 95 mmHg
Right Answer: B. 105 mmHg
Exlaination: The correct answer is 105 mmHg. In CRRT, the transmembrane pressure (TMP) reflects the pressure gradient across the filter membrane and is used as an indicator of filter performance and developing membrane fouling or clotting. (Michel T et al. Curr Opin Crit Care. 2018) TMP is calculated using the formula: TMP = [(Filter pressure + Return pressure) / 2] − Effluent pressure. TMP = Pin[(Filter pressure] + Pout [Return pressure]/2 − Peff In this vignette, the filter pressure is 180 mmHg, the return pressure is 90 mmHg, and the effluent pressure is 20 mmHg. Substituting these values into the formula gives: TMP = [(180 + 90) / 2] − 20. First, add the filter and return pressures: 180 + 90 = 270. Then divide by 2 to obtain the mean blood-side pressure across the membrane: 270 / 2 = 135 mmHg. Finally, subtract the effluent pressure: 135 − 20 = 105 mmHg. Therefore, the TMP in this circuit is 105 mmHg. This value represents the effective pressure required to drive ultrafiltration across the haemofilter membrane. The formula uses the average of the filter pressure and return pressure because these approximate the mean pressure on the blood side of the membrane, while the effluent pressure represents the pressure on the filtrate side. A rising TMP suggests increasing resistance to filtration, commonly due to haemoconcentration, protein deposition, or early clot formation within the filter. In the clinical setting described, concern about rising filter pressures and possible early filter clotting makes TMP especially relevant, because a progressively elevated TMP may indicate that the filter is becoming less permeable and may soon fail. Thus, calculating TMP not only identifies the correct numerical answer but also helps assess circuit function and anticipate the need for intervention, such as adjusting filtration fraction, reviewing anticoagulation, or preparing for filter change. CLINICAL PEARLS 1. TMP = “Filter Health Meter” • Rising TMP = early clotting / membrane fouling • Often rises before filter actually clots 2. TMP > 150–200 mmHg → Danger zone • Suggests imminent filter failure • Plan: 👉 Check anticoagulation 👉 Reduce filtration fraction 👉 Prepare filter change 3. Why average pressure? • Blood side ≠ single pressure • So we use: 👉 Mean of filter + return pressure 4. Common causes of rising TMP • Haemoconcentration (high FF) • Inadequate anticoagulation • Protein deposition (sepsis) • High ultrafiltration rate 5. TMP vs Access pressure (exam trap ) • TMP → membrane problem • Access pressure → vascular access issue Bottomline- “High TMP = High resistance → Low filter survival”
Wrong Answer: C. 115 mmHg
Wrong Answer: D. 125 mmHg
