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Yes but...
- Nevil P Hutchinson, Robert Hearn, Specialist Registrar in Anaesthesia (26 January 2006)
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Is it Wise to Use Pressure Control-Derived Parameters to Predict Fluid Responsiveness?
- Frédéric Michard, Mauricio B. Ferri, Rogério H. Passos (14 December 2005)
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Nevil P Hutchinson, Consultant Cardiac Anaesthetist Royal Sussex County Hospital, Brighton, Robert Hearn, Specialist Registrar in Anaesthesia
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This was an interesting paper, especially for those interested in predicting the possible effects of increasing preload. Unfortunately I can see little utility in such a test. With such poor ROC curves one is hardly likely to withold fluid on the basis of a negative prediction. Likewise, if the cardiac index is adequate, and ditto the arterial blood pressure, one is unlikely to increase the preload no matter how encouraging the RSVT. Furthermore, although CVP may have little value in terms of assessing where on the "Starling curve" a patient currently resides, studies that use "dynamic" assessment of the response of CVP to fluid challenges show improved outcomes.1 At what point does one stop giving fluid boluses? How many patients would develop pulmonary oedema before the RSVT suggested no more fluid? How would diastolic dysfunction influence the RSVT? What if a patient isn't ventilated? Does the RSVT need to be indexed to pulmonary compliance? What would be interesting would be to see such a technique being used in real patients, with evidence that it can influence useful endpoints. nevil.hutchinson@bsuh.nhs.uk 1. Venn R, et al Randomised controlled trial to investigate the influence of the fluid challenge on duration of hospital stay and perioperative morbidity in patients with hip fractures. BJA 88(1):65-71 (2002) Conflict of Interest:None declared |
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Frédéric Michard, MD, PhD Paris, France,, Mauricio B. Ferri, Rogério H. Passos
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We read with interest the study by Preisman et al. (1) regarding the prediction of fluid responsiveness in patients undergoing cardiac surgery. The authors compare the value of several functional haemodynamic parameters and conclude that the Respiratory Systolic Variation Test (RSVT) and the arterial Pulse Pressure Variation (PPV) are the most accurate predictors of the haemodynamic response to volume loading. So far, many clinical studies have shown that PPV is a very accurate predictor of fluid responsiveness in patients ventilated in volume control mode (2). Nevertheless, this parameter has some drawbacks, limiting its use in some clinical circumstances, for example in patients ventilated with low tidal volumes (e.g. 6 ml/kg) (2,3). In an attempt to bypass this limitation and propose a parameter independent of the settings of mechanical ventilation, Perel et al. recently introduced the slope of the RSVT (4). This parameter is obtained by applying a sequence of three consecutive mechanical breaths with inspiratory pressures of 10, 20, and 30 cm H2O. The minimal values of the systolic blood pressure during each of the three breaths are measured and plotted against the corresponding values of the inspiratory pressure to obtain a slope. This slope is assumed to be proportional to the patient’s preload reserve. However, the drop in blood pressure following each standardized mechanical breath is highly dependent on the inspiratory rise in pleural pressure and hence on the static compliance of the respiratory system (Cst,rs). Indeed, the transmission of airway pressure to the pleural space has been shown to be roughly equal to Cst,rs (when Cst,rs is 50 ml/cmH2O the percentage of transmission of airway pressure is around 50%) (5). That is, if the airway pressure is 20 cmH2O, pleural pressure is 10 cmH2O when Cst,rs is 50 ml/cmH2O (normal lung) but only 5 cmH2O if Cst,rs is 25 mL/cmH20 (ARDS). In this regard, for a given volume status, the successive drops in blood pressure during the RSVT maneuver are more market in patients with normal lungs than in patients with ARDS. One could imagine that applying three consecutive incremental level of airway pressure would settle the problem, but even the slope of the relationship between airway pressure and pleural pressure depends on Cst,rs, and so does the slope of RSVT. In other words, the slope of the RSVT depends not only on preload reserve but also directly on lung compliance. In contrast, PPV – as assessed so far, i.e. in volume control mode - has the advantage not to be directly affected by lung compliance. Indeed, for a given tidal volume, any decrease in lung compliance, and hence in transmission of airway pressure to the pleural space, is compensated by an increase in airway (plateau) pressure, such that pleural pressure remains roughly the same during mechanical insufflation. When tidal volume is 500 mL and Cst,rs is 50 mL/cmH20 (normal lung), plateau pressure is 10 cmH20 and the transmission is 50%; if Cst,rs is 25 mL/cmH20 (ARDS), plateau pressure is 20 and the transmission is 25%. Thus, in both cases pleural pressure – the main determinant of changes in blood pressure - goes up around 5 cmH2O during mechanical insufflation. In other words, the relationship between tidal volume and pleural pressure is not affected by lung compliance. In summary, volume control mode-derived PPV is dependent on tidal volume (that could be standardized), but not significantly affected by lung compliance (2). RSVT is independent of the settings of mechanical ventilation by definition (since airway pressure levels are standardized), but highly dependent on lung compliance (that cannot be controlled). Therefore, we do believe that Preisman et al. (1) were able to report such a good predictive value for RSVT because they studied a homogeneous patient population in terms of lung compliance (patients free of lung disease undergoing cardiac surgery), and we suspect that RSVT would behave very differently in critically ill patients with acute lung injury or pulmonary edema or COPD. Therefore, we do not agree with Preisman et al. when they claim that the “newly introduced RSVT seems to have a promising potential” because its dependency on lung compliance represents a significant drawback as compared to volume control-derived dynamic parameters. Frédéric Michard, MD, PhD Paris, France E-mail: michard.frederic@free.fr Mauricio B. Ferri, MD Sao Paulo, Brazil Rogerio H. Passos, MD Sao Paulo, Brazil References 1. Preisman S, Kogan S, Berkenstadt H, et al. Predicting fluid responsiveness in patients undergoing cardiac surgery: functional haemodynamic parameters including the Respiratory Systolic Variation Test and static preload indicators. Br J Anaesth 2005; 95:746-755 2. Michard F. Changes in arterial pressure during mechanical ventilation. Anesthesiology 2005; 103:419-428 3. Michard F. Volume management using dynamic parameters: the good, the bad, and the ugly. Chest 2005; 128:1902-1904 4. Perel A, Minkovich L, Preisman S, et al. Assessing fluid responsiveness by a standardized ventilatory maneuver: the respiratory systolic variation test. Anesth Analg 2005; 100:942-945 5. Teboul JL, Pinsky MR, Mercat A, et al. Estimating cardiac filling pressure in mechanically ventilated patients with hyperinflation. Crit Care Med 2000; 28: 3631-3636 Conflict of Interest:None declared |
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