Over 100 years ago Karl Ludwig stated that: The fundamental problems in the circulation derive from the fact that the supply of adequate amounts of blood to the organs of the body is the main purpose of the circulation while the pressures that are necessary to achieve it are of secondary importance; but the measurement of flow is difficult while that of pressure is easy so that our knowledge of flow is usually derivatory. Are we, in 1998, any nearer to the routine monitoring of flow? The more salient question may be whether flow measurement is actually useful in terms of clinical management or patient outcome. The wherewithal to monitor flow exists. Techniques for cardiac output measurement, albeit of varying accuracy, invasiveness and complexity, have been available commercially for the past 25 years, although their use in the UK is relatively sparse, both in intensive and coronary care units1 and operating theatres.2 The national confidential enquiry into perioperative deaths2 covering the years 1993–94 reviewed 1802 deaths occurring within 30 days of an operation. Three quarters of these patients were deemed moderate to very high risk, yet only 4.2% had a Swan-Ganz (pulmonary artery) catheter in situ during surgery. Several studies have shown outcome benefit from flow directed haemodynamic manipulation in the high risk surgical patient using either invasive3 4 or non-invasive5 6monitoring techniques. Only sporadic studies have demonstrated an advantage for the critically ill ICU patient.7 However, a recent retrospective study by Connors and colleagues8suggested that patients receiving a Swan-Ganz catheter on day 1 of their ICU admission were 39% more likely to die compared with patients matched for disease and illness severity by complex statistical manipulations who did not receive the catheter. The accompanying editorial9 advocated an immediate moratorium …