See also pp. 201–8 Meyer and colleagues are to be congratulated for writing an excellent review in this issue of Emergency Medicine Australasia of the current experience with the USCOM ultrasonic cardiac output monitor (USCOM, Coffs Harbour, NSW, Australia).1 Just searching ‘cardiac output’ recovers 9500 hits on PubMed from the last 10 years. Informed reviews such as this have an important role in disseminating scientific literature to the clinical reading public, who otherwise simply do not find the time to sift through the mass of literature available. Meyer states that cardiac output is an important parameter when assessing cardiovascular status in the critically ill. Such a point seems obvious, so why is cardiac output monitoring not a part of everyday treatment of emergency patients, in the way that oxygen saturation monitoring has become? The answer lies partly in the lack of progress these monitors have made in their clinical evaluation. The manufacturer has to demonstrate that the monitor has the technological capability to do what is claimed before a monitor is registered by bodies, such as the Therapeutic Goods Administration in Australia, or Medsafe in New Zealand. These data are not normally the subject of clinical journals such as ours. Most of the literature regarding non-invasive cardiac output monitors centres on the topic of diagnostic accuracy, and this is where problems start occurring. All of the articles reviewed by Meyer compare the USCOM monitor with thermodilution using the pulmonary artery catheter (PAC). The PAC has many well-documented issues with accuracy and reproducibility.2,3 Thus, the agreement between modalities in any comparison using the PAC as a reference method is bound to be poor, yet anecdotally the poor agreement reported in some of these studies is one reason why acceptance of non-invasive cardiac output monitors is slow. The fact that many authors use different methods for reporting agreement when comparing monitoring techniques adds to the confusion, and almost certainly delays clinical acceptance of the literature. A larger portion of the answer to the question as to why we do not use these non-invasive cardiac output monitors more lies in the almost total lack of evidence in the literature regarding the impact of non-invasive cardiac output monitors on clinical therapy and outcome. In order to improve outcomes, these monitors have to lead to a change in treatment of the critically ill. There is currently no literature on the capacity for the USCOM monitor to achieve this. However, the USCOM monitor is not alone in this paradox. There are but a handful of papers regarding the ability of the PAC to influence therapeutic decisions, and only recently has the impact of the PAC on patient outcomes been examined with mixed results.4,5 Literature on other methods of non-invasive, or minimally invasive, cardiac output monitoring is similarly lacking. While cardiac output is a vital component of oxygen delivery, it is also important to monitor the effectiveness of this oxygen delivery. Most of the literature in this field concentrates on trauma and major sepsis patients, but the lessons are applicable to all patients. Changes in lactate and base excess have been linked to patient outcome, as has the rapidity of restoration of their values to normal.6,7 The most critical point is to realize that inadequate oxygen delivery and low cardiac outputs often occur in patients with normal vital signs.8,9 Any non-invasive cardiac output monitor with a role in the ED will need to be supported by the publication of clinical trials where they are shown to lead to changes in therapy, which improves patient outcomes. Comparing these monitors to an existing and problematic reference standard (PAC), which is rarely used in the ED anyway, is simply not enough.