Cardiac output and oxygen delivery, and the treatment of high risk surgical patients
In the peri-operative period physiological changes take place in cardiac function and the amount of oxygen delivered to the tissues. Reductions in cardiac output, oxygen delivery and tissue perfusion are more pronounced in the non-survivors of surgery, usually occurring in elderly patients with comorbidity and reduced physiological reserve. Furthermore patients who do not survive surgery ultimately die from multiple organ dysfunction syndrome (MODS), triggered in part by an overwhelming inflammatory reaction to various stimuli including reduced tissue perfusion. Because normalization of physiology, as indicated by achieving goals obtained by survivors, is established treatment in anaesthesia and critical care, and because tissue hypoperfusion and hypoxia are underlying factors in the development of MODS, it was hypothesized that all higher risk surgical patients should be treated preemptively to achieve physiological goals defined by survivors. Patients are monitored with a pulmonary artery catheter or oesophageal Doppler, treatment being instituted to predefined goals for cardiac output, oxygen delivery or stroke volume using fluids, vasodilators and inotropes. Other goals for treatment have also been suggested. 21 randomized controlled clinical studies have been undertaken to test this hypothesis; 4258 patients have been studied. The combined odds ratio for reduction in mortality in the treatment groups is 0.67 (95% confidence interval 0.54-0.83). Randomised, controlled studies have been undertaken following the same approach in non-surgical patients, these failed to show any improvement. Despite identifying a number of problems with the published studies such as details of patient enrolment, adherence to trial protocols, and the effect on statistical analysis of the crossover affect (whereby some patients in the control limbs of the studies can be expected to achieve the goals of treatment spontaneously), it is concluded that the treatment of higher risk surgical patients to goals for cardiac output, oxygen delivery or stroke volume reduces postoperative mortality.