A prospective observational study to compare relationship between minimally invasive and non-invasive cardiac output measurement in septic patients with or without vasopressor therapy admitted to intensive care unit

In intensive care facilities, one of the most frequently encountered situation is sepsis. It is a life-threatening organ dysfunction caused by an unregulated response of a host which may lead to septic shock, which is a state of widespread reduction in effective tissue perfusion.1,2 This shock may be due to vasodilation or sepsis induced cardiomyopathy which depresses myocardial intrinsic contractility. Cardiac output and oxygen delivery may be impaired in sepsis due to primary myocardial dysfunction and decreased systemic vascular resistance (SVR) requiring the use of vasopressors.3

Cardiac output (CO) is the surrogate measure for myocardial contractility and decreased SVR and is often used to guide treatment and fluid therapy. Pulmonary artery cauterization (PAC) is the gold standard but is associated with complications like pneumothorax, arrythmias, infections, pulmonary artery injury, valve injury, knotting and thrombosis leading to embolism and it is not routinely recommended.4

Cardiac ultrasound is an important tool for the assessment of cardiac structures, filling status and the contractility of the heart in critically ill patients. It is  a reliable tool and considered as precise as the reference method PAC. Calculation of left ventricular volumes from apical four and two chamber views is the accepted method to calculate stroke volume which is the difference between end diastolic and end systolic left ventricular volume. But cardiac ultrasound is performer dependant and cannot give a real time measurement.5,6

Minimally invasive pulse contour analysis is a relatively newer tool which is based on the principle that area under the systolic part of the arterial pressure waveform corresponds to the stroke volume, which in turn maybe influenced by SVR. It is termed as arterial pressure based cardiac output (APCO) and requires an arterial line. It can also measure stroke volume variation and pulse pressure variation which are important in predicting fluid responsiveness of the patient. More-over it is not performer dependant and doesn’t need any external calibration. It gives beat to beat variability and real time cardiac output (CO) measurements.7,8

There are lot of studies comparing the invasive and non-invasive methods, however due to difference in measurement methods, the minimally invasive CO may not be reflective of the measurement by ultrasound in certain conditions. There are only limited comparative studies between minimally invasive and non-invasive methods in septic patients with or without vasopressor therapy admitted to  intensive care unit. Therefore, we have planned to study the relationship between the minimally invasive method with the non-invasive method of cardiac output measurement.