# The Basic Physiology Of Cardiac Output And Its Measurement

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egin{equation} DO_{2} = CO imes CaO_{2} end{equation} where CO denotes cardiac output and \$ CaO_{2} \$ denotes oxygen content of arterial blood. The oxygen content can be calculated by the formula: egin{equation} CaO_{2} = (1.36 imes Hb imes SaO_{2} )+ ( 0.0031 imes PaO_{2}) end{equation} Where Hb denotes haemoglobin, \$ SaO_{2} \$ denotes saturations from pulse oximetry, \$ PaO_{2} \$ denotes arterial oxygen tension and A major determinant of oxygen delivery, as noted earlier, is the cardiac output. I will discuss the basic physiology of cardiac output and its measurement in further detail now. subsection{Cardiac output} Cardiac output (CO) is the amount of blood pumped out of the heart in a minute. This is a product of the heart rate per minute and the stroke volume (blood volume per heart beat). Cardiac output is usually expressed as cardiac index (CI) (CO per unit body surface area). Cardiac index is crucial to ensure oxygenation of tissues. The normal range of CI in humans is between 3.5-6 l/min/\$ m^{2} \$. Studies by Starling on dogs were the first to describe the parameters that affect CO. According to Frank-Starling 's law, the force of contraction of cardiac muscle is directly proportional to the length of stretch. Blood fills the ventricle during diastole and stretches the cardiac muscle. Consequently, the amount of blood contained in the ventricle at end of diastole - the end diastolic volume - determines the length of stretch of the cardiac muscle which in turn determines the cardiac index.cite{Patterson:1914wka}cite{Starling:1927vt} The increase in force of contraction as a function of stretch is a non-linear curve. Therefore, the stroke volume initially increases with ... ... middle of paper ... ...duct guide.cite{Phillips:2009tz}} label{uscom curve} end{minipage} subsection{Oxygen consumption} Oxygen consumption depends on cellular respiration. The amount of oxygen utilised by the cells to generate ATP - in the electron transport chain in the mitochondria - is the ultimate determinant of consumption. Therefore oxygen consumption can act as a marker of the mitochondrial function and the functional status of cells. Other surrogate measures like mixed venous saturations, lactate and extit{\$ pCO_{2}\$} difference have also been used. The oxygen extraction rate (OER) is the ratio of oxygen consumption to oxygen delivery. The OER is utilised to analyse oxygenation of tissues. egin{equation} OER = {frac {VO_{2}} {DO_{2}}} end{equation} Where OER denotes oxygen extraction rate, \$ VO_{2} \$ denotes oxygen consumption and \$ DO_{2} \$ denotes oxygen delivery.