Table of Contents

Fick's principle can be summarised as " the total uptake of (or release of) a substance by the peripheral tissues is equal to the product of the blood flow to the peripheral tissues and the arterial-venous concentration difference (gradient) of the substance". In other words, CO = VO2 / (Ca - Cv). This principle underlies the gold standard of measuring cardiac output. The indirect version of this method relies on estimates for one or more of the variables, which makes it easier to use in clinical practice at the cost of some accuracy.

Question 2e from the first paper of 2000 presents the candidate with a pulmonary embolism scenario, and explores the possibility of IVC filter insertion. The college wanted a list of pros and cons. Since this early paper, the cons seem to have multiplied. The rushed exam candidate should probably limit their reading to the content of this  highly critical 2013 article from PulmCCM, as well as the excellent LITFL page on IVC filter placement. For the time-rich exam candidate, this issue of CHEST published all of the most recent (9th edition) ACCP guidelines. Whereas previously they had a specific set of guidelines for IVC filters, they have now split them all up between different indications, and one needs to hunt though the articles for the recommendations. PulmCCM link to a summary statement which describes them.  The British also have a set of discrete recommendations available (from 2006).

The most important determinants of total blood oxygen content are the effective haemoglobin concentration and oxygen saturation. Solubility plays little role -only about 1-2% of the total oxygen content is dissolved (though this increases with extreme hypothermia). The remaining 98% are bound to haemoglobin.

The arterial pressure wave (which is what you see there) is a pressure wave; it travels much faster than the actual blood which is ejected. It represents the impulse of left ventricular contraction, conducted though the aortic valve and vessels along a fluid column (of blood), then up a catheter, then up another fluid column (of hard tubing) and finally into your Wheatstone bridge transducer. A high fidelity pressure transducer can discern fine detail in the shape of the arterial pulse waveform, which is the subject of this chapter.

The pressure transducer system can be described as a second-order dynamic system, a harmonic oscillator. The natural frequency of the system is the frequency at which it will oscillate freely (in the absence of sustained stimulus) Resonance is the amplification of signal when is its frequency is close to the natural frequency of a system.

The arterial pressure wave travels at 6-10 metres/sec. The cannula in the artery is connected to the transducer via some non-compliant fluid-filled tubing; the transducer is usually a soft silicone diaphragm attached to a Wheatstone Bridge. It converts the pressure change into a change in electrical resistance of the circuit. This can be viewed as waveform.

A transducer by definition is a device which converts energy from one form to another. Medical pressure transducers are usually piezoresistive strain gauges, incorporating a Wheatstone bridge. They consist of a thin semiconductive silicon membrane, onto which resistor circuitry is etched. Deformation of the membrane results in a change of the circuit resistance, which is measured using the Wheatstone bridge. The transducer is coupled to the system being measured by means of a fluid column, contained within a length of incompressible tubing.

An electrocardiogram is a graph of cardiac voltage plotted versus time. It is measured from paired electrodes positioned on the body surface, where they can detect the faint (0.5-2.0 mV) potential difference which is produced by the electric field of the "cardiac dipole", the mathematical abstraction used to describe a summed composite electric field produced by all the action potentials of depolarising myocytes. As the external surface of the myocytes changes from a net positive to a net negative charge, the charge difference between different ends of the myocyte produces this electric field, which changes magnitude and direction over the course of the cardiac cycle.

Though all anions deserve love to the same equal extent, phosphate has risen to become exceptional in receiving attention in the form of an entire chapter.