Table of Contents

Question 1 from the second paper of 2009 and Question 24 from the second paper of 2006 asked for a detailed discussion about the utility of using CPP as a therapeutic target. Unfortunately, the college model answer was very brief; it probably reflects some sort of minimal level of competence. An ideal synopsis of this issue is offered by LITFL. No resource improves on the brevity or clarity of their short note on this concept. If brevity and clarity are not your thing, then you should probably read this 2013 review article from the BJA, by Kirman and Smith, and the canonical "Guidelines for the management of severe traumatic brain injury" from the Brain Trauma Foundation.

There are four volumes and four capacities recognised in respiratory physiology, where lung volumes are measurable gas-filled spaces in the lung, whereas capacities are combinations of two or more volumes (where the definition of capacity is the measure of the lungs' ability to hold a gas). 

The FRC is the volume of gas present in the lung at end-expiration during tidal breathing. It is composed of ERV and RV. This is usually 30-35 ml/kg, or 2100-2400ml in a normal-sized person. It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure.  The measurement of FRC is an important starting point for the measurement of other lung volumes, and its decrease has consequences for gas exchange and lung mechanics

Of the volumes and capacities, the most important to measure is the FRC, because this volume can then be used to determine all the other volumes by means of spirometry. The FRC can be determined by a number of ways. The most common are body plethysmography, nitrogen washout and inert tracer gas dilution. After determining the FRC, all other lung volumes can be determined by spirometry.

The trigger phase variable determines how a mechanical breath is initiated. This variable determines whether a mode of ventilation can be described as "mandatory" or "spontaneous". Historically, this has been a purely machine-driven affair – but with advent of microprocessor-controlled ventilators, mechanical ventilation has become more user friendly (where the user is the patient). Patient-triggered modes are generally more comfortable, and can improve the work of breathing.

Work is the product of force and distance, and is measured in Joules (1J = 1N per 1m). In respiratory physiology, work is the product of pressure and volume. Several components contribute to the total work of breathing: elastic work (work done to overcome elastic recoil of the chest and lungs) and resistive work (work done to overcome tissue resistance and airway resistance). There is also some work done to overcome respiratory inertance and to compress intrathoracic gas, but these are negligible..

This is an altered level of consciousness attributed to the consequences of acute or chronic liver failure. Ammonia is only one of the aetiological agents. But, it is certainly the one everyone always thinks of. Copious amounts of blood are being sent for ammonia levels every day. Surely, there must be some reason behind this. In order to derive some meaning from this seemingly mindless ammonia-lust, one must explore the mechanisms of metabolic derangement which arise within the liver failure patient.

This page acts as a footnote to the "Boston vs. Copenhagen" chapter from Acid-Base Physiology by Kerry Brandis. His chapter explores the epistemology of acid-base interpretation systems by means of which we might be able to determine whether a patient has a single or mixed acid base disorder; i.e. whether there is a purely metabolic or a purely respiratory disturbance, or some mixture of the two. As it happens, there are two well-accepted systems for doing this, each with its own merits and demerits. These are the Boston and Copenhagen methods of acid-base interpretation.

Posture changes everything about respiratory mechanics. Virtually all the volumes and lung compliance are at their best in the upright position, and the more supine the position becomes the poorer the lung compliance and the smaller the FRC. Going prone has the effect of improving compliance and FRC slightly, but its greatest benefit is in improving the homogeneity of pleural pressure distribution.

Pleural pressure is usually negative, due to the recoil of the chest wall, the recoil of the lungs, and the negative pressure exerted by the lymphatic system, In the upright subject, it is more negative in the apices, and less negative in the bases. The vertical pleural pressure gradient is the difference between the apical and basal pleural cavity pressures. This gradient is due to the effects of gravity (i.e. weight of the lung), pressure from mediastinal contents and pressure from abdominal contents.

Respiratory resistance can be measured in several ways. The most popular (generally viewed as a gold standard of sorts) is whole-body plethysmography. For uncooperative subjects, forced oscillation or airway interruptor techniques can be used, as they require minimal input from the subject. Rhinomanometry can also be sued, but it requires uncomfortable gag-inducing nasal probes. All methods in one way or another produce some flow data and some pressure data which can be used to calculate resistance.

The inspiratory hold manoeuvre abolishes the pressure contribution from the airway resistance and reveals the pressure in the alveoli. This is available on virtually every specialist-grade ventilator, and consists of a manual override of the expiratory valve, forcing it to close and essentially producing a super-syringe-style test of lung compliance, where the entire respiratory system (including the ventilator circuit) is challenged with a static volume..

The pressure waveform can give one information about the compliance of the different parts of the respiratory system. The waveform which is of greatest interest is the one generated when you put the patient on a mode of ventilation which features a constant inspiratory flow, such as  a volume controlled mode of ventilation. In the presence of constant flow, the waveform represents the change in circuit pressure over time.

Surface tension is the force of attraction between liquid molecules at the liquid-gas interface which tends to minimise surface area. The relationship of this force to sphere size is described by the Law of Laplace. Smaller partially deflated alveoli will have lower compliance and higher Laplace pressure at any given surface tension. With lung surfactant, which is mainly phospholipid, surface tension is reduced. The results of this are improved lung compliance. Surfactant is also party responsible for the phenomenon of lung hysteresis, and it protects the alveoli from oedema fluid..