Table of Contents

This chapter is not relevant to any recent part of CICM Second Part Exam, probably because the whole topic of pregnancy-associated physiolgoical changes has migrated to the Primary Exam section at some stage in 2007-2008. Before that exam existed, the college had asked about this in Question 18 from the first paper of 2006, but since then there have been no SAQs on it. As such, all of this sort of material has been transferred to the Part One section, leaving only a brief summary to assist with revision. 

Osmolarity is the measure of solute concentration per unit volume of solution. Osmolality is the measure of solute concentration per unit mass of solvent. You never measure osmolarity in practice, because water changes its volume according to temperature (but mass remains the same, and so it is more convenient and consistent).

In the absence of serious urgency for dialysis, the kidneys should be offered a chance to recover before the intensivist resorts to the use of dialysis. This strategy does not seem to change mortality and offers the opportunity for up to half of the patients to get away without any RRT whatsoever, without extending their ICU stay or hospital stay. Moreover it allows time for the patient to be pampered with the sort of refinements which only the ICU has to offer With careful haemodynamic resuscitation and biochemical correction a proportion of patients (those less sick at baseline) will probably never need dialysis, and be grateful for having fewer holes in their necks and groins.

This chapter deals with protecting ICU patients from their own gastric secretions. The group at greatest risk (over 20-fold) are the mechanically ventilated patients, followed by those with coagulopathy and shock. Broadly, the options include antacids (old-school), sucralfate (also outdated), H2-receptor antagonists and PPIs. Of these, the PPIs appear to have the greatest effect, but the NNT is 37 when comapred to the H2As. Considering the cost of ranitidine is just 1 US cent per tablet, the extra expense is difficult to justify.

The cardiac and vascular function curves describe the cardiac response to preload and the venous response to cardiac output as two interdependent functions. The cardiac function curve is cardiac output as a function of right atrial pressure. As contractility increases, the curve shifts up, and a plateau is seen with higher RA pressures. The vascular function curve is venous return as a function of right atrial pressure; a plateau is seen with right atrial pressure below 0 mmHg. The curves intersect at the steady-state operating point where cardiac output and venous return are equal.

Autoregulation of regional blood flow is defined as the tendency for blood flow to remain constant despite changes in arterial perfusion pressure. This is mediated by several mechanisms: myogenic, metabolic, flow or shear-associated, and conducted vasomotor responses. Additionally, paracrine mediators (eg. inflammatory mediators) can regulate regional blood flow in the setting of injury or infection. Lastly, there are multiple reulatory mechanisms which are unique to specific organ system areas, eg. tubulointerstitial feedback in the renal regional circulation

Systemic vascular resistance is one of the major regulatory mechanisms which control blood pressure, and its main determinants are the length of the blood vessels, the viscosity of the blood and the radius of the vessels. Arterioles of around 200μm diameter tend to produce most of the resistance in the systemic circulation. Their radius is under control by systemic events (eg. the arterial baroreceptor reflex) as well as a host of locally acting mechanisms.

A portal system is two capillary beds not separated by the heart, i.e. some sort of conduit "portal" vessel connects the two sets of capillaries. The three main portal systems are the hepatic, pituitary and renal.

The uteroplacental circulation is unique: it is in essence two distinct circulatory systems in parallel (foetoplacental and maternoplacental). At term, it receives about 800ml/min of blood flow, or something like 14-15% of the total cardiac output. The maternoplacental circulatory system is also unique because it has no arteriolar resistance vessels, and is therefore incapable of autoregulation. The uterus and placenta compensate for decreased perfusion pressure and flow by increasing their oxygen extraction ratio.

Pneumocephalus, pneumoencephalus, intracranial pneumatocele or aerocele are all terms for air in the head, where it is not meant to be. It can be a normal consequence of brain surgery and is usually not a major thing, except were there is too much air, or when it ends up under tension. This condition has never come up in the exam until a relatively recent radiology question (Question 14.2 from the first paper of 2016). The college presented us with a CT slice depicting the classical "Mount Fuji" sign, in a patient who has lost consciousness following a C5-6 epidural abscess drainage.

Skeletal muscle is unique among organ systems, in that it can vary its blood flow by a factor of 1,000. It can go from a maximaly vasoconstricted state in haemorrhagic shock (flow of 0.1-0.4ml/min/100g) to a maximally vasodilated state during peak exercise (400ml/min/100g). This vascular responsiveness is mediated by intrinsic myogenic mechanisms and metabolic byproducts which act as vasodilators. Under normal circumstances, the sympathetic nervous system maintains a high resting vascular tone, limiting flow to resting muscle.

The candidates have been asked to compare different CRRT modalities in numerous past paper questions. Typically, the college has expected the answers to be presented in a tabulated format. The table in this chapter makes an attempt to combine all these tables into a single reference.

The splanchnic circulation by definition consists of blood vessels which supply and drain the liver, spleen, stomach, pancreas, and the amall and large intestine. Owing to their low oxygen extraction ratio, these organs tend to have no need of blood flow autoregulation to support their metabolism, as they can just extract more oxygen instead. Thus, the splanchnic circulation mainly regulates its blood flow according to demands made by digestion, and after a meal intestinal blood flow can be up to 35% of the total cardiac output.

This chapter deals with the indications, contraindications and complications of blood transfusion in the ICU. The contents and properties of packed red blood cells, the physiology of acute haemorrhage and the physiological responses to blood transfusion are detailed in other chapters. The National Blood Authority of Australia has issued a series of practice guidelines which can be helpful in guiding the shaking bloodied hand of the ICU practitoner. The guidelines have been endorsed by our college, which means that at least some of the Australian intensivists must agree with them. The following entries are a summary of these clinical practice guidelines.