Table of Contents

Mean circulatory filling pressure (MCFP) is the pressure that would be measured at all points in the entire circulatory system if the heart were stopped suddenly and the blood were redistributed instantaneously in such a manner that all pressures were equal. Mean systemic filling pressure (MSFP) is the pressure in only the systemic circuit. The main determinants of MCFP and MSFP are total blood volume and venous resistance.

This chapter deals with the practical aspects of measuring the performance characteristics of the arterial pressure transducer system. The theoretical aspects of frequency response and damping coefficient are fascinating but likely not essential to the exam-oing candidate; as such they have been dismissed to the largely apocryphal Principles of Pressure Measurement section.

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.

Preload is defined as myocardial sarcomere length just prior to contraction. This is a suitable (memorable, short, non-insane) definition of preload which Part One writers settled on, and it relates preload directly to end-diastolic volume. In turn, the end-diastolic volume is determined by the filling pressure of the cardiac chambers, and by the compliance of those chambers.

Venous return is the rate of blood flow into the heart from the veins. At a steady state, venous return and cardiac output are equal. Venous return can be expressed as VR = (MSFP - RAP) / VR,  where MSFP is mean systemic filling pressure, RAP is right atrial pressure and VR is the venous resistance.  Factors which influence venous return include total venous blood volume, venous smooth muscle tone, right atrial and pericardial properties, blood viscosity, vasoactive drugs and autonmic tone.

Neonatal respiratory physiology is characterised by higher chest wall compliance, lower lung compliance and increased airway resistance. Work of breathing is increased, even accounting for the increase in respiratory demand and metabolic rate. Generally, a normal resting respiratory rate is around 30-50. Gas exchange is also markedly different, with a left-shifted oxygen-haemoglobin dissociation curve and an increased haemoglobin concentration.

The changes in respiratory physiology associated with pregnancy can be summarised as "higher minute volume and lower FRC", as those are the most important changes which have clinical relevance. The airway also becomes more oedematous, there is a progesterone-associated chronic respiratory alkalosis, the chest wall undergoes some remodelling, and respiratory resistance decreases.

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. 

Diabetic ketoacidosis is a state of insulin deficiency, characterised by rapid onset, extreme metabolic acidosis, a generally intact sensorium, and only mild hyperglycaemia. DKA comes up frequently in the CICM SAQs, but usually as an ABG interpretation exercise. This chapter focuses on the medical side of DKA, including its causes, manifestations, complications, and management strategies.

Bioavailability is the fraction of the dose which reaches systemic circulation intact. IV bioavailability is by definition 100%. "Absolute" bioavailability compares one non-IV route with IV administration, and "relative" bioavailability compares one non-IV route or formulation with another (instead of using IV route as a reference). Bioavailability is measured using the area under the concentration-time curve; according to Dost's Law the ratio of AUCs is the bioavailability value. 

Control of ventilation is a function of the medulla, with descending voluntary control from the cerebral cortex via the pons. The medullary respiratory centres integrate input from the sensory organs such as the carotid glomus and aortic chemoreceptors, as well as central chemoreceptor regions. These inputs modulate the rhythmic activity of the central respiratory pacemaker regions. Efferent fibres travel via the vagus nerve, phrenic nerve, glossopharyngeal nerve and the upper motor neuron fibres of the spinal cord to innervate the muscles of respiration.

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.

Pregnancy results in several pharmacokinetic and pharmacodynamic changes. Oral administration is affected by delayed gastric emptying; distribution is affected by changes in the fat and water content of the body, and metabolism is afected by changes in hepatic blood flow and enzyme activity. Clearance of many substances is increased by the increased glomerular filtration rate. Pharmacodynamic changes are also numerous, eg. increased sensitivity to both local and general anaesthetics. Discussion of adverse effects and dosing adjustments must take into account toxicity to the foetus.