A free online resource for Intensive Care Medicine.
An unofficial Fellowship Exam (CICM Part 2) preparation resource.
Deranged Physiologyis a slowly growing archive of discussions and study notes relevant (or if not relevant, then at least interesting) to the practice of intensive care medicine. The content provides an introduction to the fundamental themes in intensive care: mechanical ventilation, vasopressors, electrolyte management, hemodynamic monitoring, dialysis, and so forth. Attention is directed at equipment in intensive care, and there are attempts to revisit interesting pharmacology and physiology. The aim of this resource is to supplement the bedside teaching of senior staff, and to consolidate resources for intensive care trainees in the initial stages of their training.
End-tidal capnography has appeared multiple times in the CICM exams. Whereas the Part I questions are typically concerned with how it is measured, in Part II the candidates are expected to interpret the waveforms and comment on the utility of the practice. This chapter is more concerned with EtCO2waveform interpretation.
Assessment for extubation and weaning from mechanical ventilation is a topic which has appeared in numerous past paper SAQs. The college also loves to use this topic for hot cases. Issues regarding post-extubation stridor, tracheostomy and emergency cricothyroidotomy are explored in the Airway Management section; this chapter is more concerned with spontaneous breathing trial, RSBI and the various indices used in the assessment of readiness for liberation from the ventilator.
The expired CO2 waveform can identify a variety of pulmonary and airway pathology. It all but eliminates the need to auscultate the lung, for the lazy intensivist who never lays his hands on the patient. Do you really need to hear a wheeze? The end-tidal trace, sloping up, not only alerts you to the bronchospastic airways disease, but also to the fact that it is improving with your nebs.
Positioned on the end of the ETT, the capnograph should be able to pick up the expired carbon dioxide (EtCO2 ) in whatever gas happens to be wafting past it, and this concentration is recorded on a graph. This is plotted as a very useful waveform. The pattern of CO2 concentration over time has features which give us some information about the gas movement in the airways and in the alveoli. These features, though they have no standardised names, are well recognised, and sometimes crop up in the fellowship exams as questions demanding certain waveforms to be graphed.
The expiratory hold pauses the breath in expiration, preventing the delivery of more breaths. With the flow stopped, alveolar pressure equilibrates with the ventilator circuit and can be measured. The circuit pressure at the end of an expiratory hold is therefore thought to be representative of intrinsic PEEP in at least some of the lung units. The volume which escapes during the inspiratory hold is the trapped gas volume. One limitation is the need to perform this test with a paralysed sedated patient. Another limitation is the fact that dynamic airway closure prevents the equlibration of pressure between the ventilator circuit and the lung units with the highest pressure. Thus, the expiratory hold manoeuvre underestimates intrinsic PEEP.
Amperometry, or galvanometry, or polarography (turns out these are not interchangeable terms) is used around this site seemingly interchangeably to describe a sensory process where changes in current are measured ( though it turns out these are not interchangeable terms). Unlike the potentiometric electrodes, which measure the change in potential difference which is generated when ions migrate across a membrane, the amperometric chain subjects the migrating ions to a potential difference, and then measures the current which is generated.
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..
Diffusion describes solute transport across a semi-permeable membrane generated by a concentration gradient. The major determinant of diffusion rate in dialysis is the concentration gradient; however several other factors influence the rate of diffusion. These factors include the characteristics of the membrane, the temperature of the solution, the available surface area and the diffusivity coefficient of the molecule (which is a complex composite measure of how fast a substance diffuses across a solvent volume, expressed in length squared per second).
Typically, when one thinks of flow-volume loops, one refers to the classical loops of forced expiratory spirometry. Most of what is written about flow-volume loops refers to these. Indeed, both the LITFL entry on flow-volume loops and the AnaesthesiaUK revision article use the formal pulmonary function test standard of flow-volume loop interpretation. The shape of the curves is quite similar, but the fundamental difference is in the fact that in the ventilator loop is by convention upside-down.
Pressure-volume loops can inform us about changes in the patient's lung compliance, air leaks, patient-ventilator dyssynchrony, and increased work of breathing. For instance, they may reveal alveolar overdistension, or help determine the optimal level of PEEP (the so-called "critical opening pressure") for a patient with ARDS.
The changing relationship of pressure and volume over the course of a breath can provide us with information about the compliance of the respiratory circuit. Inspiration creates a negative pressure, which gradually trends to zero as the lungs fill to the full capacity of the tidal volume. At expiration, the elastic recoil of the chest wall and lung tissue creates a positive pressure, which decreases towards zero as the volume is exhaled.
The flow waveform is the most interesting waveform. Much information can be derived from its shape. When flow is being used to generate a controlled level of pressure, the shape of the inspiratory flow waveform is informative regarding the necessary inspiratory time (if flow reaches zero, then the inspiratory time could be shorter without compromising volume). The expiratory flow pattern is also informative, as a slow return to baseline is an indication of the resistance to airflow.