Andrew Farmery

University of Oxford | OX · Nuffield Department of Clinical Neurosciences

This article first outlines definitions and descriptions of key terms that will be used in the discussions that follow, including osmolarity, osmolality, osmotic and oncotic pressure. The physicochemical properties of water, ions and organic molecules are discussed in terms of their biological roles. Similarly, the interactions of amphipathic molec...

The degree of specific ventilatory heterogeneity (spatial unevenness of ventilation) of the lung is a useful marker of early structural lung changes which has the potential to detect early-onset disease. The Inspired Sinewave Test (IST) is an established noninvasive ‘gas-distribution’ type of respiratory test capable of measuring the cardiopulmonar...

The traditional approach to acid–base physiology is based on the Henderson–Hasselbalch equation which is derived from the CO2/HCO3− buffer system. However, it is becoming increasingly recognized that this is an incomplete analysis as it focuses on only one of the six reactions involving H⁺ and can lead to the incorrect assumption that CO2 and HCO3−...

Background: Tidal recruitment/derecruitment (R/D) of collapsed regions in lung injury has been presumed to cause respiratory oscillations in the partial pressure of arterial oxygen (PaO2). These phenomena have not yet been studied simultaneously. We examined the relationship between R/D and PaO2 oscillations by contemporaneous measurement of lung-...

Humans are homeotherms, i.e. they fix their temperature regardless of their environment. This is vital for normal cellular function and for metabolism to be independent of external temperature. The body has a warm ‘core’ and a cooler peripheral ‘shell’ whose role is to regulate heat transfer in and out of the core. Body temperature is controlled by...

This article first outlines definitions and descriptions of key terms that will be used in the discussions that follow, including osmolarity, osmolality, osmotic and oncotic pressure. The physicochemical properties of water, ions and organic molecules are discussed in terms of their biological roles. Similarly, the interactions of amphipathic molec...

The traditional approach to acid–base physiology is based on the Henderson–Hasselbalch equation which is derived from the CO2/HCO3⁻ buffer system. However, it is becoming increasingly recognized that this is an incomplete analysis as it focuses on only one of the six reactions involving H⁺ and can lead to the incorrect assumption that CO2 and HCO3⁻...

New findings: What is the central question of this study? We present a new non-invasive medical technology, the inspired sine-wave technique, which involves inhalation of sinusoidally fluctuating concentrations of a tracer gas. The technique requires only passive patient cooperation and can monitor different cardiorespiratory variables, such as en...

The inspired sinewave technique is a noninvasive method to measure airway dead space, functional residual capacity, pulmonary blood flow, and lung inhomogeneity simultaneously. The purpose of this paper was to assess the repeatability and accuracy of the current device prototype in measuring functional residual capacity, and also participant comfor...

Arterial oxygen partial pressure can increase during inspiration and decrease during expiration in the presence of a variable shunt fraction, such as with cyclical atelectasis, but it is generally presumed to remain constant within a respiratory cycle in the healthy lung. We measured arterial oxygen partial pressure continuously with a fast intra-v...

Method: The new method used integration of flow and concentration. A computer algorithm sought an appropriate value of deadspace to satisfy the mass balance equation for each breath. A modern gas mixing apparatus with rapid mass flow controllers was used to verify the procedure. Result Experiments on a tidally ventilated bench lung showed that the...

Routine estimation of functional residual capacity (FRC) in ventilated patients has been a long held goal, with many methods previously proposed, but none have been used in routine clinical practice. This paper proposes three models for determining FRC using the nitrous oxide concentration from the entire expired breath in order to improve the prec...

Measurement allows us to quantify various parameters and variables in natural systems. In addition, by measuring the effect by which a perturbation of one part of the system influences the system as a whole, insights into the functional mechanisms of the system can be inferred. Clinical monitoring has a different role to that of scientific measurem...

We report the monitoring of oxygen partial pressure (pO2) in the liquid phase in cell cultures with a luminescence quenching based fiber optic oxygen sensor. The sensitivity of the fiber optic oxygen sensor was evaluated in the gas phase and the sensor was calibrated in the liquid phase before monitoring cell culture pO2 with a luminescence phase d...

Conventional methods for monitoring lung function usually require complex gas analysers and the cooperation of the patient. Therefore, they are not compatible with the crowded environment of the intensive care unit (ICU) or operating theatre, where patient cooperation is usually impossible. However, it is precisely these patients that would benefit...

Very fast sensors that are able to track rapid changes in oxygen partial pressure (PO2) in the gas and liquid phases are increasingly required in scientific research – particularly in the life sciences. Recent interest in monitoring very fast changes in the PO2 of arterial blood in some respiratory failure conditions is one such example. Previous a...

Background. There is considerable interest in oxygen partial pressure (PO2) monitoring in physiology, and in tracking PO2 changes dynamically when it varies rapidly. For example, arterial PO2 (PaO2 ) can vary within the respiratory cycle in cyclical atelectasis (CA), where PaO2 is thought to increase and decrease during inspiration and expiration,...

The development of a clinically useful fiber-optic oxygen sensor based on oxygen fluorescence quenching is described in this paper. The fiber optic oxygen sensor was formed by coating a thin polymer matrix, which contains an oxygen sensitive fluorophore, on the tapered end of a polymer optical fiber. Three acrylate polymers have been used for the m...

Humans are homeotherms i.e. they fix their temperature regardless of their environment. This is vital for normal cellular function and for metabolism to be independent of external temperature. The body has a warm ‘core’ and a cooler peripheral ‘shell’ whose role is to regulate heat transfer in and out of the core. Body temperature is control by a f...

Background: Ketamine has a rapid antidepressant effect in treatment-resistant depression (TRD). The effects on cognitive function of multiple ketamine infusions and of concurrent antidepressant medication on response rate and duration are not known. Method: Twenty-eight patients with uni- or bipolar TRD were treated over three weeks with either...

Intracranial pressure (ICP) is determined by the volumes of brain, blood and cerebrospinal fluid within the skull, which is of course of fixed volume. The Monro–Kellie hypothesis states that an increase in volume of one of these components must be compensated for by a reduction in volume of one or both of the others. If this compensation is insuffi...

The development of a clinically useful fiber-optic oxygen sensor based on oxygen fluorescence quenching is described in this paper. The fiber optic oxygen sensor was formed by coating a thin polymer matrix, which contains an oxygen sensitive fluorophore, on the tapered end of a polymer optical fiber. Three acrylate polymers have been used for the m...

Two challenges in the management of Acute Respiratory Distress Syndrome are the difficulty in diagnosing cyclical atelectasis, and in individualising mechanical ventilation therapy in real-time. Commercial optical oxygen sensors can detect [Formula: see text] oscillations associated with cyclical atelectasis, but are not accurate at saturation leve...

The development and construction of a tapered-tip fibre-optic fluorescence based oxygen sensor is described. The sensor is suitable for fast and real-time monitoring of human breathing. The sensitivity and response time of the oxygen sensor were evaluated in vitro with a gas pressure chamber system, where oxygen partial pressure was rapidly changed...

Conventional methods for monitoring lung function can require complex, or special, gas analysers, and may therefore not be practical in clinical areas such as the Intensive Care Unit (ICU) or operating theatre. The system proposed in this article is a compact and non-invasive system for the measurement and monitoring of lung variables, such as alve...

Inspired Sinewave is a novel technique to measure dead space, alveolar volume, and pulmonary blood flow noninvasively. In this paper, we describe a brief introduction to the principle of the technique, which involves forcing inspired concentrations to oscillate sinusoidally and measuring responding expired concentrations. Then, we give some updates...

The water contained in the body is divided amongst compartments of differing sizes and compositions. The dynamic balance across these compartments is an essential component of normal physiology. Here, the calculation of these volumes by measuring the dilution of markers able to permeate specific compartments is considered. Furthermore, the potentia...

This article first outlines definitions and descriptions of key terms that will be used in the discussions that follow including osmolarity, osmolality, osmotic and oncotic pressure. The physicochemical properties of water, ions and organic molecules are discussed in terms of their biological roles. Similarly, the interactions of amphipathic molecu...

The traditional approach to acid–base physiology is based on the Henderson–Hasselbalch equation which is derived from the 2CO/−3HCOCO2/HCO3− buffer system. However, it is becoming increasingly recognized that this is an incomplete analysis as it focuses on only one of the six reactions involving H+ and can lead to the incorrect assumption that CO2...

The development of a methodology for testing the time response, linearity and performance characteristics of ultra fast fibre optic oxygen sensors in the liquid phase is presented. Two standard medical paediatric oxygenators are arranged to provide two independent extracorporeal circuits. Flow from either circuit can be diverted over the sensor und...

A miniature (200 μm in diameter) cylindrical-core fiber-optic oxygen sensor has been developed for measuring rapid change in oxygen partial pressure (pO2). The fiber-optic sensing element is based on a cylindrical-core waveguide structure formed by coating a thin medical grade polymer sensing film that contains immobilized Pt(II) complexes on silic...

Apnoea due to airway obstruction is an ever present concern in anaesthesia and critical care practice and results in rapid development of hypoxaemia that is not always remediable by manual bag-mask ventilation. As it is often difficult or impossible to study experimentally (although some historical animal data exist), it is useful to model the kine...

A reliable, robust and low cost fibre optic oxygen sensor for measuring rapid changes in oxygen partial pressure has been developed using a polymer optical fibre and a polymer sensing matrix which is biocompatible with human tissue. These materials have been used in many medical and biomedical applications. The polymer fibre optic oxygen sensor is...

This article establishes the basic mathematical models and the principles and assumptions used for inert gas transfer within body tissues-first, for a single compartment model and then for a multicompartment model. From these, and other more complex mathematical models, the transport of inert gases between lungs, blood, and other tissues is derived...

The development of a cylindrical-core fiber optic oxygen sensor for fast measurement of oxygen partial pressure (pO2) is described. The fibre sensing element is based on a cylindrical-core waveguide structure formed by a polymer sensing film that contains immobilized Pt (II) complexes. The performance of the fibre optic oxygen sensor was evaluated...

A test system has been developed that can be used to calibrate and determine the time response, linearity and temperature sensitivity of a fibre optic oxygen sensor. The simple system obviates the need for precision gas standards and the requirement to generate a true square wave step response, which is seldom achievable. The sensor is mounted in a...

The development of a fibre optic oxygen sensor system based on an unsymmetrical fibre coupler for the measurement of oxygen partial pressure (PO2) is reported. The principle of the oxygen sensor is based on the luminescence quenching of a luminophore by oxygen and the sensor consists of an unsymmetrical silica fibre coupler and a PMMA sensing mediu...

The development of a rapid response time, plastic optical fiber (POF) based oxygen sensor for the measurement of oxygen partial pressure (pO2) is described. The POF oxygen sensor is based on the luminescence quenching of a luminophore by oxygen and consists of PMMA plastic optical fibre and a polymer matrix medium that contains immobilized Pt (II)...

In a patient whose airway is likely to become obstructed upon loss of consciousness, anesthesia may be induced using an inhaled vapor. If the airway occludes during such an inhalational induction, the speed of patient awakening is related to the rate at which anesthetic gas redistributes away from lung and brain to other body compartments. To deter...

Clonidine is an alpha(2) adrenoreceptor and imidazoline receptor agonist, which has analgesic, sedative, and minimum alveolar anesthetic concentration-sparing effects. It has been used orally, IV, and epidurally. In spinal surgery, there is a reluctance to use local anesthetic-based epidural analgesia postoperatively because of fears of masking imp...

To analyse the false positive referrals by community optometrists to a glaucoma case-finding clinic over a 3-year period. A retrospective study was undertaken of the 531 patients who were referred by community optometrists and discharged after their first visit to the glaucoma case-finding clinic at the Oxford Eye Hospital between 2003 and 2005. Th...

This paper describes the design of a control system for the gas composition of the inhaled breath during anaesthesia. A model of the gas flow through the anaesthetic system has been developed which is used as a disturbance predictor in the control system. To cope with the considerable delays in the system, internal model control is used and found t...

This paper describes the development of a model of an anesthetic circle system. The different components of the model are validated by a series of experiments. The results of these experiments show that the simulated system closely follows the actual system. The mathematical model is used for controlling the gas concentrations during inspiration, b...

The electrochemical reduction of the inhalation anaesthetic agent isoflurane was investigated at a commercially available Au microelectrode (5 μm) in DMSO solvent, individually, and also as a component of a simple vapour mixture with oxygen using both traditional voltammetric and potential step chronoamperometric techniques.In a binary gas mixture...

To describe a method that permits optical coherence tomography scanning to be performed under general anesthesia. Observational case report. A 7-year-old child underwent optical coherence tomography scanning under general anesthesia. He was positioned prone, with the neck extended to enable vertical facial alignment on the scanner. An anti-Trendele...

We compared the effects of sub-Tenon's local anaesthetic block and placebo on peri-operative opioid requirement and cardiovascular stability and on postoperative pain, nausea and vomiting in patients undergoing vitreo-retinal surgery under general anaesthesia. We studied 43 patients undergoing vitreo-retinal surgery under general anaesthesia in a r...

We develop tidal-ventilation pulmonary gas-exchange equations that allow pulmonary shunt to have different values during expiration and inspiration, in accordance with lung collapse and recruitment during lung dysfunction (Am. J. Respir. Crit. Care Med. 158 (1998) 1636). Their solutions are tested against published animal data from intravascular ox...

The relationship between the larynx and the subclavian arteries was studied in a series of magnetic resonance images (MRIs) from 50 patients without neck pathology. The vertical distances of the excursion of the subclavian arteries into the neck was measured, as was the distance from the cricoid cartilage to the highest point of this excursion. Sta...

A simple mathematical model of electron flow along the mitochondrial respiratory cytochrome assembly and the transfer of electrons to molecular oxygen is presented. First, an expression for the current-voltage relationship for a biological oxygen electrode is derived, and from this the relationship between oxygen consumption rate and oxygen partial...

The pupils in brainstem‐dead patients are classically fixed and dilated. We present a case of a brainstem‐dead patient whose pupils displayed persistent asynchronous pupillary constriction and dilatation independent of external physical stimuli. Central causes for the phenomenon were excluded leaving an unexplained peripheral cause as the most like...

The use of sidestream infrared and paramagnetic clinical gas analyzers is widespread in anesthesiology and respiratory medicine. For most clinical applications, these instruments are entirely satisfactory. However, their ability to measure breath-by-breath volumetric gas fluxes, as required for measurement of airway dead space, oxygen uptake, and s...

A healthy young woman is described in whom the left chest was unable to be inflated after intubation. The differential diagnosis and management are discussed. Severe unilateral bronchospasm was probably caused by topical lidocaine injected at the vocal cords and, inadvertently, into the left main bronchus with a Laryngojet device. Br J Anaesth 20...

Tidal ventilation gas-exchange models in respiratory physiology and medicine not only require solution of mass balance equations breath-by-breath but also may require within-breath measurements, which are instantaneous functions of time. This demands a degree of temporal resolution and fidelity of integration of gas flow and concentration signals t...

In this paper a description is given of the anesthetic circle system and of an experimental setup to be used in research in anaesthesia. A mathematical model of the circle system is developed. The different components of the model are validated by a series of experiments and the results of these experiments show that the simulated system closely fo...