Variation in performance of oxygen therapy devices

Factors associated with intubation in patients with acute hypoxaemic respiratory failure treated with high-flow nasal cannula oxygen therapy: A prospective, observational study

BACKGROUND

High-flow nasal cannula (HFNC) oxygen is an alternative to conventional oxygen in acute hypoxaemic respiratory failure. Some patients require intubation, with a risk of delay; thus, early predictors may identify those requiring earlier intubation. The "ROX" index (ratio of pulse oximetry/fraction of inspired oxygen to respiratory rate) predicts intubation in patients with pneumonia treated with HFNC therapy, but this index has not been validated in non-pneumonia causes of acute hypoxaemic respiratory failure.

AIM/OBJECTIVE

The aim of this study was to identify factors associated with intubation in a heterogeneous group of patients with acute hypoxaemic respiratory failure treated with HFNC oxygen.

METHODS

This prospective observational study was undertaken in an Australian tertiary intensive care unit and included patients over 18 y of age with acute hypoxaemic respiratory failure who were treated with oxygen via HFNC. Vital signs and arterial blood gases were recorded prospectively at baseline and regular prespecified intervals for 48 h after HFNC initiation. Multivariate logistic regression was used to identify the factors associated with intubation.

RESULTS

Forty-three patients were included (N = 43). The multivariate factors associated with intubation were admission Sequential Organ Failure Assessment score (odds ratio [OR]: 1.94 [95% confidence interval {CI}: 1.06-3.57]; p = 0.032) and Pneumonia Severity Index (OR: 0.95 [95% CI: 0.90-0.99]; p = 0.034). The ROX index was not independently associated with intubation when adjusted for Sequential Organ Failure Assessment score (OR: 0.71 [95% CI: 0.47-1.06]; p = 0.09). There was no difference in mortality between patients intubated early (<24 h) compared to those intubated late.

CONCLUSIONS

Intubation was associated with admission Sequential Organ Failure Assessment score and Pneumonia Severity Index. The ROX index was not associated with intubation when adjusted for admission Sequential Organ Failure Assessment score. Outcomes were similar irrespective of whether patients were intubated late rather than early.

True intratracheal oxygen concentration delivered by SentriO Oxy™ masks under various respiratory conditions: a bench study

SentriO Oxy™ is a newly available, Food and Drug Administration-approved oxygenation mask system that provides high oxygenation, even on low-flow (5-10 L/min) oxygen. This study aimed to accurately measure the intratracheal fraction of inspired oxygen (FiO2) using SentriO Oxy™ masks under relatively low oxygen flow rates. A manikin-ventilator-test lung simulation system was used. We measured FiO2 at the level of the carina, 5 minutes after applying 45 different respiratory parameter combinations using SentriO Oxy™ masks. Tidal volume (TV) was set to 300, 500, and 700 mL; respiratory rate (RR) was set to 8, 12, 16, 20, and 24 breaths per minute; and oxygen flow rate was set to 6, 8, and 10 L/min. Our hypothesis was that FiO2 would be proportional to the difference between oxygen flow rate and minute ventilation. FiO2 measured by smaller TV, lower RR, or higher oxygen flows revealed a significantly higher value, confirming our hypothesis. In addition, using linear regression analysis, we found that TV, RR, and oxygen flow were all significant factors influencing the measured FiO2. Our experiment proposed two prediction equations considering the oxygen flow rate, TV, and RR. The results of our study may provide information and prediction of FiO2 for clinicians to use SentriO Oxy™ masks during sedative anesthetic procedures under low oxygen flow rates.

Fraction of Inspired Oxygen With Low-Flow Versus High-Flow Devices: A Simulation Study

Purpose: The fraction of inspired oxygen while administering oxygen to patients must be measured as it represents the alveolar oxygen concentration, which is important from a respiratory physiology viewpoint. Therefore, the purpose of this study was to compare the fractions of inspired oxygen obtained through different oxygen delivery devices.

Methods: A simulation model of spontaneous respiration was used. The fractions of inspired oxygen obtained through low- and high-flow nasal cannulas and a simple oxygen mask were measured. The fraction of inspired air was measured every second for 30 s after 120 s of oxygen administration. This was measured three times under each condition.

Results: With a low-flow nasal cannula, airflow reduced both the intratracheal fraction of inspired oxygen and extraoral oxygen concentration, indicating that exhalatory respiration occurred during rebreathing and may be involved in increasing the intratracheal fraction of inspired oxygen.

Conclusion: Oxygen administration during expiratory flow may lead to an increased oxygen concentration in the anatomical dead space, which may be involved in the increase in the fraction of inspired oxygen. With a high-flow nasal cannula, a high fraction of inspired oxygen can be achieved even at a flow rate of 10 L/min. When determining the optimum amount of oxygen, it is necessary to set an appropriate flow rate for patients and specific conditions without being bound by the fraction of inspired oxygen values alone. It might be difficult to estimate the fraction of inspired oxygen while using a low-flow nasal cannula and simple oxygen mask in clinical situations.

Capnographic Analysis of Minimum Mandatory Flow Rate for Hudson Face Mask: A Randomized Double-blind Study

BACKGROUND

Oxygen supplementation through Hudson type face mask is frequently used in perioperative settings. Hudson mask is a variable rate performance device with the risk of rebreathing. Studies using capnography to find out an actual fraction of rebreathing in spontaneously breathing patients are not available.

AIMS

In this study, we analyzed the effects of different flow rates through Hudson mask with capnography on fractional inspired carbon dioxide (FiCO₂).

SETTING AND DESIGNS

Forty patients posted for minor surgeries under monitored anesthesia care were divided into four groups. They received oxygen flow rate of 3 L/min, 4 L/min, 5 L/min, and 6 L/min as per group division, through Hudson mask.

MATERIALS AND METHODS

Parameters such as pulse rate, noninvasive blood pressure, oxygen saturation, respiratory rate (RR), end-tidal carbon dioxide (EtCO₂), and FiCO₂ were noted at baseline. After starting oxygen supplementation, these parameters were monitored every minute for ten minutes. Statistical analysis was done using analysis of variance and Kruskal-Wallis test. Pearson correlation was used to establish a relation between RR and FiCO₂.

RESULTS AND CONCLUSIONS

EtCO₂ and FiCO₂ were comparable in all four groups with no statistical significance. There was strong positive correlation seen between RR and FiCO₂ at a flow rate of 3 L/min while negative correlation in other groups. We conclude that FiCO₂ is similar at all flow rates while breathing through Hudson mask. Increase in RR can cause increase in FiCO₂ at lower flow rate, which is within physiological tolerance limits.

Guidelines for diagnosis and management of chronic obstructive pulmonary disease: Joint ICS/NCCP (I) recommendations

Chronic obstructive pulmonary disease (COPD) is a major public health problem in India. Although several International guidelines for diagnosis and management of COPD are available, yet there are lot of gaps in recognition and management of COPD in India due to vast differences in availability and affordability of healthcare facilities across the country. The Indian Chest Society (ICS) and the National College of Chest Physicians (NCCP) of India have joined hands to come out with these evidence-based guidelines to help the physicians at all levels of healthcare to diagnose and manage COPD in a scientific manner. Besides the International literature, the Indian studies were specifically analyzed to arrive at simple and practical recommendations. The evidence is presented under these five headings: (a) definitions, epidemiology, and disease burden; (b) disease assessment and diagnosis; (c) pharmacologic management of stable COPD; (d) management of acute exacerbations; and (e) nonpharmacologic and preventive measures. The modified grade system was used for classifying the quality of evidence as 1, 2, 3, or usual practice point (UPP). The strength of recommendation was graded as A or B depending upon the level of evidence.

Evaluation of a Humidified Nasal High-Flow Oxygen System, Using Oxygraphy, Capnography and Measurement of Upper Airway Pressures

In this study, we evaluated the performance of a humidified nasal high-flow system (Optiflow™, Fisher and Paykel Healthcare) by measuring delivered FiO2 and airway pressures. Oxygraphy, capnography and measurement of airway pressures were performed through a hypopharyngeal catheter in healthy volunteers receiving Optiflow™ humidified nasal high flow therapy at rest and with exercise. The study was conducted in a non-clinical experimental setting. Ten healthy volunteers completed the study after giving informed written consent. Participants received a delivered oxygen fraction of 0.60 with gas flow rates of 10, 20, 30, 40 and 50 l/minute in random order. FiO2, FEO2, FECO2 and airway pressures were measured. Calculation of FiO2 from FEO2 and FECO2 was later performed. Calculated FiO2 approached 0.60 as gas flow rates increased above 30 l/minute during nose breathing at rest. High peak inspiratory flow rates with exercise were associated with increased air entrainment. Hypopharyngeal pressure increased with increasing delivered gas flow rate. At 50 l/minute the system delivered a mean airway pressure of up to 7.1 cmH2O. We believe that the high gas flow rates delivered by this system enable an accurate inspired oxygen fraction to be delivered. The positive mean airway pressure created by the high flow increases the efficacy of this system and may serve as a bridge to formal positive pressure systems.

Accuracy of a novel system for oxygen delivery to small children

OBJECTIVE

Oxygen therapy for infants and small children in developing countries is often not available. Entrainment devices may provide an accurate and precise concentration of oxygen when used at the flow rates appropriate for infants and small children.

METHODS

A continuously adjustable entrainment device was tested to determine the concentrations and flows of oxygen delivered by using low inlet flow rates suitable for therapy for infants and small children and 3 distinct oxygen delivery systems that varied in their resistive load.

RESULTS

The use of long and large bore, low resistance tubing (similar to a mask) resulted in the delivery of oxygen concentrations that tracked closely (accurate and precise) to values indicated by the entrainment device. The directly connected system with lower resistance (similar to a hood) produced a similar profile of concentrations and flow rates to the large bore tubing but with even greater accuracy. The use of a long and narrow tubing with higher resistance (similar to a cannula) did not deliver accurate oxygen concentrations. In fact, this high-resistance system failed to work as intended, and instead of entraining air, a large proportion (sometimes >50%) of the oxygen delivered to the entrainment device was ejected through its vents.

CONCLUSIONS

Entrainment devices can deliver accurate oxygen concentrations at low flow rates if used with low resistance delivery systems; however, entrainment devices are not suitable for use with high resistance delivery systems such as a standard nasal cannula.

Performance of six types of oxygen delivery devices at varying respiratory rates

The administration of a known concentration of oxygen is an important part of routine care of the sick patient. Many devices are currently available. The actual concentration of oxygen that can be delivered by these devices can be affected by several factors, both from the patient as well as the device itself. Measuring the F(i)o(2) delivered to the lungs in vivo can be both difficult and potentially uncomfortable for the subjects. We constructed a model using a resuscitation manikin, a ventilator and a set of bellows to simulate ventilation. With this model we tested a series of devices - variable performance, fixed performance and high flow - at two fixed tidal volumes. The respiratory rate was increased and its effect on the oxygen concentration assessed. Variable performance systems such as the Hudson mask deliver a significantly reduced oxygen concentration at high respiratory rates. Fixed performance systems delivering 24-40% oxygen deliver appropriate oxygen concentrations across the range of respiratory rates, whereas those delivering 60% show a reduction in performance. High flow systems show no failure of performance at increased respiratory rates.

Effects of short-term isocapnic hyperoxia and hypoxia on cardiovascular function

Both hypoxia and hyperoxia have major effects on cardiovascular function. However, both states affect ventilation and many previous studies have not controlled CO2 tension. We investigated whether hemodynamic effects previously attributed to modified O2 tension were still apparent under isocapnic conditions. In eight healthy men, we studied blood pressure (BP), heart rate (HR), cardiac index (CI), systemic vascular resistance index (SVRI) and arterial stiffness (augmentation index, AI) during 1 h of hyperoxia (mean end-tidal O2 79.6 ± 2.0%) or hypoxia (pulse oximeter oxygen saturation 82.6 ± 0.3%). Hyperoxia increased SVRI (18.9 ± 1.9%; P < 0.001) and reduced HR (−10.3 ± 1.0%; P < 0.001), CI (−10.3 ± 1.7%; P < 0.001), and stroke index (SI) (−7.3 ± 1.3%; P < 0.001) but had no effect on AI, whereas hypoxia reduced SVRI (−15.2 ± 1.2%; P < 0.001) and AI (−10.7 ± 1.1%; P < 0.001) and increased HR (18.2 ± 1.2%; P < 0.001), CI (20.2 ± 1.8%; P < 0.001), and pulse pressure (13.2 ± 2.3%; P = 0.02). The effects of hyperoxia on CI and SVRI, but not the other hemodynamic effects, persisted for up to 1 h after restoration of air breathing. Although increased oxidative stress has been proposed as a cause of the cardiovascular response to altered oxygenation, we found no significant changes in venous antioxidant or 8-iso-prostaglandin F2α levels. We conclude that both hyperoxia and hypoxia, when present during isocapnia, cause similar changes in cardiovascular function to those described with poikilocapnic conditions.

The 'fixed performance' venturi: effect of downstream pressure on outflow and FIO2

Fixed performance venturi devices should provide a predetermined oxygen concentration at an outflow which exceeds an adult's peak resting inspiratory flow rate (approximately 30 l.min(-1)). Campbell's original description mentioned the sensitivity of the venturi device to downstream resistance but gave no further details. This study examined outflow and oxygen concentration from the five standard venturi devices (24-60% O(2)) when downstream pressure increased. Outflow was exquisitely sensitive to small increases in pressure. The outflow at zero downstream pressure for the 24-40% O(2) venturi devices ranged from 40 to 50 l.min(-1) but only 2-3 mmH(2)O was needed to halve this flow and increase oxygen concentration. The 60% O(2) venturi delivered a maximum of only 30 l.min(-1) at zero downstream pressure and flow was reduced further by increasing this pressure. An increase in downstream pressure of only a few mmH(2)O increased oxygen concentration and decreased outflow of all the venturi devices tested, in most to less than normal peak tidal flow in adults.

Cardiovascular effects of acute oxygen administration in healthy adults

Supplementary oxygen is commonly administered in current medical practice. However, attention has recently been drawn to the potentially disadvantageous hemodynamic consequences in certain patients. Possible mechanisms underlying the cardiovascular responses to acute hyperoxia are unclear. The effects of acute oxygen administration on heart rate, blood pressure, cardiac output, systemic vascular resistance, and baroreflex sensitivity were studied in a series of randomised, placebo-controlled studies in healthy individuals, using validated, non-invasive techniques. The effects of oxygen administration on forearm blood flow responses to locally administered acetylcholine, an endothelium-dependent vasodilator, sodium nitroprusside, an endothelium-independent vasodilator, and l-NG-monomethylarginine, a nitric oxide synthase inhibitor, were studied using venous occlusion plethysmography. Oxygen administration for 1 hour caused a reduction in heart rate (P < 0.01) and cardiac index (P < 0.05), and an increase in mean arterial pressure (P < 0.01), systemic vascular resistance (P < 0.05), large artery stiffness (P < 0.05), and baroreflex sensitivity (P < 0.05). There were no effects on vascular responses in the isolated forearm bed. These findings indicate that oxygen administration causes acute effects on cardiovascular function, which might be important in the context of acute illness.

Performance of standard and reservoir-type Hudson masks in pregnant and non-pregnant subjects

We have assessed the performance of the Hudson-type mask without reservoir and the Hudson non-rebreathing mask with reservoir bag in 10 pregnant and 10 non-pregnant women at different oxygen flow rates by measuring end-expired oxygen fraction (FEO(2)). The Hudson mask without reservoir behaved similarly in pregnant and non-pregnant women, providing a mean FEO(2) of 0.32 with oxygen flow of 4 L.min(-1) which increased to 0.46 at 15 L.min(-1). The reservoir mask produced a lower FEO(2) in pregnant subjects than in non-pregnant subjects, with a mean FEO(2) of 0.33 and 0.64 at 4 L.min(-1) and 15 L.min(-1) in pregnant subjects compared to 0.44 and 0.72, respectively, in non-pregnant women. Neither mask delivers the expected maximum inspired oxygen fraction quoted by its manufacturer, assuming that inspired oxygen fraction quoted by its manufacturer, assuming that inspired oxygen fraction is 0.06 higher than expired and correcting for humidification of inhaled gases within the respiratory tract. The importance of these findings in the care of the obstetric patient and the treatment of acute fetal hypoxia is discussed.

Evaluation of five oxygen delivery devices in spontaneously breathing subjects by oxygraphy

Oxygen supply systems may be divided into constant and variable performance systems. As the variable performance systems are widely used, it is relevant to investigate the variation in performance between devices and the influence of oxygen supply on the inspired oxygen fraction. Data were collected from 10 healthy volunteers during the use of one constant performance system and four variable performance systems at different gas flows and inspired oxygen fractions. A thin sampling catheter was placed in the nasopharynx to allow the measurement of the end-tidal oxygen fraction. When oxygen was supplied to variable performance systems, end-tidal oxygen fraction values measured in this way varied less and were more easily quantifiable than inspired oxygen fraction. End-tidal oxygen fraction was used to calculate inspired oxygen fraction. With the variable performance systems, inspired oxygen fraction varied considerably between subjects whereas a constant and equal rise was found for each subject with the fixed performance system. A large nasal catheter was capable of delivering the highest inspired oxygen fraction, whereas the Venturi mask delivered the most precise inspired oxygen fraction. We found oxygraphy useful in the interpretation of measurements made in patients receiving unknown inspired fractions of oxygen.

Oxygen administration during transport and recovery after outpatient surgery does not prevent episodic arterial desaturation

STUDY OBJECTIVE

To compare the efficacy of two different oxygen (O2) delivery systems in preventing episodic arterial desaturation in the immediate postoperative period.

STUDY DESIGN

Randomized, prospective, nonblinded comparison in patients.

SETTING

Operating room and postanesthesia care unit (PACU) of a university outpatient surgery center.

PATIENTS

100 ASA status I and II adults, male and female, age greater than 18 years, undergoing outpatient surgical procedures not involving the upper airway.

INTERVENTIONS

Group I received supplemental O2 administered by bag-valve-mask during transport, followed by 40% face shield in the PACU. Group 2 received supplemental O2 by nasal cannula at 4 L/min both during transport and during PACU stay.

MEASUREMENTS AND MAIN RESULTS

Arterial O2 saturation (SpO2) was collected by computer from a recording pulse oximeter at 15-second intervals beginning before extubation and continuing until O2 administration was discontinued in PACU. Neither mode of therapy was successful in completely eliminating arterial desaturation defined as SpO2 less than 90%, nor was there any difference in efficacy between the two treatment groups. Group 1 had 8 patients desaturate on 9 different occasions (5 times during transport, 4 times in PACU). Group 2 had 5 patients desaturate on 9 different occasions (4 times during transport, 5 times in PACU).

CONCLUSION

Routine O2 administration during transport and PACU stay did not abolish episodic desaturation, even in healthy patients undergoing minor surgical procedures. Given the marked difference in acquisition cost, it would appear that O2 administration by nasal cannula is a more cost-effective alternative for routine postoperative O2 administration in certain groups of patients undergoing general anesthesia for outpatient surgery.

Oxygraphy in spontaneously breathing subjects

BACKGROUND

Continuous monitoring of O2 and CO2 in the airways of spontaneously breathing patients can be carried out by sampling air to a gas monitor through a catheter placed in the upper airway. The graphical display of O2 (oxygraphy) is a rather new facility.

OBJECTIVE

To describe the photo-acoustic and magneto-acoustic technique for CO2 and O2 monitoring in the open unintubated airway, to evaluate the efficacy of oxygen therapy by oxygraphy and to determine alveolar gas tensions and alveolar-arterial partial pressure gradients.

DATA SOURCES

O2 and CO2 fractions in the airways were monitored in 9 healthy subjects. Blood samples were drawn from the radial artery.

METHODS

The Multigas Monitor 1,304 (Brüel and Kjaer, Naerum, Denmark) was used; end-expiratory measurements were considered as representative for the alveolar gas composition. Arterial blood was analysed by ABL520 (Radiometer Medical A/S, Copenhagen, Denmark).

RESULTS

Reliable tracings of gas fractions (FCO2 and FO2) were obtained during the respiratory cycle in all subjects. When oxygen was supplied, FO2 of the airway varied considerably during the inspiratory phase whereas it remained almost constantly during the expiratory phase. The end-expiratory FO2 increased from 0.15 breathing atmospheric air to 0.41 breathing oxygen 15 L/min through a Hudson mask. Alveolar-arterial partial pressure differences were: pO2(A-a): 1.07 +/- 0.85 kPa and pCO2(A-a): -0.04 +/- 0.33 kPa during normoventilation in atmospheric air.

CONCLUSION

Continuous monitoring of CO2 and O2 in the airway gives information about the pulmonary gas exchange and the efficacy of oxygen supply. Combined with arterial blood gas analysis the method allows determination of alveolar-arterial CO2 or O2 gradients.

Nitrous Oxide-Oxygen Analgesia: The Performance of the MC Mask Delivery System

Nitrous oxide administration via an oxygen facemask such as the Mary Catterall (MC) is an accepted technique for supplementing regional anaesthesia. This study assessed the range of nitrous oxide and oxygen concentrations delivered to the trachea using a mechanical lung model. The inspired concentrations of these two gases were determined by the total fresh gas inflow rates, their concentrations in the fresh gas mixture and the peak inspiratory flow rate of the analogue lung. A 1: 1 nitrous oxide-oxygen mixture in the fresh gas with an inflow rate of 4–6 1/min is recommended to ensure adequate oxygen delivery into the trachea as well as achieving the therapeutic concentrations of nitrous oxide (20–30%).

Variability of inspired oxygen concentration with nasal cannulas

BACKGROUND

The likely values of inspired oxygen concentration (FIO2) of patients with chronic obstructive pulmonary disease breathing via nasal cannulas have not been assessed previously.

METHODS

Seven patients with chronic obstructive lung disease and seven healthy subjects were studied while breathing oxygen via nasal cannulas or fixed performance (Venturi) or uncontrolled (MC) oxygen masks. Breath to breath values of FIO2 were calculated by extrapolation from expired oxygen and carbon dioxide concentrations on the basis of the oxygen-carbon dioxide relationship and on the assumption of a respiratory exchange ratio (R) of 0.8.

RESULTS

In both groups of subjects the average values of FIO2 with nasal cannulas at 1 and 2 l min-1 were of a similar order to those achieved with 24.5% and 28% Venturi masks, but variations within and between subjects in both groups breathing via nasal cannulas were considerable and similar to those found with MC masks. In the seven patients with chronic obstructive lung disease breathing via nasal cannulas at 2 l min-1 the average FIO2 varied from 23.7% to 34.9%.

CONCLUSIONS

"Typical" values of FIO2 quoted with nasal cannulas can mislead. The results confirm that this mode of oxygen delivery is unsatisfactory if precise control of inspired oxygen is desired.

An evaluation of oxygen delivery using nasal prongs

Oxygen delivery using nasal prongs was assessed using a lung model for spontaneous ventilation. The analogue lung was attached to a manikin, which provided a model of the 'face and pharynx' to which the nasal prongs were applied. Oxygen concentrations were measured in the model trachea at varying fresh gas inflow and peak inspiratory flows. The study demonstrated enormous variability in the both the peak-inspired (26.3-90.0%) and end-expired concentrations (25.2-78.6%) of oxygen delivered to the trachea. There was a regular relationship between the ratio of peak inspiratory flows, expressed over fresh gas inflow and the end-expired oxygen concentrations which could allow estimation of inspired oxygen concentration.

Oxygen delivery systems--a comparison of two devices

Two low-volume, variable performance oxygen delivery systems were compared in conscious spontaneously breathing volunteers. Oropharyngeal oxygen concentrations were measured during periods of nose and mouth breathing. The systems were studied at oxygen flow rates of 2 or 4 litres/minute. The performance of both systems was similar under the test conditions but the nasal catheter is preferable in terms of cost.

'Safe oxygen' in acute asthma: prospective trial using 35% Ventimask prior to admission

There is still some uncertainty as to the best inspired oxygen concentration to use in the treatment of acute asthma before measurement of arterial blood gases can be made. In the absence of published data, we report a prospective study in which 35% oxygen was the initial therapy given to patients with moderate to severe asthma, and arterial blood gases were taken to assess the adequacy of oxygenation and the safety of this method. Forty-five episodes were analyzed and a wide range of PaO2 was observed (8.8-21.3 kPa 66-160 mmHg). No relationship was shown between PaCO2 and either PaO2 or duration of oxygen treatment. It was concluded that 35% oxygen given in acute asthma is both safe and probably adequate. A lesser concentration of oxygen could expose asthmatics to an unacceptable risk of significant hypoxaemia.

The oxygen delivery characteristics of the Hudson Oxy-one face mask

The inspired oxygen fraction (FIO2) delivered by the Hudson Oxy-one face mask was measured under changing conditions of ventilation, oxygen flow rate to mask, and mask fit. A single trained subject sat in a body plethysmograph to measure ventilation and breathed at a constant rate of 15 per minute at three different tidal volumes, of approximately 0.3, 0.6, and 1.2 litres, from the mouthpiece in the plethysmograph. The Oxy-one face mask was fitted to a plaster-of-Paris face model on the outside of the plethysmograph in a loose and then in a tight fashion. Oxygen concentration was continuously monitored from a point in the metal tube connecting the face model to the mouthpiece. The tightly fitting mask demonstrated an orderly reduction in FIO2 as ventilation increased and oxygen flow rate to the mask decreased. The mean FIO2 at a ventilation of 4.5 l.min-1 and 8 l.min-1 oxygen flow was 78% and this fell to 27% at a ventilation of 16 l.min-1 and oxygen flow of 2 l.min-1. The loosely fitting mask demonstrated larger SD of measurements and lower mean maximum FIO2 values of 46 to 49% and these fell in an irregular fashion to similar minimum values as ventilation increased and oxygen flow decreased. Although the precise definition of the FIO2 for each breath from the changing concentration during each inspiration was not possible, these results indicate that FIO2 changes in a predictable way as a function of ventilation and oxygen flow, if the mask is close fitting. This method could be conveniently used to study other oxygen delivery systems.

A comparison of oxygen therapy devices used in the postoperative recovery period

Seventy-one patients scheduled to undergo upper or lower abdominal surgical procedures were allocated at random to one of seven treatment groups: in the recovery room they were to receive oxygen via a 40% Ventimask with 10 litres/minute oxygen flow, or via either a Hudson mask or a nasal cannula with 3, 6 or 9 litres/minute oxygen flow. The 40% Ventimask gave the most consistent, satisfactory postoperative values of PaO2 but the much cheaper nasal cannula at 6 or 9 litres/minute was generally adequate in conscious patients. The performance of the intermediately priced Hudson mask was similar to that of the nasal cannula at these flows. The unconscious state was associated with a 45% lower PaO2 than the rousable or awake states. Differences between the treatments with regard to postoperative PaCO2 were small and non-significant. The nasal cannula with 6 litres/minute humidified oxygen flow is recommended for routine treatment, and the Ventimask for unconscious patients.

Postoperative nitrous oxide analgesia. Administration from air entrainment oxygen masks with a primary flow of Entonox

Nitrous oxide is occasionally used as an analgesic agent in the postoperative period. The feasibility of administering a known concentration from an air entrainment oxygen mask, using a primary flow of Entonox, (50% oxygen/50% nitrous oxide) was investigated. Accurox (C.R. Bard Canada Inc.) blenders, disconnected from their facemasks, were studied using a primary flow of Entonox. An increase in air entrainment of approximately 13% was demonstrated. Concentrations of about 20% nitrous oxide in oxygen enriched air, appropriate for postoperative analgesia, can be produced, but the methods are extravagant in the use of Entonox and are likely to be slightly unreliable.

Quantitative effects of respired helium and oxygen mixtures on gas flow using conventional oxygen masks

A model lung system with a variable artificial constriction was used to quantify the changes in flow produced by the addition of helium to the gas mixture in the system. The tracheal concentration of helium obtained when administering 79% helium/21% oxygen to the system via conventional, disposable face masks was determined using a resuscitation dummy. The greatest proportion of the increase in flow through the orifice occurred at concentrations of helium up to 40%. A tracheal concentration of 40% helium could be obtained with all oxygen masks at fresh gas flow rates greater than 8 litres/minute. These findings will help to rationalise the administration of 79% helium/21% oxygen to patients with upper respiratory tract obstruction.

An air-entrainment device for preparing precision gas mixtures

Three low-cost venturi's built from readily available materials are described and evaluated to determine whether they can be used to prepare precision gas mixtures for the calibration of gas analysers. Using pure oxygen (O2) and nitrogen as the priming gases the venturi's generated mixtures with an O2 concentration within the range 12-53% O2. Over a two-week period, the variability was found to be less than 0.25% O2. The mixtures produced were found to vary according to the density of the priming gas, but were virtually independent of the priming flow rate. We conclude that the venturi may offer a simple and inexpensive method of preparing precision gas mixtures suitable for the calibration of gas analysers.

Effect of breathing pattern on oxygen concentration received from standard face masks

Standard face masks are widely used for patients who require high oxygen concentrations. A mechanical model was designed to reproduce various respiratory manoeuvres while oxygen was delivered through a standard variable-performance face mask. Over a range of tidal volumes from 300 to 1200 ml and of frequencies from 10 to 30 breaths/min, the received oxygen concentration was lower than the delivered concentration. The disparity between delivered and received oxygen increased with increasing rate and depth of simulated breathing but could be reduced by increasing the flow rate up to 60 l/min. When a turbulent environment was created, the difference between delivered and received oxygen concentrations was exaggerated. Disparities between delivered and received concentrations can be lessened by increasing the flow rate.

Fixed performance oxygen masks. Hypoxic hazard of low-capacity drugs

Four recently introduced low-capacity fixed performance oxygen therapy masks have been compared with the established Ventimask design. Under conditions of varying peak inspiratory flow rate the low-capacity devices all permit a variable amount of air admixture with a consequent fall in the inspired oxygen concentration. It is concluded that low-capacity venturi masks are not true fixed performance devices under all circumstances. The Ventimasks satisfy their specifications under all test conditions. A case is made for more rigorous assessment of new so-called "fixed performance" oxygen therapy devices before marketing is permitted.

Performance of venturi oxygen delivery devices

The oxygen blenders used in disposable oxygen masks from five manufacturers (Hudson, Inspiron, McGaw, Medishield and Vickers) were tested for delivered oxygen concentration and flow. In all concentration ranges the Medishield and McGaw were more accurate than the others. With some exceptions the flow output of these two devices was also adequate. The Medishield blenders which were tested over a wide range of oxygen inflows proved very stable in their delivered oxygen concentration.

An analysis of the performance of a variable venturi-type oxygen mask

The theoretical performance of the Hudson Multivent mask is considered. A method is described of determining the flow-weighted mean inspired oxygen concentration produced by the mask. Using this method, it was found that the inspired oxygen concentration was predictable only at the 24% setting using the recommended flow rates, but that increasing the supplied-oxygen flow rates above the recommended levels resulted in the return of predictable function. At settings above 30% predictable function was found to be unlikely, whatever the supplied oxygen flow rate. Recommendations are made regarding the use of this device.

Controlled oxygen therapy at 60% concentration. Why and how

In a number of circumstances pulmonary function is so disturbed that inspired oxygen concentrations of about 60% are needed to combat arterial hypoxaemia. Rational therapy requires that the inspired concentration be controlled and not vary with the patient's breathing. An exploration of the interaction between gas flow, ventilation, breathing pattern, and mask volume shows that a mask of 300 ml volume supplied with a flow of 30 1/min (15 1 O2, 15 1 air) can meet the need.

Respiratory care following open heart surgery

Respiratory care of patients undergoing open heart surgery should begin in the preoperative period. Patients must stop smoking, and if obese they are encouraged to lose weight. Pulmonary infection is treated and secretions must be eliminated. Postoperative hypoxemia, which is an expected event following anesthesia and surgery, is aggravated by circulatory instability and pulmonary complications. Following open heart surgery pulmonary complications such as atelectasis, congestion, edema, postperfusion lung, pneumothorax, pleural effusion, and hemothorax are common. Respiratory care should be planned to avoid these complications and to treat them promptly should they occur. Routinely every patient is mechanically ventilated for at least 12 to 18 hours following surgery. The type of ventilator used and its parameters are adjusted according to the clinical condition of the patient to maintain adequate oxygenation and to prevent any respiratory acidosis. When indicated, PEEP is applied to improve arterial oxygenation. Respiratory care is extended for at least 5 days after termination of artificial ventilation. Oxygen therapy is given with either a nasal catheter or a mask, according to the patient's need. IPPB and physiotherapy are continued until the patient shows no signs of pulmonary infection and is capable of effectively eliminating secretions. This routine management and extended postoperative respiratory care definitely contribute to the successful outcome of open heart surgery.

Oxygen therapy for pneumatosis coli

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Oxygen therapy for spontaneous pneumothorax

The rate of absorption of gas from a pneumothorax was studied in 12 patients breathing air and in 10 patients breathing air and a high concentration of oxygen alternately. The mean rate of absorption while breathing air was the same in both groups of patients. In the second group the rate consistently increased during periods of oxygen therapy and decreased again on breathing air. The mean rate of absorption increased fourfold during periods of oxygen therapy. This increase was most pronounced in patients with a large pneumothorax.