Central Haemodynamics, Oxygen Transport and Oxygen Consumption During Three Methods for CPAP

During the weaning period after 18 hours of mechanical ventilation following open heart surgery, central haemodynamics, systemic oxygen transport and total oxygen consumption were assessed in a total of 17 patients, receiving continuous positive airway pressure (CPAP) therapy. Three different means of providing CPAP were studied: an electronically controlled method (mode S); a method based on the Venturi principle along with a continuous flow of gases via the T-piece connector attached to the intubation tube (mode B); and a method employing an elastic inspiratory gas reservoir (mode C). The inspiratory pressure plateau was adjusted to be equal under the different methods for CPAP. The airway pressure range, the expiratory and mean airway pressures and the mean oesophageal pressure were seen to be highest during mode S. The pulmonary vascular resistance was higher during mode S than during modes B and C, and the mean pulmonary arterial pressure was higher during mode S than mode C. While the cardiac index remained statistically unchanged under the three methods for CPAP, the stroke index (SI) and the left ventricular stroke work index (LVSWI) were higher during mode C than mode S. As the transmural right atrial and pulmonary capillary wedge pressures were unchanged, these alterations in SI and LVSWI might be a consequence of airway pressure-induced changes on the myocardial performance. Arterial oxygenation was good and unchanged during the study; mixed venous oxygen tension (Pvo₂) was higher and the arteriovenous oxygen content difference (avDo₂) was smaller during mode B than during mode C. During mode S, the total oxygen consumption was higher than during mode B. On the basis of the observations made, there seem to be no major differences in the effects of the three methods for CPAP on the parameters studied. The Venturi principle, employing a continuous flow of gases seems, however, to produce a slightly lower oxygen consumption, while the myocardial performance was best preserved with the system based on the elastic inspiratory gas reservoir.     

EVALUATION OF THE AMBU CPAP SYSTEM

We have assessed a Venturi driven device for delivering continuouspositive airway pressure (CPAP) using a reservoir bag and expiratoryvalve under conditions of continuous flow and simulated spontaneousbreathing. The system performed well and was economical, consumingonly 3.5 litre min^–1 of fresh gas. One Venturi was partiallyblocked and performed inadequately, but the function of a secondone was close to the manufacturer's specification (inspiredoxygen 33%, flow 20 litre min^–1 against end-expiratorypressures of 0–1.8 kPa). Compliance curves for two reservoirbags (new and old) were defined; these showed that complianceincreased as the pressure in the circuit increased. The characteristicsof the expiratory valve approached those of a threshold resistor.Small fluctuations in airway pressure occurred at all settingsof CPAP and decreased with the increasing compliance of thecircuit at higher values of CPAP. The method provided to monitorthe airway pressure was inaccurate and overestimated the truepressure by 20% at pressures greater than 1 kPa.     

Exposure to noise during continuous positive airway pressure: influence of interfaces and delivery systems

Background: We measured noise intensity and perceived noisiness during continuous positive airway pressure (CPAP) performed with two interfaces (face-mask, helmet) and four delivery systems.
Methods: Eight healthy volunteers received CPAP in random order with: two systems provided with a flow generator using the Venturi effect and a mechanical expiratory valve (A: Venturi, Starmed; B: Whisperflow-2, Caradyne Ltd); one 'free-flow' system provided with high flow O₂ and air flowmeters, an inspiratory gas reservoir, and a water valve (C: CF800, Drägerwerk, AG); and a standard mechanical ventilator (Servoventilator 300, Siemens-Elema). Systems A, B, and C were tested with a face-mask and a helmet at a CPAP value of 10 cm H₂O; the mechanical ventilator was only tested with the face mask. Noise intensity was measured with a sound-level meter. After each test, participants scored noisiness on a visual analog scale (VAS).
Results: The noise levels measured ranged from 57±11 dBA (mechanical ventilator plus mask) to 93±1 and 94±2 dBA (systems A and B plus helmet) and were significantly affected by CPAP systems (A and B noisier than C and D) and interfaces (helmet CPAP noisier than mask CPAP). Subjective evaluation showed that systems A and B plus helmet were perceived as noisier than system C plus mask or helmet.
Conclusions: Maximum noise levels observed in this study may potentially cause patient discomfort. Less noisy CPAP systems (not using Venturi effect) and interfaces (facial mask better than helmet) should be preferred, particularly for long or nocturnal treatments.     

Pressure characteristics of the Ambu CPAP system and the Servo Ventilator 900C in CPAP mode

Spontaneous breathing was stimulated in the Ambu continuous positive airway pressure (CPAP) system and the Servo Ventilator 900C by means of a lung model programmed to mimic the respiratory flow patterns of a healthy volunteer and a patient in severe respiratory distress. Changes in airway pressure, flow and volume were recorded during "breathing" with CPAP at 0.5, 1.0 and 1.5 kPa. In the Ambu system, the airway pressure decreased during inspiration and increased during expiration, while the mean airway pressure was close to the pre-set CPAP value. The pressure changes were minimal when the fresh gas flow was increased from 15 to 25 1 X min-1. The higher fresh gas flow is recommendable during deep or rapid breathing. In the Servo ventilator 900C, there was a short initial inspiratory pressure drop, succeeded by a pressure rise above the CPAP value. The expiratory airway pressure was somewhat higher than CPAP. Both systems were found to be recommendable for clinical use.     

Easy alteration of the PEEP-system to CPAP by using exclusively original bird parts (author's transl)]

In various situations breathing with positive-end expiratory pressure (PEEP) and spontaneous respiration with continously positive airway pressure (CPAP) is of advantage to the patient. Not all intensive care units are equipped with the modern apparatus that makes possible alternative application of PEEP and CPAP. We have developed a system using a BIRD Mark 7 or 8 that allows alteration of PEEP to CPAP by only few manipulations. As a safety measure we exclusively apply original parts of the same firm (BIRD). The system allows exact adjustment and controllable application of a continuous positive airway pressure, exact proportion of oxygen in the inspired gas, moistening of the in spired air and nebulization of medicaments. If the patient should show inadequate ventitation after a certain times then artificial respiration with the PEEP system is quickly possible with only a few manipulations.     

Gas,tubes and flow

Gases behave as ‘fluids’ under flow conditions. There are two main flow patterns: laminar and turbulent. Here, we review the flow characteristics of gases and how they relate to the airway and endotracheal tubes. An understanding of these characteristics can be manipulated to improve flow in clinical situations; for example, using a gas with a lower density than air such as heliox reduces turbulent flow and may be helpful in patients with airway obstruction. The Bernoulli principle and Venturi effect have been used to develop fixed-performance masks, jet ventilators and suction devices.     

A new CPAP system

The "AMBU-CPAP system" is a new, simple and reliable circuit for administering positive airway pressure in spontaneous-breathing therapy. Some disadvantages of other CPAP devices are avoided, and the use of the system with low gas flow is possible. The change of airway pressure during respiration was measured and was found to be less than 5 millibars.     

Use of the Inspiron nebuliser during continuous positive airway pressure ventilation

The Inspiron Nebuliser 002305 with air entrainment was assessed as a gas delivery device in a continuous positive airway pressure system. Inspired oxygen concentrations, total gas flows and pressures within the system were measured over a range of settings, with and without positive and expiratory pressure. Inspired oxygen concentrations and total flows were completely disrupted when a positive and expiratory pressure valve was applied, and the system failed to generate continuous positive airway pressure. We would not therefore recommend the Inspiron nebuliser as a gas delivery system for continuous positive airway pressure.     

Ventilatory support by continuous positive airway pressure breathing improves gas exchange as compared with partial ventilatory support with airway pressure release ventilation

In acute lung injury, airway pressure release ventilation (APRV) with superimposed spontaneous breathing improves gas exchange compared with controlled mechanical ventilation. However, the release of airway pressure below the continuous positive airway pressure (CPAP) level may provoke lung collapse. Therefore, we compared gas exchange and hemodynamics using a crossover design in nine pigs with oleic acid-induced lung injury during CPAP breathing and APRV with a release pressure level of 0 and 5 cm H(2)O. At an identical minute ventilation (V(E) 8 L/min) spontaneous breathing averaged 55%, 67%, and 100% of V(E) during the two APRV modes and CPAP, respectively. Because of the concept of APRV, mean airway pressure was highest during CPAP and lowest during APRV with a release pressure of 0 cm H(2)O. Shunt was reduced to almost half during CPAP (6.6% of Q(t)) compared with both APRV-modes (13.0% of Q(t)). Cardiac output and oxygen consumption, in contrast, were similar during all three ventilatory settings. Thus, in our lung injury model, CPAP was superior to partial ventilatory support using APRV with and without positive end-expiratory pressure. This may be attributable to beneficial effects of spontaneous breathing on gas exchange as well as to rapid lung collapse during the phases of airway pressure release below the CPAP level. These findings may suggest that the amount of mechanical ventilatory support using the APRV mode should be kept at the necessary minimum. IMPLICATIONS: Oxygenation is better with continuous positive airway pressure breathing than with partial mechanical ventilatory support using airway pressure release ventilation. Therefore, mechanical ventilatory support achieved by a cyclic release of airway pressure during APRV should be kept at the minimum level that enables enough ventilatory support for patients to avoid respiratory muscle fatigue.     

THE HIGH OXYGEN MIXTURES DELIVERED BY THE AIR-MIX CONTROL OF THE BIRD MARK 7 VENTILATOR

The reasons for the high oxygen mixtures delivered by the BirdMark 7 ventilator were investigated by measuring the flow ofgases through various parts of the ventilator. The major causeswere the constant flow of 100 per cent oxygen from the nebulizer,the decrease and eventual cessation of flow through the venturias the airway pressure rose, and the decrease in the proportionof entrained air at low flow settings.     

Constant positive airway pressure. A review.

Constant positive airway pressure (CPAP) represents a major advance in the treatment of hypoxaemia. By increasing the functional residual capacity (FRC), airway closure and airway resistance are minimized. Ventilation and perfusion are more evenly matched so that gas exchange is made more efficient. In addition, the lung remains on a favourable portion of the compliance curve so that the work of breathing is reduced. The increased oxygenation is achieved at the lowest possible inspired oxygen concentration (FiO2) to minimize oxygen toxocity. A unified approach to the terms used, as well as their abbreviations, is presented.     

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 FiO, 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, F(E)O2, F(E)CO2 and airway pressures were measured. Calculation of FiO2 from F(E)O2 and F(E)CO2 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 cm H20. 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.     

Augmentierte Spontanatmung

Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hyercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devices; it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory ventilation (IMV) allows spontaneous breathing of the patient and augments alveolar ventilation by periodically applying positive airway pressure tidal volumes, which are generated by the respirator. Potential advantages include lower mean airway pressure (P_AW), as compared with controlled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fatigue of the respiratory muscles and CO₂ retention. During pressure support ventilation (PSV), inspiratory alterations of P_AW or gas flow (trigger) are detected by the respirator, which delivers a gas flow to maintain P_AW at a fixed value (usually 5–20?cm H₂O) during inspiration. PSV may be combined with other modalities of respiratory therapy such as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of oxygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveolar ventilation is not guaranteed during PSV; thus, close observation of the patient is mandatory to avoid serious respiratory complications. Continuous positive airway pressure breathing (CPAP) maintains P_AW above atmospheric pressure throughout the respiratory cycle, which may increase functional residual capacity and decrease the effort of breathing. CPAP has been conceptually designed for the augmentation of spontaneous breathing and requires the intact central and peripheral regulation of the respiratory system. Airway pressure release ventilation (APRV) improves alveolar ventilation by intermittent release of P_AW, which is kept above atmospheric pressure by means of a high-flow CPAP system. The opening of an expiratory valve for 1–2?s induces a decreased P_AW and lung volume, which increases rapidly to pre-exhalation values after closure of the valve due to the high gas flow within the circuit (90–100?l/min). APRV may improve haemodynamics and V_A/Q distribution as compared with conventional mechnical ventilation. Biphasic positive airway pressure (BIPAP) is characterized by the combination of spontaneous breathing and time-regulated, pressure-controlled mechanical ventilation. During the respiratory cycle the ventilator generates two alternating CPAP levels, which can be modified with regard to time and pressure. As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved V_A/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.     

Xenon inhalation increases airway pressure in ventilated patients

BACKGROUND: The inert gas xenon, known as an anaesthetic for nearly 50 years, is also used as a contrast agent during computerised tomography (CT)-scanning. As xenon has a higher density and viscosity than air, xenon inhalation may increase airway resistance. METHODS: In a retrospective study we investigated the effects of 33% xenon/67% oxygen on airway pressure and cardio-respiratory parameters in 37 long-term mechanically ventilated patients undergoing cerebral blood flow (rCBF) measurements by means of stable xenon-enhanced CT. RESULTS: Xenon administration caused a significant increase in peak airway pressure from 31.6+/-8.0 cm H2O to 42.7+/-16.9 cm H2O. This effect was reproducible, did not occur after reduction of inspiratory flow rate by 50% from 0.56+/-0.15 L x s(-1) to 0.28+/-0.08 L x s(-1), and vanished immediately after termination of xenon delivery. CONCLUSION: Due to the higher density and viscosity of this gas mixture, ventilation with xenon/oxygen produces a higher Reynolds' number than oxygen/air when given at the same flow rate. This means that during xenon ventilation the zone of transition from turbulent to laminar gas flow may be located more peripherally (in smaller airways) than during oxygen/air ventilation with a subsequent increase in airway resistance. Our results indicate that xenon inhalation may cause a clinically relevant increase of peak airway pressure in mechanically ventilated patients.     

Pressure support ventilation versus continuous positive airway pressure with the laryngeal mask airway: a randomized crossover study of anesthetized adult patients

BACKGROUND: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway. METHODS: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate. RESULTS: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP. CONCLUSION: The authors concluded that PSV provides more effective gas exchange than does unassisted ventilation with CPAP during LMA anesthesia while preserving leak fraction and hemodynamic homeostasis.     

Continuous-flow apneic ventilation during thoracotomy

Continuous-flow apneic ventilation (CFAV) by endobronchial insufflation of conditioned gas was evaluated in dogs during thoracotomy. In Group 1 (n = 6), dogs were anesthetized with pentobarbital (25 mg/kg). An endobronchial catheter (2.5 mm ID) was introduced into each mainstem bronchus using a fiberoptic bronchoscope and held in place by an endotracheal tube. Before the onset of CFAV (total flow 1.01 X kg-1 X min-1, the animals were paralyzed with pancuronium bromide and muscle relaxation was monitored with a peripheral nerve stimulator. The CFAV delivery system consisted of a flow meter, air/oxygen blender, oxygen analyzer, heated humidifier, and ultrasonic spirometer. Blood gas values were measured after 30 min of spontaneous ventilation, and CFAV with: 1) closed chest, fractional inspired O2 concentration (FIO2) 0.21; 2) open chest, FIO2 0.21; 3) open chest, FIO2 0.21, continuous positive airway pressure (CPAP) 5 mmHg; and 4) open chest FIO2 0.4, CPAP 5 mmHg. This last combination resulted in a mean PaO2 of 113.1 +/- 5.5 (SEM) mmHg and a PaCO2 of 35.0 +/- 2.1 (SEM) mmHg. In Group 2 (n = 6), animals with open chests were ventilated with CFAV (FIO2 0.4 and CPAP 5 mmHg) for 5 h. Adequate oxygenation and ventilation were achieved. PaCO2 after 5 h of CFAV was 41.8 +/- 1.9 (SEM) mmHg compared with 40.8 +/- 1.9 (SEM) mmHg during spontaneous breathing. PaO2 after 5 h of CFAV was 138.1 +/- 11.7 (SEM) mmHg. There were no significant changes observed in vascular pressures. Significant differences in other hemodynamic parameters were probably due to pentobarbital anesthesia. Adequate gas exchange can be achieved during CFAV in dogs with open chests for 5 h.     

Delivery characteristics of a combined nitric oxide nasal continuous positive airway pressure system

BACKGROUND: Nitric oxide (NO), when inhaled, has a synergistic effect with airway recruitment strategies such as positive endexpiratory pressure (PEEP) or continuous positive airway pressure (CPAP) in improving oxygenation in lung injury. METHODS: We modified a commercially available nasal CPAP (nCPAP) system to enable the concomitant delivery of inhaled NO (iNO) and nCPAP to neonates and term babies. Oxygen, NO and nitrogen dioxide (NO2) concentrations were measured, comparing the effects of using 50 or 1000 parts per million (p.p.m.) NO stock gas cylinders. RESULTS: Stable and accurate delivery of iNO was found for both stock gas concentrations. Using a 50 p.p.m. NO stock gas resulted in limited NO2 formation, with a maximum inspired NO2 concentration of < or = 0.3 p.p.m. (dose range up to 37 p.p.m. iNO), which was interpreted as the result of progressive dilution with nitrogen. In contrast, using a 1000 p.p.m. NO stock gas cylinder, inspired NO2 levels increased nonlinearly as expected with an increasing inspired concentration of NO. CONCLUSIONS: Inhaled NO can be safely and reliably delivered by the system we describe. The NO2 levels generated by the system are low, at least up to a dose of 37 p.p.m. NO, regardless of a stock gas concentration of 50 or 1000 p.p.m. NO. Using a 50 p.p.m. NO stock gas concentration, up to 80% oxygen can be given at 10 p.p.m. iNO.     

Continuous positive airway pressure. The use of low levels in adult patients with artificial airways

Twelve adult surgical patients recovering from acute respiratory failure (ARF) had a catheter inserted for measurement of pulmonary capillary wedge pressure, pulmonary artery pressure, and cardiac output; and for calculation of arterial-mixed venous oxygen content difference, physiologic shunt, and systemic and pulmonary vascular resistances. Measurements were made with 5 cm H2O continuous positive airway pressure (CPAP), during spontaneous respiration at ambient airway pressure, and then again with 5 cm H2O CPAP. During breathing without CPAP, the total physiologic shunt increased, and the PaO2/fraction of inspired oxygen concentration (FiO2) ratio decreased significantly from control values. After CPAP therapy was again introduced, the physiologic shunt decreased and the PaO2/FiO2 ratio increased significantly. This improvement in oxygenation took place without any significant change in cardiovascular status. Thus, low levels of CPAP are helpful in maintaining the pulmonary status in intubated adult patients recovering from ARF.     

Pressure Support Ventilation versus Continuous Positive Airway Pressure with the Laryngeal Mask Airway: A Randomized Crossover Study of Anesthetized Adult Patients

Background: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway.

Methods: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate.

Results: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP.     

Unilateral high-frequency jet ventilation during one-lung ventilation for thoracotomy

One-lung ventilation is indicated during thoracic operations for bronchopleural fistula, pulmonary abscess, and pulmonary hemorrhage in spite of the possibility of the development of severe hypoxemia. To evaluate methods for improving oxygen transport during one-lung ventilation, we applied high-frequency jet ventilation (HFJV) and continuous positive airway pressure (CPAP) to the nondependent lung following deflation to atmospheric pressure in each procedure, and measured the effects on cardiac output and arterial oxygenation. In each case, the dependent lung was ventilated with conventional intermittent positive pressure ventilation (IPPV).

Eight patients were studied during posterolateral thoracotomy using double-lumen endobronchial tubes. HFJV or CPAP to the nondependent lung improved arterial oxygenation significantly during both closed and open stages of the surgical procedures (p < 0.008). When the chest was open, HFJV maintained satisfactory cardiac output, whereas CPAP usually decreased cardiac output (p < 0.008). There were no significant differences in mean partial pressure of arterial carbon dioxide between HFJV, CPAP, and deflation to atmospheric pressure.

In conclusion, HFJV to the nondependent lung provides not only satisfactory oxygenation but also good cardiac output, thereby maintaining better oxygen transport than CPAP or deflation to atmospheric pressure, while the dependent lung is ventilated with IPPV during one-lung ventilation for thoracotomy.