Measurement of Pressure-Time Product during Spontaneous Assisted Breathing by Rapid Interrupter Technique

Background: Measuring the work of breathing of patients undergoing spontaneous assisted ventilation can be useful to monitor and titrate ventilatory support. The aim of this study was to obtain measurements of the pressure generated by the respiratory muscles (PMUSC) and the derived pressure-time product (PTP; a good indicator of the metabolic work of breathing), performing the rapid interrupter technique with a commercial ventilator.

Methods: A Draeger Evita 4 ventilator (Draeger Medical, Lubeck, Germany) was controlled by a personal computer to rapidly interrupt the airway flow at different times and volumes of the respiratory cycle during pressure-support ventilation. From the airway pressure tracing after the occlusion, the authors estimated the alveolar pressure and PMUSC; the integration of PMUSC values over the inspiratory time yields the measurement of PTP. Esophageal pressure measurements were used as a reference. After a bench study of the valves' performance, the authors performed 11 measurement sequences in eight patients.

Results: The closure times for the inspiratory and expiratory valves were 74 +/- 10 and 61 +/- 13 ms, respectively. The interrupter technique provided a reliable estimate of PMUSC (PMUSC, occl = 1.00 [middle dot] PMUSC, pes + 0.19; r = 0.88; 95% confidence interval for agreement, +5.49/-5.32 cm H2O). PTPoccl tightly correlated with PTPpes (PTPoccl = 0.95 [middle dot] PTPpes + 0.13; r = 0.96; 95% confidence interval, 1.94/-1.61 cm H2O [middle dot] s).     

Permissive hypercapnia

The term permissive hypercapnia defines a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure. The aim of permissive hypercapnia is to minimize lung damage during mechanical ventilation; its limitation is the resulting hypoventilation and carbon dioxide (CO2) retention. In this article we discuss the rationale, physiologic implications, and implementation of permissive hypercapnia. We then review recent clinical studies that tested the effect of various approaches to permissive hypercapnia on the outcome of patients with acute respiratory failure.     

Danger of helmet continuous positive airway pressure during failure of fresh gas source supply

Objective To assess the behavior of different helmets after discontinuation of fresh gas flow by disconnection at the helmet inlet, flow generator, or gas source. Design and setting Randomized physiological study in a university research laboratory. Patients Five healthy volunteers. Intervention CPAP (FIO₂ 50%, PEEP 5 cmH₂O) delivered in random sequence with three different helmets: 4Vent (Rüsch), PN500 (Harol), CaStar (StarMed) with antisuffocation valve open or locked. For each helmet all three disconnections were randomly employed up to 4 min. Measurements and results During flow disconnection we measured: respiratory rate and tidal volume by respitrace; inspiratory and expiratory CO₂ concentration, and FIO₂ from a nostril; SpO₂ by pulse oxymetry. Independently of the site of disconnection we observed a fast increase in CO₂ rebreathing and minute ventilation, associated with a decrease in inspired O₂ concentration. In the absence of an operational safety valve, larger helmet size and lower resistance of the inlet hose resulted in slower increase in CO₂ rebreathing. The presence of the safety valve limited the rebreathing of CO₂, and the increase in minute ventilation but did not protect from a decrease in FIO₂ and loss of PEEP. Conclusions While the use of a safety valve proved effective in limiting CO₂ rebreathing, it did not protect from the risk of hypoxia related to decrease in FIO₂ and loss of PEEP. In addition to a safety antisuffocation valve, a dedicated monitoring and alarming systems are needed to employ helmet CPAP safely.     

Measurement of pressure-time product during spontaneous assisted breathing by rapid interrupter technique

BACKGROUND: Measuring the work of breathing of patients undergoing spontaneous assisted ventilation can be useful to monitor and titrate ventilatory support. The aim of this study was to obtain measurements of the pressure generated by the respiratory muscles (PMUSC) and the derived pressure-time product (PTP; a good indicator of the metabolic work of breathing), performing the rapid interrupter technique with a commercial ventilator. METHODS: A Draeger Evita 4 ventilator (Draeger Medical, Lubeck, Germany) was controlled by a personal computer to rapidly interrupt the airway flow at different times and volumes of the respiratory cycle during pressure-support ventilation. From the airway pressure tracing after the occlusion, the authors estimated the alveolar pressure and PMUSC; the integration of PMUSC values over the inspiratory time yields the measurement of PTP. Esophageal pressure measurements were used as a reference. After a bench study of the valves' performance, the authors performed 11 measurement sequences in eight patients. RESULTS: The closure times for the inspiratory and expiratory valves were 74 +/- 10 and 61 +/- 13 ms, respectively. The interrupter technique provided a reliable estimate of PMUSC (PMUSC, occl = 1.00 . PMUSC, pes + 0.19; r = 0.88; 95% confidence interval for agreement, +5.49/-5.32 cm H2O). PTPoccl tightly correlated with PTPpes (PTPoccl = 0.95 . PTPpes + 0.13; r = 0.96; 95% confidence interval, 1.94/-1.61 cm H2O . s). CONCLUSION: The rapid interrupter technique can be performed by means of a commercial ventilator, providing reliable measurement of PMUSC and PTP.     

Head helmet versus face mask for non-invasive continuous positive airway pressure: a physiological study

Objective To assess selected physiological effects of non-invasive continuous positive airway pressure delivered by head helmet (CPAP_H), a special interface device designed to completely contain the head of the patient, compared to face mask (CPAP_M).Design Randomized physiological study.Setting University research laboratory.Patients Eight healthy volunteers.Intervention Continuous positive airway pressure delivered by face mask and CPAP_H in random order. Three gas flow rates (20-30-40 l/min and 30-45-60 l/min, respectively, for CPAP_M and CPAP_H) and four CPAP levels (0-5-10-15 cmH₂O) were employed in a randomized sequence.Measurements and results In each patient we monitored airway pressure, esophageal pressure, expiratory flow, and inspiratory and expiratory CO₂ concentration. End-expiratory lung volume changes from CPAP 0 were measured by inductance plethysmography. The application of increased levels of CPAP resulted in a significant increase in end-expiratory lung volume, similar for CPAP_H and CPAP_M. Inspiratory changes of airway pressure were comparable for the two CPAP modes. Inspiratory CO₂ concentration was higher during CPAP_H (significantly decreased at increased gas flow rates), compared to CPAP_M.Conclusions Continuous positive airway pressure delivered by head helmet is as effective as CPAP_M in increasing end-expiratory lung volume and in compensating for airway pressure changes without the need of a reservoir bag. Higher gas flow rates are necessary to maintain a relatively low inspiratory CO₂ concentration.     

Daily nursing care on patients undergoing venous–venous extracorporeal membrane oxygenation: a challenging procedure!

Daily nursing in critical care patients may alter vital parameters, especially in the most critically ill patients. The aim of our study was to evaluate feasibility and safety of daily nursing on patients undergoing venous–venous extracorporeal membrane oxygenation (vv-ECMO) for severe respiratory failure. Daily nursing was performed following defined phases (sponge bath, elevation with scooping stretcher, change position of endotracheal tube, dressing replacement). We recorded physiological and ECMO parameters before and during daily nursing in 5 patients for several days (total: 25 daily nursing) and adverse events: desaturation, hypertension, reduction of mixed venous oxygen saturation, arterial oxygen saturation or ECMO blood flow and elevation in minute ventilation. Sedative drug dosage and additional bolus were recorded. Daily nursing was performed in 92 % of cases (23/25), with a minimum of two adverse events per daily nursing. Hypertension and tachycardia were mostly recorded at the beginning, while desaturation, reduction in mixed venous oxygen saturation and blood flow were recorded during elevation with scooping stretcher. Increase in minute ventilation was frequent in spontaneous breathing patients. Additional bolus of sedation was required before and/or during nursing. Daily nursing significantly alters physiologic parameters; thus, it should be performed only when physicians are readily available to treat adverse events.     

Performance of different continuous positive airway pressure helmets equipped with safety valves during failure of fresh gas supply

Purpose  

We assessed the performance of different continuous positive airway pressure (CPAP) helmets equipped with a safety valve during discontinuation of fresh gas flow.