 Show
Review ArticlePositive pressure ventilation: what is the real cost?AbstractPositive pressure ventilation is a radical departure from the physiology of breathing spontaneously. The immediate physiological consequences of positive pressure ventilation such as haemodynamic changes are recognized, studied, and understood. There are other significant physiological interactions which are less obvious, more insidious, and may only produce complications if ventilation is prolonged. The interaction of positive pressure with airway resistance and alveolar compliance affects distribution of gas flow within the lung. The result is a wide range of ventilation efficacy throughout different areas of the lung, but the pressure differentials between alveolus and interstitium also influence capillary perfusion. The hydrostatic forces across the capillaries associated with the effects of raised venous pressures compound these changes resulting in interstitial fluid sequestration. This is increased by impaired lymphatic drainage which is secondary to raised intrathoracic pressure but also influenced by raised central venous pressure. Ventilation and PEEP promulgate further physiological derangement. In theory, avoiding these physiological disturbances in a rested lung may be better for the lung and other organs. An alternative to positive pressure ventilation might be to investigate oxygen supplementation of a physiologically neutral and rested lung. Abandoning heroic ventilation would be a massive departure from current practice but might be a more rationale approach to future practice. Keywordscomplications, respiratory lung, compliance lung, pathophysiology ventilation, positive end-expiratory pressure Cited by (0)Copyright © 2008 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved. Disclaimer Oxford University Press makes no representation, express or implied, that the drug dosages in this book are correct. Readers must therefore always … More Oxford University Press makes no representation, express or implied, that the drug dosages in this book are correct. Readers must therefore always check the product information and clinical procedures with the most up to date published product information and data sheets provided by the manufacturers and the most recent codes of conduct and safety regulations. The authors and the publishers do not accept responsibility or legal liability for any errors in the text or for the misuse or misapplication of material in this work. Except where otherwise stated, drug dosages and recommendations are for the non-pregnant adult who is not breastfeeding. You do not currently have access to this chapter.
Book In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan. 2022 Apr 28. Affiliations
Free Books & Documents Book Ventilation Assist ControlAndres L. Mora Carpio et al. Free Books & Documents ExcerptMechanical ventilation is a lifesaving procedure that is often performed when patients require respiratory support. Assist-control (AC) mode is one of the most common methods of mechanical ventilation in the intensive care unit. AC ventilation is a volume-cycled mode of ventilation. It works by setting a fixed tidal volume (VT) that the ventilator will deliver at set intervals of time or when the patient initiates a breath. The VT delivered by the ventilator in AC always will be the same regardless of compliance, peak, or plateau pressures in the lungs. When AC mode is selected in the ventilator, four parameters may be quickly modified: Tidal Volume (VT) This is the set amount of volume that will be delivered with each breath. Changing the VT will, in turn, change the minute ventilation (VT x RR); an increase in minute ventilation will result in a decrease in carbon dioxide (CO2), by the same token, a decreased VT will result in a decreased minute ventilation and increase in the patient’s blood CO2. Respiratory Rate (RR) This is the set rate for delivering breaths per minute (bpm). For example, if the set rate is 15, then the delivery is 15 bpm or 1 breath every 4 seconds. This is called time-triggered control. In AC, this set rate can be overturned by the patient, meaning that if the patient inhales, the ventilator will sense the drop in pressure and deliver that breath, even if the patient is breathing above the set rate. For example, if a patient is breathing at 20 bpm and the ventilator is set at 15 bpm, the ventilator will follow the patient and deliver 20 bpm (one each time the patient initiates a breath). This is called patient-triggered breaths. The ventilator will only deliver breaths at the set RR if the patient does not trigger it faster. As with VT, increasing RR will increase minute ventilation and decrease the patient’s blood CO2. A caveat on this is that by increasing the RR, the dead space is also increased, so increasing RR may not be as effective as increasing VT in improving ventilation. The ventilator in AC mode is programmed to sense changes in the system pressure when a patient initiates a breath. When the diaphragm contracts, the intrathoracic pressure becomes more negative. The negative pressure is transmitted to the airways and then to the ventilator tubing, where sensors detect the change in pressure and deliver a breath to the set tidal volume. The amount of negative pressure needed to trigger a breath is called the trigger sensitivity and is usually set up by the respiratory therapist. The Fraction of Inspired Oxygen (FiO2) This is the percentage of oxygen in the air mix delivered by the ventilator during each respiratory cycle. Increasing the FiO2 will increase the patient's oxygen saturation. Positive End Expiratory Pressure (PEEP) The positive pressure that will remain in the system at the end of the respiratory cycle (end of expiration) is the PEEP. As with FiO2, PEEP can be used to increase oxygenation. By Henry’s law, we know that the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution. This applies to mechanical ventilation in that increasing PEEP will increase the pressure in the system. This increases the solubility of oxygen and its ability to cross the alveolocapillary membrane and increase the oxygen content in the blood. PEEP also can be used to improve ventilation-perfusion mismatches by opening or “splinting” airways to improve ventilation throughout the system. Apart from these four main parameters, the way the ventilation is delivered also can be adjusted. For every setting, regardless of the rate and volume, the breath will always be delivered to the patient in the same way. The ventilator allows flow change; the flow may be constant through the inhalation (square waveform) or decelerating as the breath is delivered (ramp waveform).
The speed at which this flow is delivered also can be controlled by setting inspiratory and expiratory times. This can be adjusted for patient comfort or to prevent auto-PEEP. After the inspiration is finished, the expiratory valve of the ventilator opens, and the air is allowed to come out until the pressure in the system reaches PEEP. (figure 1) Copyright © 2022, StatPearls Publishing LLC. Sections
Similar articles
References
Publication typesLinkOut - more resources
Which physical law explains Elastance?Hooke's law explains elastance. Hooke's law explains elastance. The intra-alveolar and intrapleural pressures would increase during a positive pressure breath from a mechanical ventilator. The intra-alveolar and intrapleural pressures would increase during a positive pressure breath from a mechanical ventilator.
Which of the following is one of the pressures that influences ventilation?Pulmonary ventilation is dependent on three types of pressure: atmospheric, intra-alveolar, and interpleural. Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body.
What is the formula for lung compliance?The following formula is useful to calculate compliance: Lung Compliance (C) = Change in Lung Volume (V) / Change in Transpulmonary Pressure {Alveolar Pressure (Palv) – Pleural Pressure (Ppl)}.
What is the reciprocal of compliance?Elastance, the reciprocal of compliance, is the pressure required to inflate the lungs. One half of this pressure is spent to inflate the lungs, and the other half is used to inflate the chest wall in normal lungs. Normally, the elastance of the lungs and chest wall is similar.
|
Which law best explains the basic operation of the negative pressure ventilator?
zusammenhängende Posts
Urheberrechte © © 2024 ketiadaan Inc.
