Deadspace Ventilation and Acute Respiratory Distress Syndrome

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Table of Contents

Introduction
Definition of Terms Relevant to Topic
Associated Disease
Clinical/Physiological Effects
Current Therapy
Effects of Therapy
Role of the Respiratory Therapist
Summary
Reference List

Introduction

Acute Respiratory Distress Syndrome (ADRS) is a chronic reaction to acute infections or injuries of the lungs. ADRS is not a disease but rather a syndrome triggered by diverse direct and indirect factors. The parenchyma of the lung becomes inflamed, causing impairment in the process of gas exchange. Concomitant release of mediators that are responsible for inflammation takes place systematically leading to hypoxemia, and severe organ failure (Aboab et al. 2000). The condition is often acute and chronic necessitating the patient to be admitted in an intensive care unit and be put under mechanical ventilation (Sundaresan et al 2011, p.1). The lung of a person suffering from ADRS is referred to as a baby lung because it is smaller and stiffer.

Dead space ventilation involves use of gas that does not interact with pulmonary blood at any one time. The clinical importance of Deadspace Ventilation is the lack of physiologic benefit of the energy utilized to move the gas.Inefficient and inadequate flow of pulmonary blood results to an increase in dead space ventilation due to inadequate flow of blood in the lungs to exchange with the ventilation gas.

Dead space ventilation ratio is increased when the patient is undergoing mechanical ventilation and a ratio of 0.50 is considered normal. A ratio below 0.60 does not warrant any reason to obstruct natural respiration while a ratio of 0.60-0.80 portends chronic disease and indicates that the patient cannot handle prolonged natural respiration. This paper is an analysis of the relationship between ARDs and Deadspace ventilation.

ARDs deplete the capacity of lungs for ventilation. The patients are therefore admitted into the intensive care unit where they require mechanical ventilation (Sundaresan et al 2011, p.1). Through artificial management of ventilation, the mortality of patients suffering from ARDs has been increased. This has been achieved through strictly limiting the tidal volume and the maintenance of plateau pressure (Pplat) below 30cmH₂O. A bronchorial collapse partially or totally excludes certain compartments of the lung from ventilation. ARDS does not respond to administration of high concentration of inspiratory oxygen (Niklason, 2008). Dead space ventilation helps in delivering information about the relationship between body tissue and gas in terms of quantity.

Definition of Terms Relevant to Topic

Dead space ventilation is a form of mechanical ventilation. It is recommended for patients requiring support in elimination of carbon dioxide and in maintenance of oxygen. Ventilated patients usually suffer from abnormalities in the lung structure, obstructions in the airways, and damaged tissues of the lung (Forel et al. 2012).Mechanical ventilation is anchored on the concept that the behavior of air is similar to that of fluid since both air and fluid follow the path that has least resistance as they enter a surface.

The maximal pressure in the airways during respiration is known as Peak Inspiratory Pressure (PIP). PIP is used to measure the pressure in the major air paths in the lungs. Acute or Rapid changes in PIP normally indicate severe complications such as bronchospasm or plugging of mucus. Plateau Pressure (P_plat) on the other hand measures the pressure of airways at the last stages of inspiration and normally indicates the pressure in the alveoli.P_plat determines complications that are brought about by the ventilator such as volutrauma and must always be kept between 30-35 cm H₂O pressure.

The volume of air that is inhaled and exhaled during a respiratory cycle is referred to as tidal volume (Vt). Minute ventilation determines (MV) the levels of carbon dioxide in the blood. It can be calculated by multiplying the tidal volume with the respiratory rate (Charron et al., 2011, p.2). Increasing Minute Ventilation decreases the level of carbon dioxide in the blood by increasing the rate at which elimination of carbon dioxide from the blood takes place. Decreasing the M.V increases the level of pulmonary carbon dioxide by reducing the rate at which carbon dioxide is eliminated from the blood. The non-perfused areas in the natural respiratory tract are referred to as Dead Space (V_DS).Dead space describes the parts and components of the respiratory system that do not indulge in elimination of carbon dioxide.

The mean Distribution time (MDT) defines the time available for alveolar diffusion and distribution of tidal gas (Aboab et al., p.1). The ratio of the dead space versus that of the tidal volume determines the lungs capability to transport carbon dioxide. This process is affected by pathology as well as by settings of the ventilator. When blood perfusion and air ventilation do not match, there is an abnormal deadspace, which manifests itself in the form of disorders such as pulmonary embolism (Bhadade et al. 2011). The condition is characterized by ventilation of the alveolar while blood perfusion is not taking place. An increased VDS/VT ratio causes abnormal oxygenation and irregular ventilation.

The Fraction of Inspired Oxygen (FiO₂) connotes the percentage of oxygen in the air that the ADRS patient inhales. The FiO₂of room air is 21%. Increase of The Fraction of Inspired Oxygen to levels beyond 60% is attributed to increase in production of free radical oxygen, which could harm the cells due to the toxicity of oxygen. Arterial carbon dioxide (PACO₂₎decreases because of reduction in ventilation (Charron et al., 2011, p.2). Patients suffering from ARDS have poor respiratory systems and require The Fraction of Inspired Oxygen levels to be above 60% (Forel et al., 2012). High levels of Fraction of Inspired Oxygen are recommended for them even when they are in danger of oxygen toxicity. Mechanical ventilators are used in order to reduce The Fraction of Inspired Oxygen to safe levels.

Dead space ventilation allows the alveoli that do not take part in the ventilation process to expand and increase the surface are available for oxygenation and ventilation. The method is referred to as alveolar recruitment and it is achieved through maximizing the capillaries of the alveolar (Niklason et al., 2008). This way the deadspace alveoli are capable of remaining open and function effectively.

Associated Disease

ADRS is associated with ALI (Acute Lung Injury). This disease is characterized by injury of the lungs due to hypoxemic related disorders (Bhadade et al., 2011). It occurs when sepsis triggers systematic inflammation of the lung. Sepsis is a negative response by the body to a disease or an infection. It is caused by invasion and quick spread of bacteria in the bloodstream. The normal response of the bodys immune system is to fight diseases but on occurrence of sepsis, the immune system becomes agitated and overwhelmed. Primary ALI occurs when the liver is injured directly. For instance, it occurs when a person suffers from an infection of pneumonia. Secondary ALI is normally caused by indirect injury on another organ of the body such as an infection of the pancreas. It is severe but is not as fatal as ADRS.

Clinical/Physiological Effects

It is estimated that one-third of the people who suffer from ARDS end up dying from the disease. The survivors are able to recover the normal functioning of their lungs though most of them contract mild permanent damage of the lungs. During the time that the lungs are not functioning properly, the brain does not receive sufficient oxygen as a consequence brain damage occurs (Forel et al., 2012). ARDS patients therefore suffer from memory loss and a host of other psychological problems.

Current Therapy

Dead space ventilation can take various forms. In Controlled Mechanical Ventilation, the ventilator takes up the complete role of breathing. A rate and volume is set for the ventilator. The patient cannot breathe naturally in this mode because he is completely sedated and almost paralyzed. This mode is not comfortable for the patient and is highly discouraged.

In intermittent mechanical ventilation, the mechanical ventilator is set in such a way that it conveys a certain number of breaths each minute with a regulated tidal volume. The patient has the freedom to breathe in and out without depending on assistance from the ventilator. Pressure is added to the breaths generated by The Intermittent Mandatory Ventilation so that the extra pressure supports the patient when taking own breaths because a lot of energy is expended in inhalation. By increasing the pressure, the workload of breathing is reduced and the patient is able to generate high spontaneous tidal pressure (Bhadade et al., 2011).

The Intermittent Mechanical Ventilator method was traditionally used to wean the patient but modern physicians have stopped its use as it causes tremendous muscle fatigue on a respiratory system that has not fully recovered.

Pressure control ventilation modes are recommended over volume control ventilation modes because they pose less risk of injuring the alveolar. This is because they decrease the level of stretching that the alveolar undergo in weak lungs such as those of people suffering from ARDS (Niklason et al., 2008).The tidal volume is not set but it is achieved through changes in the pressure. Patients are encouraged to breathe spontaneously when pressure is at the highest level.

High-frequency ventilation employs a technique similar to the Airways Pressure Ventilation but small breaths are rapidly delivered to the patient (Aboab et al., 2012). The rapid frequency of delivering breath keeps the alveoli open allowing oxygen to be delivered easily and carbon dioxide to be eliminated without complications. This method requires the patient to be sedated and paralyzed. In the pressure support method, the ventilator is set to deliver a regulated amount of pressure when the patient initiates natural breath (Sundaresan et al., 2011).

Positive End Expiratory pressure (PEEP) has been hailed as one of the most important mechanisms in management of ADRS patients. It promotes alveolar recruitment at the termination of expiration by maintaining the unstable units of the lung in an open state (Sundaresan et al., 2011, p.2)

Effects of Therapy

Dead space ventilation mechanisms usually create complications such as volutrauma, hypotension, and in some cases Ventilator Associated Pneumonia. Volutrauma increases the risk of death and multiple organ failure and is associated with high chances of death in the intensive care unit (Forel et al., 2011, p.8). Volutrauma can be avoided by keeping plateau pressures as low as possible. Hypotension is caused by reduced pleural pressure resulting from introduction of positive pressure. It can be reversed by administering fluids and adjusting the ventilator. Ventilator associated pneumonia is a fatal complication that arises from deadspace ventilation. It increases the patients mortality, morbidity, and the time-span during which the patient is supported by the ventilator.

Ventilator associated pneumonia is usually treated by use of antibiotics that act on the pathogens that are under suspicion and employment of bronchoscopy mechanisms. The condition can be prevented by shortening the periods during which the patient undergoes mechanical ventilation (Niklason et al., 2008). The patient should also be places in a semi recumbent position as opposed to a supine position.Patients also suffer from deep vein thromboses, decline in nutritional condition and pressure ulcers. Non-invasive ventilation procedures are being called for and they will soon phase out mechanical ventilation procedures. The methods use nose and mouth masks in place of tubes.

Role of the Respiratory Therapist

The respiratory therapist should be actively involved in provision of the appropriate nutrients to patients who have undergone deadspace ventilation. The therapist should design regimens of nutrition that are patient-specific (Bhadade et al., 2011). In addition, they should always ensure that adequate oxygenation is provided, ensure that the hemodynamic function is supported and that the airway is maintained. Perfusion of the ARDS patient must be maximized in the blood capillary system and this can only be done by increasing fluids to ensure that oxygen is readily transported between the pulmonary capillaries and the alveoli. The therapist needs to constantly evaluate the patients blood pressure, pulse pressure of the arteries, cardiac index, and the level of the oxygen saturation.

Positioning of the patient is also integral to recovery. The Prone Position (PP) has been advocated for as the most efficient one in critical care. This is because it permits the slow compartments of the lungs that had been excluded from respiration by ADRS to be recruited (Charron et al., 2011, p.2). The therapist should implement kinetic therapy, and the lateral rotational therapy. The therapist should also keep monitoring and evaluating the patient for changes in the respiratory cycle and status such as reduced oxygenation, decreased saturation, increase in the rate of respiration and quick breath sounds (Niklason et al., 2008).

The therapist should also provide dexterous skin care of the patient to avoid pressure ulcers, utilize devices that relieve pressure such as air mattresses, and continuously monitor the patients nutrition condition.

Summary

Physicians and respiratory therapists working with ARDS patients undergoing deadspace ventilation should have extensive knowledge of the entire ventilation process so that they are in a position to provide the best medical care. The medication taken by the patient is affected by the mode of deadspace mechanical ventilation that the patient has undergone.

Pharmacologic and ventilation technologies and therapies are evolving rapidly and physicians must be on the lookout for the new regimens and their advantages over traditional approaches. Respiration therapists must always keep in mind that the mode of mechanical ventilation used affects delivery of medication, analgesia, and sedation to the patient. Weaning is very important in shortening the time that the patient spends in the intensive care unit. Non-invasive ventilation involving the use of masks rather than tubes contributes to optimum critical care of an ARDS patient, thus it is highly recommended.

Reference List

Aboab, J. et al. (2012). Dead space and CO2 elimination related to pattern of inspiratory gas delivery in ARDS patients. Critical Care, 16 (R39), 1-8. Web.

Charron, C. et al. (2011). PaCO2 and alveolar dead space are more relevant than PaO2/FiO2 ratio in monitoring the respiratory response to prone position in ARDS patients: a physiological study. Critical Care, 15 (R175), 1-10. Web.

Forel, J. et al. (2012). Ventilator-associated pneumonia and ICU mortality in severe ARDS patients ventilated according to a lung-protective strategy. Critical Care, 16(R65), 1-10. Web.

Sundaresan, A. et al. (2011). Model-based optimal PEEP in mechanically ventilated ARDS patients in the Intensive Care Unit. Biomedical Engineering OnLine, 10(64), 1-18. Web.

Bhadade, R. et al. (2011). Clinical Characteristics and Outcomes of Patients with acute lung Injury and ARDS. Journal of Post Graduate Medicine, 574 (286).

Niklason, J. et al. (2008). The Influence of Venous Admixture on alveolar dead space and carbon dioxide exchange in acute respiratory distress syndrome; Computer modeling. Critical Care, 12 (R53), 1-7. Web.

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