Airway Pressure the Effects of Airway Pressure essay

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Airway Pressure









Mechanical Ventilation refers to the process of helping the normal breathing process of an individual when his breathing patterns are compromised due to either a pathology within the lung or due to a collective collapse of the related organs. Mechanical ventilation can be of two types: negative pressure ventilation where the air is made to suck into the lungs or positive ventilation where air is forced into the lungs through the airway. During the process of providing ventilation to the lungs it becomes important to "secure the airway" which means that the air should go through the airway passages namely trachea, bronchi and then to lungs rather then going to the esophagus and then to the stomach. This can be done by the process of endotracheal intubation, tracheostomy or simply by airway maneuvers. Mechanical ventilation does have its own complications which include injury to the airway, pneumothorax, alveolar damage, and pneumonia associated to the ventilator.

Many airway complications indicate to mechanical ventilation which include Acute respiratory distress syndrome (ARDS), Apnea due to various conditions, chronic obstructive pulmonary disease (COPD), hypotension due to shock, sepsis or congestive heart failure, diaphragm paralysis leading to respiratory acidosis that maybe caused from muscle relaxant or anesthetic drugs, myasthenia gravis, spinal cord injury or Guillain-Barre syndrome, neurological diseases like Amyotrophic Lateral Sclerosis & Muscular Dystrophy.

Ventilators used for mechanical ventilation can be divided into the following categories: conventional ventilation by breath termination or breath initiation, high frequency ventilation (HFV), airway pressure release ventilation (APRV), non-invasive positive pressure ventilation (NIPPV), proportional assist ventilation (PAV), Adaptive Support Ventilation (ASV), and Neurally Adjusted Ventilatory Assist (NAVA). This paper will focus on the study of two ventilator mechanisms which is APRV and HFV.


Congestive heart failure refers to the inadequacy of the heart to supply blood to meet the needs of the entire body. (dorland's medical dictionary, 2009) congestive heart failure can be caused due to various conditions affecting the heart like myocardial infarction, valvular heart diseases, cardiomyopathies or hypertension. The symptoms include leg swelling, exercise intolerance and most importantly shortness of breathe. Usually a blood test or an echocardiograph can diagnose this condition. It is a costly, immobilizing, common and potentially deadly condition (McMurray JJ, 2005) the percentage of people suffering from heart failure in developed countries is 2% which increases to 6-10% in the age group of 65 and above. (Dickstein K, 2008)

The pathophysiology of CHF explains that the type of CHF most commonly affecting ventilation of the lungs is the left sided heart failure which is connecting the heart with the lungs. In this kind of heart failure a condition known as cardiogenic pulmonary edema results which causes fluid to retain within the lungs. As a result the ventilation spare capacity reduces, the lungs stiffen and gaseous blood exchange is impaired because of a greater distance between the two as a result of edema. This pathophysiology exhibits itself in the form of shortness of breath, paroxysmal nocturnal dyspnea and orthopnea as its main symptoms.

Congestive heart failure can be managed in acute conditions by maintaining the airway, breathing and circulation. This is where ventilators come into play. Where as chronically they can be managed by certain drugs and behavioural modifications.


Airway pressure release ventilation has been defined as continuous positive airway pressure (CPAP) with brief regular, intermittent releases in airway pressure (H., 1996) APRV facilitates complete removal of carbon dioxide during the release phase and also provides oxygenation and hence has been termed as an improved mode of respiratory support for acute lung injury patients. (Stock MC, 1987) APRV can completely aid the ventilation process for an apneic patient or slightly enhance the ventilation for a spontaneously breathing patient. (Garner W, 1988) APRV provides room for uninterrupted spontaneous breathing through out the ventilation process along with a continuous positive airway pressure that facilitates the oxygenation of the blood and a timed process whichprovides clearance of carbondioxide. Although the advantages of APRV are many but the major ones include the capacity of providing the chance of spontaneous breathing throough out the cycle, at a given tidal volume providing a plateau level which is much lower comparatively, and a lower level of sedation need along with almost complete elimination of neuromuscular blockers.


Congestive heart failure manifests itself by an augmentation of blood circulation throughout the body leading to edema which in left side heart failure is mostly in the lung. Pulmonary edema blocks the usual passage of airway in the alveoli. Hence APRV provides a major advantage in these conditions of opening of collateral airway circulation to provide a continous and improved ventilator function rather than an intermittent one. This collateral circulation provides a considerable increase in functional residual capacity. (L., 1989). APRV works on this principal of collateral circulation by mainting sufficient pressure in the alveoli to keep these collaterals patent.

A literature review of the effect of APRV on the heart and the circulatory function demonstrate it to be a beneficial adjunct in cardiologically compromised patients. In one research Radermacher & Calzia (Calzia E, 1997) proved that APRV had no harmful effects on the heart or the circulatory system through a systemic literature review spanned over 10 years. There has also been a single case report that has demonstrated instead an increase in the blood pressure and cardiac output in a patient using APRV. (Falkenhain SK, 1992) they further proceed to suggest its use in patients who are compromised hemodynamically and require mechanical ventilation as an adjunct to the conventional pharmacologic therapy.

A comparision of APRV with CPPV on animals indicates a significant impairment in the tissue oxygenation and circulatory function in patients using CPPV, hence further strengthening the role of APRV in such compromised patients. (Rasanen J, 1988). This reduction has been owed to the suppression of spontaneous breathing that occurs with CPPV that has an effect of decreasing the venous return hence impairing the cardiac function.


High-Frequency Ventilation itself means providing ventilation at a pace considerably higher then the natural ventilation ( usually 250 -- 950 breathes per minute). High frequency ventilation can further be divided into 3 fundamental types namely high-frequency jet ventilation (HFJV), high-frequency oscillatory ventilation (HFOV) and high-frequency flow interruption (HFFI).

High frequency oscillatory ventilation while operating at the similar high ventilation rates also oscillates the pressure between the two ends of the mean airway pressure or the constant distending pressure. This phenomenon of oscillation provides the air to be sucked in within the lungs during the process of inspiration and then pushed out during the process of expiration. The rates used in HFOV depend greatly on the age, size, and medical condition of the patient under consideration. HFOV operates by providing very low tidal volumes, which are even lesser then the dead space. This tidal volume can also be adjusted according to the specific needs of the patient. The gas deliver mechanisms used in HFOV include convective (direct bulk flow), Pendelluft effect, Taylorian dispersion, Asymmetrical velocity profiles, molecular diffusion and cardiogenic mixing principles. Indications for the use of HFOV include conditions exhibiting refractory hypoxemia like severe acute respiratory distress syndrome and acute lung injury. It is also more generally used in neonates where the use of conventional ventilators will predispose the neonate to potential injury.

High-frequency oscillatory ventilation is a successful alternative tool to provide controlled pressure at low tidal volumes in patients with ARDS. (Cartotto R, 2004)

HFOV has the advantage of improving the process of ventilation and oxygenation without any injury to the lungs. Sensormedics 3100B (Viasys Healthcare, Yorba Linda, CA). is the only HFOV approved for use in adults.

The advantages of HFOV include the use of lower tidal volume then other modes of ventilation, maintaining the patency of the alveoli at a constant pressure and keeping them open for ventilation, hence preventing the adverse effects of barotrauma and atelectrauma to the lungs. (Fort P, 1997), and uniform aeration that increases the ventilation/perfusion matching.


HFOV, just like APRV operates on the principle of open lung concept which allows the lung to be distended for larger periods throughout the cycle. This allows the lung tissue to rid itself of any edematous material present within it. In case of congestive heart failure the major issue predisposposing to use of ventilators is pulmonary edema and this Open lung concept of HFOV can benefit the elimination or atleast the control of this condition. As for its effect on cardiac output, Gulberg et al. demonstrated a minute but significant effect of HFOV on cardiac output in neonates and infants. (Gullberg N, 2004) where as Mayers…[continue]

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