Effects of Positive Pressure Ventilation on the Pulmonary System Flashcards
-Cardiovascular Effects of Mechanical Ventilation
-The primary cardiovascular complication of MV is reduction of cardiac output (CO)
-Cardiovascular Effects of Mechanical Ventilation
-Lung compliance
-Modest increases in intrapleural pressures will not alter the cardiac output as much as high increases in intrapleural pressure
-Increased lung compliance=Higher intrapleural pressures
-Decreased lung compliance = Lower intrapleural pressure
-Alos Pt with stiff lungs, such as those with ARDS, are less likely to experience hemodynamic changes with high ventilating pressure -With stiff lungs, less of the alveolar pressure is transmitted to the intrapleural space -On the other hand, patients with compliant lung are more likely to have higher intrapleural pressure during PPV
-Cardiovascular Effects of Mechanical Ventilation
-Increased Airway Resistance
-PT who have increased Airway resistance (RAW), the peak inspiratory pressure (Ppeak, PIP) may be very high
-Much of the pressure, however, is lost to the resistive airways and less pressure actually reaches the alveolar levels and the intrapleural space -Therefore, high peak inspiratory pressure does not always mean that cardiac output will be affected
-Effects of MV on intracranial pressure
-Since MV increases central venous pressure (CVP), this may decrease venous return from the head, which could increase intracranial pressure (ICP)
-Clinically, this could be observed by an increase in jugular distention -The greatest clinical risk related to cerebral perfusion is for PT who already have an increased ICP and who may develop cerebral edema -PT with head injuries, cerebral tumor, and post neurosurgery PT fall into this category
• Effects of Mechanical Ventilation on Intracranial Pressure Cont.
• With normal intracranial dynamics, patients do not seem to develop increased ICP with PPV
• While those with abnormal cerebral function do appear affected by changes in cerebral perfusion and pressures
• These patients are often mechanically hyperventilated to reduce cerebral perfusion by lowering PaCO2 to 25-30 mmHg
• Alkalosis from low PaCO2 can constrict cerebral vessels…..this reduces ICP
• Some patients with head injury or cerebral dysfunction require PEEP for the treatment of refractory hypoxemia
• PEEP can increase ICP, on the other hand, if PEEP is needed to maintain oxygenation, it may be lifesaving and should be used
• It is important to monitor ICP in this patient group
• NOTE: It is important to note that the actual benefits of using hyperventilation in patients with closed head injury remains controversial and is not always recommended as a standard practice for these patients.
• Renal Effects of Mechanical Ventilation
• Since the arterial blood pressure is usually compensated where PPV is used, decreased blood pressure is probably not a significant factor leading to decreased urinary output during mechanical ventilation
• The redistribution of blood inside the kidney, on the other hand, may be responsible for cardiovascular-induced changes in kidney function during mechanical ventilation
• The importance of this for patients on PPV is if arterial blood pressure is not maintained, water retention may result
• This would increase plasma volume and might contribute to the formation of pulmonary edema
• Thus, urinary output changes with mechanical ventilation are probably related to changes in ..blood pressure, cardiac output, and intrarenal perfusion
• Effects of Mechanical Ventilation on Ventilatory Status
• Causes of ventilatory problems associated with PPV include:
• The primary goal of maintaining normal ventilatory status for a patient on assisted ventilation is not achieved when the ventilator is mismanaged or when it causes abnormalities to the patient such as air trapping
• The patient’s ABG’s end up being abnormal
• Causes of ventilatory problems associated with PPV include:
• Hypoventilation
• Hyperventilation
• Air Trapping
• Body Position
• Gastric Distention
• Complications associated with 02 therapy during PPV
• Hypoventilation
• Acute hypoventilation can occur in patients receiving ventilatory assistance if adequate alveolar ventilation is not provided
• This results in a PaCO2 above the patient’s normal level and an acidotic pH, from the rising Paco2
• Evaluation of clinical signs and symptoms as well as ABG analysis will lead to recognition of the problem
• Clinical Changes Associated with Respiratory Acidosis
• Normally the kidneys can compensate for respiratory acidosis within 18 to 36 hours
• Obviously, it is more desirable to correct the cause of the problem by increasing alveolar ventilation than it is to wait for renal compensation
• This can be done by:
• increasing Vt…or
• increasing rate
• Hyperventilation
• Just as hypoventilation is inappropriate in mechanically ventilated patients, so is hyperventilation in most situations
• Hyperventilation results in a lower than normal PaCO2 and a rise in pH
• Evaluation of the patient’s clinical picture and ABG’s will reveal the problem
• Some clinical signs associated with respiratory alkalosis are cool skin (decreased PaCO2 and Pa02), twitching and hypokalemia (decreased potassium, which can lead to cardiac arrhythmias
• Hyperventilation in mechanically ventilated patients reduces the drive to breathe and leads to apnea
• This has the advantage of preventing the patient from trying to fight the ventilator, or from experiencing feelings of dyspnea
• The disadvantage is that weaning becomes more difficult if the patient is kep in respiratory alkalosis for a prolonged period of time
• Body Position and Mechanical Ventilation
• Hospitalized patients, especially those on mechanical ventilatory support, are often immobilized
• It is important to turn these patients frequently during the day to help prevent such pulmonary complications as atelectasis and hypoxemia
• Body position is also important in patients with certain types of pulmonary disorders to ensure that optimal ventilation and oxygenation is maintained
• It has been found that when one lung is affected by some process such as atelectasis or consolidation and the affected lung is in the dependent position, this is usually associated with a lower Pa02 than when the normal lung is in the dependent position
• In most cases…oxygenation is best when the unaffected side is down(Good lung Down)***
• Auto-PEEP (Air Trapping During Mechanical Ventilation)
• When airway resistance is increased in spontaneously breathing persons, both inspiratory and expiratory flows are impeded
• This can occur in patients with severe COPD, status asthmaticus, and similar problems
• Small or medium airways close off or collapse during exhalation and the air in the lung does not empty completely
• This trapping of air in the alveoli can occur if expiration is not long enough
• Auto-PEEP is defined as an unintentional PEEP that occurs with mechanically ventilated patients when a new inspiratory breath is delivered before expiration has ended
• Auto-PEEP is also called…occult PEEP, inadvertent PEEP, gas trapping, breath stacking, and intrinsic PEEP
• Factors That Increase the Risk of Auto-PEEP
• COPD
• A minute ventilation of greater than or equal to 10 LPM
• Increased age (the minute ventilation requirements for Auto-PEEP decrease with age)
• Increase in compliance
• Increase in respiratory rate
• Increases in Vt, particularly in patients with airway obstruction
• Reduced inspiratory flow leading to shorter expiratory time
• Decreases in ET tube size
• Factors That Increase the Risk of Auto-PEEP Cont.
• To reduce Auto-PEEP, use higher inspiratory flow rates to shorten inspiratory time and allow a longer time for exhalation
• Longer expiratory times can also be accomplished by using smaller Vt settings and low respiratory rates
• Large ET tubes may also reduce air trapping
• The use of bronchodilators in patients with reactive airways may also be beneficial
• Barotrauma in Mechanical Ventilation
• Barotrauma is a term used to describe several conditions that occur due to pressure or volume damage in the lung
• In normal lungs, the possibility of damaging lung tissue or barotraumas is probably higher than we expect
• In lungs where blebs may be present, or in patients with chest wall injury, the risk of rupture to the lung is much greater
• The rupture of lung tissue leads to such complications as pneumothorax, pneumomediastinum, subcutaneous emphysema, pneumoperitoneum
• Conditions Predisposing to Barotrauma
• High peak airway pressures
• Bullous lung disease (emphysema & history of TB)
• High levels of PEEP with high Vt’s
• Necrotizing pneumonias
• ARDS
• Hazards of Oxygen Therapy with Mechanical Ventilation
• Oxygen-Induced Bradypnea
• In patients who are breathing on a hypoxic drive, such as those with COPD and chronic CO2 retention, high oxygen levels can suppress the drive to breathe. This is a problem
ONLY if an adequate alveolar ventilation is not provided by the ventilator
• Absorption Atelectasis
• High oxygen concentrations (>70%) often lead to rapid absorption atelectasis in hypo-ventilated lung units. This increases intrapulmonary shunt (perfusion in excess of ventilation). In mechanically ventilated patients this is especially a problem of low Vt settings or pressure-cycled ventilators with low cycling pressures
• Hazards of Oxygen Therapy with Mechanical Ventilation Cont.
• Oxygen Toxicity
• The use of high oxygen concentrations (100%) can induce pulmonary changes in humans in as little as 6 hours. Pulmonary changes include the following:
• decrease tracheal mucus flow
• decrease macrophage activity
• decrease vital capacity
• decrease surfactant production
• decrease compliance
• decrease diffusing capacity
• endothelial cell damage & accompanying increased lung water
• progressive formation of absorption atelectasis
• Exposure for more than 72 hours causes development of a pattern similar to ARDS
• Invasive Devices That Increase Risk For Nosocomial Infections:
• Intubated patients on mechanical ventilation are usually very ill and at high risk for nosocomial infections and pneumonias
• Invasive Devices That Increase Risk For Nosocomial Infections:
• Endotracheal Tubes
• Intravenous Lines
• Central Venous Lines
• Arterial Monitors
• Intracranial Pressure Monitors
• Pulmonary Artery Catheters
• Indwelling Urinary Catheters
• Infections Cont.
• Factors Predisposing Patients to Nosocomial Infections & Pneumonias:
• Severe illness
• Major surgery
• Long duration of hospital stay
• Use of antibiotics
• Use of invasive devices (see preceding slide)
• Infections Cont.
• Common Pathogenic Organisms from Nosocomial Pneumonias:
• Gram Neg.
• Pseudomonas
• Escherichia Coli (E Coli)
• Enterobacter
• Klebsiella
• Hemophilus Influenza
• Infections Cont.
• Common Pathogenic Organisms from Nosocomial Pneumonias:
• Gram Pos.
• Staphylococcus
• Streptococcus
• Other
• Legionella
• Candida
• Ventilator-Associated Pneumonia
• Defined as pneumonia that develops 48 hours after a patient has been placed on mechanical ventilation.
• VAP is one of the most frequent hospital-acquired infections in critically ill patients receiving mechanical ventilation.
• VAP has been linked to the aspiration of oropharyngeal secretions and esophageal/gastric contents.
• Successful management of VAP requires early diagnosis and appropriate use of antibiotic therapy to avoid the emergence of multidrug-resistant microorganisms.
• Nonpharmacological interventions, such as ETs and routine care of ventilator circuits, are potential sources of infectious material.
• The most prevalent microorganisms in VAP are gram-negative bacilli.
-Prevention includes: regular oral hygiene, head of bed elevated, daily sedation. vacation to evaluate readiness to wean, etc.
