Mechanical Ventilation: Cruise Control for the Lungs

By Diane Byrum, RN, CCNS, CCRN, MSN, FCCM, and Cherri Crabtree, RRT, RCPNursing made Incredibly Easy! September/October 20092.5 ANCC contact hoursOnline: www.nursingcenter.com

© 2009 by Lippincott Williams & Wilkins. All world rights reserved.

Respiratory Failure

Hypoxemic respiratory failure: Decreased PaO2; most common cause for mechanical ventilation, including:• Pneumothorax• Atelectasis• Pulmonary edema• Pneumonia• Pulmonary fibrosis• early ARDS• smoke inhalation

Hypercapnic respiratory failure: Decreased PaO2 and normal or increased PaCO2, including:• acute COPD exacerbation• head trauma• spinal cord injury

Other Conditions Requiring Mechanical Ventilation

Bradypnea or apnea with respiratory arrest

Acute lung injury

Tachypnea (respiratory rate of greater than 30 breaths/minute)

Clinical deterioration for another condition unrelated to the lungs

Respiratory muscle fatigue

Coma

Hypotension

Neuromuscular disease

Airway protection in patients with altered mental status or in clinical conditions in which the risk of pulmonary complications or respiratory failure is high

Indications for Mechanical Ventilation

PaO2 less than 50 mm Hg with FiO2 greater than 0.60

PaO2 greater than 50 mm Hg with pH less than 7.25

Vital capacity less than two times the tidal volume

Negative inspiratory force less than 25 cm H2O

Respiratory rate greater than 35 breaths/ minute

Endotracheal Intubation

An ET tube is necessary

Cuff pressure should be the lowest possible pressure to allow delivery of adequate tidal volume and prevent pulmonary aspiration; usually, at less than 25 mm Hg to prevent injury and at more than 15 mm Hg to prevent aspiration

Monitor cuff pressure at least every 8 hours by attaching a pressure gauge monitoring device or using a minimal occlusion volume technique

After the ET tube is in place, mechanical ventilation is initiated

Mechanics of Mechanical Ventilation

The mechanical ventilator rate is defined as breaths delivered to the patient per minute; the initial rate is set to provide the needed ventilation to achieve a normal PaCO2 value.

Tidal volume is defined as the volume of gas exchanged during each ventilated breath, or the volume of air exhaled per minute; in mechanically ventilated patients, the tidal volume value depends on the patient’s lung condition and ideal body weight

Minute ventilation is calculated by multiplying the ventilator rate by the tidal volume

Mechanics of Mechanical Ventilation

PEEP is the pressure remaining in the lungs at end expiration; used to maintain the patency of the alveoli

FiO2 is the fraction of inspired oxygen (always expressed as a decimal, not a percentage)

The P/F ratio is obtained by dividing the PaO2 by the FiO2; a normal P/F ratio is greater than 300, a value of less than 200 indicates refractory hypoxemia

The initial ventilator rate will be adjusted based on the patient’s response, his ABG values, and the P/F ratio

Ventilator Modes 101 PSV mode delivers a set pressure that’s held during

the entire inspiration while decreasing the patient’s work of breathing; patient must be spontaneously breathing

AC mode delivers a set volume with each patient-triggered breath, as well as the set mechanical rate

SIMV mode delivers a set number of breaths and tidal volume while allowing the patient to take spontaneous breaths with a patient-determined tidal volume and rate

PC mode delivers a pressure-limited breath at a set rate; the tidal volume is determined by the set pressure limit

Dual-control mode delivers a set tidal as a pressure-limited breath until the desired tidal volume is achieved

Initial Ventilator Settings

Set the machine to deliver the tidal volume required.

Adjust the machine to deliver the lowest concentration of oxygen to maintain normal PaO2.

Record PIP.

Set the mode, rate, PEEP, and pressure support as ordered.

Adjust sensitivity so that the patient can trigger the ventilator with a minimal effort.

Initial Ventilator Settings

Record minute volume and obtain ABG values to measure PaCO2, pH, and PaO2 after 20 minutes of continuous mechanical ventilation.

Adjust the setting (FiO2 and rate) according to the results of ABG analysis to provide normal values or those set by the healthcare provider.

If the patient suddenly becomes confused or agitated or begins bucking the ventilator for some unexplained reason, assess for hypoxia and manually ventilate on 100% oxygen with a bag-valve mask.

Hazards of the Road

Barotrauma—injury or damage to the lung tissue that can lead to entry of air into the pleural space (pneumothorax) or the tracking of air along the vascular bundle to the mediastinum (pneumomediastinum); high tidal volumes and peak plateau pressures are risk factors

Volutrauma—damage to the lungs caused by too large a volume, leading to a syndrome of symptoms similar to ARDS

Decreased cardiac output and BP— increased pressure in the lungs leads to increased pressure surrounding the heart and major vessels, which leads to decreasing blood return to the heart, decreased cardiac output, and eventually decreased BP

Hazards of the Road

Oxygen toxicity—a combination of increased high levels of oxygen and prolonged use causing an inflammatory-like response in the lungs similar to ARDS; use the lowest FiO2 setting that accomplishes the needed oxygenation to prevent

Ventilator-associated pneumonia—as evidenced by new or changing pulmonary infiltrate on the chest X-ray in conjunction with fever, leukocytosis, and a change in the color and amount of secretions

Fluid retention, stress-related erosive syndrome, and increased ICP may also occur

Ventilator Alarms

High PIP—increased airway pressure or decreased lung compliance, most often caused by secretions, coughing, or patient intolerance of the ventilator; always assess breath sounds for increased consolidation, wheezing, bronchospasm, or the possibility of a pneumothorax and check to see if the patient is biting on the tube, if the tubing is kinked, or if the tubing contains increased condensation

Low minute ventilation—if the patient is on control mode, check for a disconnection or leak in the circuit; if he’s being weaned off the ventilator, assess for decreased respiratory effort

Apnea—always check for patient effort and check for circuit disconnections that mimic apnea

A Team of Mechanics

Nursing and respiratory therapy must collaborate when assessing for changes in the patient’s condition and administering ordered ventilator setting changes

Most patients require some form of sedation; sedation holidays occur daily to assess the patient’s neurologic status and reorient him

The patient’s readiness for weaning off the ventilator is assessed by conducting spontaneous breathing trials

Road Map to Success

Elevate the head of the bed to greater than 30 degrees

Provide mouth care every 4 hours, teeth brushing every 12 hours, and deep oropharyngeal suctioning every 12 hours

Initiate stress ulcer prophylaxis using either histamine2-receptor blocking agents or proton-pump inhibitors

Initiate deep vein thrombosis prophylaxis