An Overview of  Critical Care


Respiratory Failure


As we have discussed, the primary function of the cardio-respiratory system is to deliver oxygen and nutrients to the tissues and remove carbon dioxide and other waste products. Oxygen is extracted, by the lungs, from the atmosphere, and carbon dioxide is excreted by the lungs in the opposite direction. The air-blood interface is where the small air sacs, the alveoli, meet the pulmonary capillaries. There are three reasons why one’s respiratory apparatus would fail: 1. failure to protect one’s airway (i.e. airway obstruction), 2. Failure to ventilate, characterized by a high arterial CO2 level, 2. Failure to oxygenate, characterized by a low arterial PO2 level.

Patients fail to ventilate due to loss of sensitivity of the brain to the main stimulus to ventilation, CO2. It may also occur due to neurological, neuromuscular junction or muscular problems. In addition, injuries to the chest wall, diaphragm or pleura will prevent chest expansion and ventilation.

Failure to Protect the Airway

Failure to Ventilate

Failure to Oxygenate


Failure to oxygenate occurs when there is an obstruction of gas exchange between the alveoli and capillaries – this may be due to an interstitial process (pulmonary edema) – a diffusion defect- or due a mismatch of ventilation and perfusion. In many lung injuries, there are a mixture of problems, alveolar dead space, shunt and diffusion defects. Patients who are hypoxemic, refractory to oxygen therapy, may have right to left intrapulmonary shunting.

Diffusion abnormality

Ventilation-Perfusion Mismatch

Right to left shunt

“Acute Lung Injury” (ALI) is a term used to describe a pulmonary syndrome, characterized by non cardiogenic pulmonary edema, caused by a number of insults. The most common cause of acute lung injury is systemic inflammation, and the injury to the alveolar-capillary interface is similar to that in sepsis: there is a widespread  increase in capillary permeability, with resultant exudation of protein rich fluid into the interstitium and alveoli. In addition, there is a deficiency of surfactant, and the wet soggy lungs become less compliant and have a tendency to collapse (in dependent zones). CT scan of the lungs displays heterogenous injury (22), with damaged lung tissue surrounded with normal looking tissue. The lungs’ reservoir of oxygen, the functional residual capacity, falls, and the work of breathing increases.

Acute Lung Injury

Acute Respiratory Distress Syndrome

ALI has a specific definition (23): a PaO2/FiO2 ratio of less than 300 (i.e. if the patient is on 30% O2, the PaO2 is less than 90mmHg), bilateral infiltrates on chest x-ray (CXR), and a pulmonary capillary wedge pressure (PCWP) of less than 18mmHg. A severe form of ALI is ARDS – acute respiratory distress syndrome, this has the same definition as ALI except that the PaO2/FiO2 ratio is less than 200 (i.e. on 60% O2, the patient’s PaO2 is less than 120mmHg).


Target: return the PaO2 to what is normal for this patient?

The treatment for all lung injuries is, initially, oxygen therapy. The goal of oxygen therapy is to return the patient’s PaO2 to what is normal for them (be it 90mmHg in most of us, or 50mmHg in a patient with severe COPD). If the patient is requiring more than 60% oxygen, they usually have a significant diffusion defect or ventilation perfusion mismatch. The treatment is to apply a positive pressure to the airway, increasing the pressure gradient to the alveoli, increasing mean interthoracic pressure, recruiting collapsed airways (and preventing derecruitment), and reducing the work of breathing. We call this CPAP (continuous positive airway pressure). Due to the fact that the greatest effect of CPAP is to prevent airway collapse at the end of expiration, this is often called PEEP (positive end expiratory pressure), but actually exists during both phases of ventilation.

Continuous Positive Airway Pressure - Prevention of Derecruitment

Volume Control

Pressure Control

Pressure Support

Click here to learn more about modes of ventilation

Most patients require more inspiratory support than CPAP alone. There are, effectively, two ways of assisting inspiration – by using positive pressure to deliver a certain amount of volume (volume control ventilation), or by delivering a certain amount of pressure (pressure control ventilation). Volume control means volume constant and pressure variable. Pressure control means pressure constant (or limited) volume variable ventilation. The mode of ventilation, is the way in which the ventilator uses volume or pressure to bump the patient up the pressure-volume curve. Patients may be given mandatory breaths (controlled ventilation) or may have their spontaneous breaths assisted (assist control ventilation). An alternative mode (which is often used with controlled modes) is pressure support, which allows a patient breath spontaneously, start and finish breaths and determine the tidal volume. We use this for maintenance and weaning of ventilation. Regardless of the inspiratory mode used, and whether or not a patient breaths spontaneously, it is now conventional to limit the magnitude of the positive pressure breath, as large tidal volumes damage the lungs (24).

Ventilator Induced Lung Injury (VILI)

Frequently it is difficult to ventilate patients with ARDS, due to the trade off between hypoxemia and ventilator induced lung injury (VILI) (25). We are now using lung protective strategies, which involve allowing the PaCO2 to climb (permissive hypercapnia) (26), and increasing mean airway pressures using higher levels of PEEP (the open lung approach) (27), prolonged inspiratory times (inverse I:E ratio) or full tidal volume ventilation (high frequency oscillation). In addition, there is anecdotal evidence that turning the patient prone, administering inhaled nitric oxide (preferential pulmonary vasodilator) and using partial liquid ventilation (with perfluocarbons), improves oxygenation.



There are many methods of weaning from ventilation, they all involve training of respiratory muscles with lower than comfortable levels of ventilatory support. Extubation requires evidence that the patient can deal with the switch from positive pressure (PPV) to negative pressure ventilation (NPV) – there are extensive cardiovascular changes, and can protect their airway. If extended periods of ventilation are anticipated, tracheostomy is performed. This prevents pressure damage to the larynx  (28) and facilitates weaning by allowing rapid reintroduction of PPV if patients fail spontaneous NPV breathing trials.




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