- Section 1 ICU organization and management
- Section 2 Pharmacotherapeutics
- Section 3 Resuscitation
- Section 4 The respiratory system
- Part 4.1 Physiology
- Part 4.2 Respiratory monitoring
- Chapter 72 Blood gas analysis in the critically ill
- Chapter 73 Pulse oximetry and capnography in the ICU
- Chapter 74 Respiratory system compliance and resistance in the critically ill
- Chapter 75 Gas exchange principles in the critically ill
- Chapter 76 Gas exchange assessment in the critically ill
- Chapter 77 Respiratory muscle function in the critically ill
- Chapter 78 Imaging the respiratory system in the critically ill
- Part 4.3 Upper airway obstruction
- Part 4.4 Airway access
- Part 4.5 Acute respiratory failure
- Part 4.6 Ventilatory support
- Part 4.7 Weaning ventilatory support
- Part 4.8 Extracorporeal support
- Part 4.9 Aspiration and inhalation
- Part 4.10 Acute respiratory distress syndrome
- Part 4.11 Airflow limitation
- Part 4.12 Respiratory acidosis and alkalosis
- Part 4.13 Pneumonia
- Part 4.14 Atelectasis and sputum retention
- Part 4.15 Pleural cavity problems
- Part 4.16 Haemoptysis
- Section 5 The cardiovascular system
- Section 6 The gastrointestinal system
- Section 7 Nutrition
- Section 8 The renal system
- Section 9 The neurological system
- Section 10 The metabolic and endocrine systems
- Section 11 The haematological system
- Section 12 The skin and connective tissue
- Section 13 Infection
- Section 14 Inflammation
- Section 15 Poisoning
- Section 16 Trauma
- Section 17 Physical disorders
- Section 18 Pain and sedation
- Section 19 General surgical and obstetric intensive care
- Section 20 Specialized intensive care
- Section 21 Recovery from critical illness
- Section 22 End-of-life care
(p. 325) Respiratory monitoring
Arterial blood gases allow the assessment of patient oxygenation, ventilation, and acid-base status. Blood gas machines directly measure pH, and the partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2) dissolved in arterial blood. Oxygenation is assessed by measuring PaO2 and arterial blood oxygen saturation (SaO2) in the context of the inspired oxygen and haemoglobin concentration, and the oxyhaemoglobin dissociation curve. Causes of arterial hypoxaemia may often be elucidated by determining the alveolar–arterial oxygen gradient. Ventilation is assessed by measuring the PaCO2 in the context of systemic acid-base balance. A rise in PaCO2 indicates alveolar hypoventilation, while a decrease indicates alveolar hyperventilation. Given the requirement to maintain a normal pH, functioning homeostatic mechanisms result in metabolic acidosis, triggering a compensatory hyperventilation, while metabolic alkalosis triggers a compensatory reduction in ventilation. Similarly, when primary alveolar hypoventilation generates a respiratory acidosis, it results in a compensatory increase in serum bicarbonate that is achieved in part by kidney bicarbonate retention. In the same way, respiratory alkalosis induces kidney bicarbonate loss. Acid-base assessment requires the integration of clinical findings and a systematic interpretation of arterial blood gas parameters. In clinical use, traditional acid-base interpretation rules based on the bicarbonate buffer system or standard base excess estimations and the interpretation of the anion gap, are substantially equivalent to the physicochemical method of Stewart, and are generally easier to use at the bedside. The Stewart method may have advantages in accurately explaining certain physiological and pathological acid base problems.
Access to the complete content on Oxford Medicine Online requires a subscription or purchase. Public users are able to search the site and view the abstracts for each book and chapter without a subscription.
Please subscribe or login to access full text content.
If you have purchased a print title that contains an access token, please see the token for information about how to register your code.
For questions on access or troubleshooting, please check our FAQs, and if you can't find the answer there, please contact us.
- Section 1 ICU organization and management
- Section 2 Pharmacotherapeutics
- Section 3 Resuscitation
- Section 4 The respiratory system
- Part 4.1 Physiology
- Part 4.2 Respiratory monitoring
- Chapter 72 Blood gas analysis in the critically ill
- Chapter 73 Pulse oximetry and capnography in the ICU
- Chapter 74 Respiratory system compliance and resistance in the critically ill
- Chapter 75 Gas exchange principles in the critically ill
- Chapter 76 Gas exchange assessment in the critically ill
- Chapter 77 Respiratory muscle function in the critically ill
- Chapter 78 Imaging the respiratory system in the critically ill
- Part 4.3 Upper airway obstruction
- Part 4.4 Airway access
- Part 4.5 Acute respiratory failure
- Part 4.6 Ventilatory support
- Part 4.7 Weaning ventilatory support
- Part 4.8 Extracorporeal support
- Part 4.9 Aspiration and inhalation
- Part 4.10 Acute respiratory distress syndrome
- Part 4.11 Airflow limitation
- Part 4.12 Respiratory acidosis and alkalosis
- Part 4.13 Pneumonia
- Part 4.14 Atelectasis and sputum retention
- Part 4.15 Pleural cavity problems
- Part 4.16 Haemoptysis
- Section 5 The cardiovascular system
- Section 6 The gastrointestinal system
- Section 7 Nutrition
- Section 8 The renal system
- Section 9 The neurological system
- Section 10 The metabolic and endocrine systems
- Section 11 The haematological system
- Section 12 The skin and connective tissue
- Section 13 Infection
- Section 14 Inflammation
- Section 15 Poisoning
- Section 16 Trauma
- Section 17 Physical disorders
- Section 18 Pain and sedation
- Section 19 General surgical and obstetric intensive care
- Section 20 Specialized intensive care
- Section 21 Recovery from critical illness
- Section 22 End-of-life care