7.13: Hypoxemia (hypoxia)
- Page ID
- 59716
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Hypoxaemia is an oxygen saturation less than 90% or arterial oxygen of less than 60 mmHg,or a fall in oxygen saturation of more than 5%.Anaesthesia will reduce the body’s cardiac and respiratory response to hypoxaemia, making the clinical detection of hypoxaemia even more difficult. Only the late signs of hypoxemia may occur including: bradycardia, tachycardia, arrhythmias, hypotension and cardiac arrest.
Cyanosis
Patient oxygenation (SaO2) is usually measured with an oxygen saturation monitor (pulse oximeter).Clinically, hypoxaemia is detected by looking for cyanosis. This can be difficult to detect and is much less accurate than an oxygen saturation monitor. Cyanosis can be detectable if there is more than 5 g/100 ml of reduced haemoglobin (SaO2 of only less than 85% or arterial oxygen of 40 to 45 mmHg). Peripheral cyanosis (finger tips) can occur from just poor peripheral blood flow without true hypoxaemia. Central cyanosis(tongue) is a true sign of hypoxaemia.Hypoxaemia may rapidly cause death and is an emergency.The anesthetist should use a saturation monitor at all times.
Causes of Hypoxia
There are several causes of hypoxaemia including:
Decreased inspired oxygen due to empty oxygen supply, low oxygen flow rate, breathing system disconnected or broken, obstructed breathing system or airway or endotracheal tube in the wrong place (oesophagus/endobronchial).
Decreased ventilation due to decreased conscious state, high spinal, increased airway resistance (asthma, upper airway obstruction, endobronchial intubation), wrong ventilator settings (low tidal volume, low respiratory rate) and pneumothorax.
Lung disease
Decreased cardiac output
Cardiac shunt
Increased oxygen requirement due to severe infection, thyrotoxicosis
Prevention
Always check the anaesthetic machine, oxygen supply and oxygen failure alarms before giving any anaesthetic or sedation.
Always have a spare supply of oxygen and an alternative method to ventilate a patient (e.g. self inflating resuscitation bag).
Always use an oxygen saturation monitor if available.
Always monitor the patient for signs of cyanosis.
Management
Increase the inspired oxygen concentration.
Check the oxygen pressure gauges and flow meters.
Check that ventilation is adequate. (Check end-tidal CO2 if available). If hypoxia occurs soon after endotracheal intubation immediately check that the endotracheal tube is in the trachea and not the oesophagus.
Hand ventilate the patient, with large tidal volumes, to assess lung compliance and exclude leaks or obstructions in the breathing system. Check the endotracheal tube is not blocked or dislodged.
Listen to the chest for equal air entry and bronchospasm. Exclude a pneumothorax.
Check for low cardiac output. Check the blood pressure and heart rate.
Consider lung problems: aspiration, pulmonary oedema, consolidation, atelectasis and pulmonary embolism.