Oxygen Therapy & Ventilation

 

Oxygen therapy and ventilation

• Oxygen therapy

  General approach

  • O2 is a drug and should always be prescribed if it is used.
  • Aim for sats of 94-98%, or 88-92% in COPD. The latter is also used in suspected COPD e.g. long-term smoker with unexplained SOB.
  • Titrate up and down between FiO2 and masks as needed. Allow at least 5 minutes on each dose before changing, unless very acutely ill.
  • Should only be used if hypoxic, especially in MI, stroke, and panic attack, as unnecessary O2 therapy may be harmful.

  Mask types and uses

  Nasal cannulae

  • FiO2 and flow rates: 24-36% O2, at 2-6 L/min.
  • Often used in hypoxia (type 1 respiratory failure).

  Fixed percentage oxygen mask (Venturi)

  • FiO2 and flow rates: 24% (2-4 L/min), 28% (4-6 L/min), 35% (8-10 L/min), 40% (10-12 L/min), or 60% (15 L/min).
  • Often used in type 2 respiratory failure.

  Simple oxygen face mask (Hudson)

  • FiO2 and flow rates: 40-60% FiO2, at 5-10 L/min.
  • Often used in hypoxia (type 1 respiratory failure) requiring higher levels of FiO2.
  • Not 1st line at lower levels (vs. nasal cannulae) as it is obtrusive and uncomfortable, but useful for patients with nasal problems such as epistaxis.

  Reservoir mask

  • Aka non-rebreather mask.
  • FiO2 and flow rates: around 80% FiO2, at high flow of 15 L/min. Ensure reservoir is inflated before placing. Give to all acutely unwell patients, including initial treatment to hypoxic COPD patients.
  • Patient should be monitored throughout the time high flow O2 is on.

• Non-invasive ventilation

  Definition

  • Ventilatory support without an endotracheal tube.
  • Basically synonymous with non-invasive positive pressure ventilation (NPPV). Negative pressure ventilation is the 'iron lung', which is hardly used at all now.

  Contraindications

  • Respiratory arrest.
  • Relative contraindications: unprotected airway, medically unstable (e.g. hypotensive shock), excess secretions.
  • Do CXR to check for pneumothorax first. If present, chest drain should be inserted before starting NIV.

  Continuous positive airway pressure (CPAP)

  • Continuous pressure helps keep airway open to improve oxygenation in hypoxic patients.
  • Used in type 1 respiratory failure – HF, pneumonia, and ARDS – where it opens up the fluid-filled alveoli. Also used in obstructive sleep apnea, where it opens up the collapsed pharynx.
  • It provides positive end expiratory pressure (PEEP), the positive pressure in the lungs at the end of expiration which keeps the airways open. This is extrinsic PEEP, versus the intrinsic PEEP resulting from incomplete exhalation.
  • FiO2 typically set at 100%.

  Biphasic positive airway pressure (BIPAP)

  • Two pressure phases: expiratory positive airway pressure (EPAP) and a higher inspiratory positive airway pressure (IPAP).
  • Helps ventilation in hypercapnic patients (type 2 respiratory failure), commonly COPD (especially if acidotic). Also used in neuromuscular and chest wall disease, and pulmonary oedema unresponsive to CPAP.
  • Respiration is triggered by the patient.
  • EPAP is synonymous with PEEP i.e. what you get in CPAP. Must be low enough for them to be able to breath out fully. Increase if patient remains hypoxic.
  • IPAP is synonymous with the pressure support ventilation (PSV) of mechanical ventilation. This is what reduces the CO2, so IPAP is titrated to PaCO2 levels.
  • FiO2 may be set lower than in CPAP e.g. 40%, especially for COPD.

• Mechanical ventilation

  Mechanism

  • Air (including oxygen) is intermittently pushed into the airways using positive pressure to replicate inspiration, interspersed with no (or lower) pressure to allow passive expiration.
  • Typically refers to 'invasive' ventilation, whereby an endotracheal tube or tracheostomy connects the ventilator to the patient's airways.

  Indications

  • Patient unable to maintain airway, oxygenation, or ventilation by less invasive means.
  • This most commonly occurs during general anaesthesia, other causes of coma (GCS ≤8), and acute respiratory failure.

  Modes

  Ventilator modes can be defined by what starts and ends inspiration:

  • The start of inspiration can be triggered by the patient (spontaneous) or ventilator (mandatory aka controlled). Spontaneous breaths may then be assisted or unassisted by the ventilator.
  • The end of inspiration, or 'cycling' from the inspiratory to the expiratory phase, is determined by a pre-specified tidal volume (volume-cycled ventilation [VCV] or volume control) or pre-specified pressure (pressure-cycled ventilation or pressure control), with the machine automatically determining the other parameter to achieve that specified target.

  Common modes:

  • The hybrid assist-control (A/C) is a common initial mode, and usually implies VCV. Inspiration is triggered if the patient makes inspiratory effort (then assisted by the ventilator), but the machine will automatically trigger otherwise.
  • Synchronized intermittent mandatory ventilation (SIMV) is a form of VCV often used in weaning, allowing patients to take unassisted, spontaneous breaths, while ensuring a minimum number of breaths are provided by the machine.
  • Pressure support ventilation (PSV) is another mode used in weaning, in which the patient determines volume and frequency by spontaneous breathing, but pressure is added to overcome ET tube resistance and increase volume.

  Parameters and settings

  Minute volume:

  • Tidal volume and respiratory rate are set to determine the minimum minute volume. In practice, this may be exceeded if the patient's spontaneous rate is faster in assisted or A/C mode.
  • Common initial settings involve a tidal volume of 8-12 ml/kg and respiratory rate of 10-12. Lower volumes (6-8 ml/kg) should be used in ARDS.

  FiO2:

  • Initially set at 100% then titrated to lowest level that maintains target SaO2 or PaO2.
  • Practice varies on what is an acceptable minimum SaO2 (90%/92%/94%), though clear evidence that very high targets (97-100%) are harmful.

  Positive end expiratory pressure (PEEP):

  • Airway pressure at end of expiratory phase, maintaining airway patency.
  • PEEP from a ventilator is extrinsic PEEP, in contrast to intrinsic PEEP (aka auto-PEEP) which results from incomplete expiration before next inspiratory phase begins.
  • Higher PEEP increases PaO2.
  • Most patients have small amount provided (5 cm H2O), with higher levels in ARDS.

  Peak and plateau pressure:

  • More force is required to open airways and alveoli than to keep them open, so in volume control ventilation there is a pressure waveform with an initial peak followed by a (lower) plateau.
  • An elevated peak pressures suggests an airway problem, while elevated plateau pressure suggests an alveolar compliance problem.
  • In pressure control ventilation the peak pressure and inspiratory time are set values, with the plateau pressure the same as the peak.

  Adjunctive therapies

  • Sedation (e.g. propofol, benzodiazepine) and analgesia (e.g. morphine) often needed to ensure patient comfort and tolerance of ET tube. However, tracheostomy allows long-term ventilation without sedation.
  • Neuromuscular blockade, while useful during general anaesthesia when patient immobility is needed, is generally not continued in ventilated ICU patients beyond the initial intubation.

  Complications

  • Ventilator-associated pneumonia: new-onset pneumonia ≥48h post-intubation.
  • Barotrauma and volutrauma cause alveolar distention and rupture, leading to air leak (pneumothorax, pneumomediastinum, subcutaneous emphysema) and acute lung injury.
  • Oxygen toxicity.
  • Gas trapping: typically occurs in asthma/COPD patients with long expiratory phase. Failure to complete exhalation leads to auto-PEEP, with resulting increased airway pressure, barotrauma, and possible cardiovascular compromise due to high intrathoracic pressure.
  • Tracheal stenosis.