Page 45 - TNFlipTest

Toronto Notes 2019
Airway Management
Anesthesia A11
■ neuromuscular complications
◆ muscle atrophy
◆ increased intracranial pressure
■ metabolic
◆ decreased CO2 due to hyperventilation
◆ alkalemia with over correction of chronic hypercarbia
Ventilator Strategies
• modeandsettingsaredeterminedbasedonpatientfactors(e.g.idealbodyweight,compliance, resistance) and underlying reason for mechanical ventilation
• hypoxemic respiratory failure: ventilator provides supplemental oxygen, recruits atelectatic lung segments, helps improve V/Q mismatch, and decreases intrapulmonary shunt
• hypercapnicrespiratoryfailure:ventilatoraugmentsalveolarventilation;maydecreasetheworkof breathing, allowing respiratory muscles to rest
Modes of Ventilation
• assist-controlventilation(ACV)orvolumecontrol(VC)
■ every breath is delivered with a pre-set tidal volume and rate or minute ventilation
■ extra controlled breaths may be triggered by patient effort; if no effort is detected within a specified
amount of time the ventilator will initiate the breath • pressurecontrolventilation(PCV)
■ a minimum frequency is set and patient may trigger additional breaths above the ventilator
■ all breaths delivered at a preset constant inspiratory pressure
■ in traditional PCV, tidal volume is not guaranteed thus changes in compliance and resistance affect
tidal volume
• synchronousintermittentmandatoryventilation(SIMV)
■ ventilator provides controlled breaths (either at a set volume or pressure depending on whether in VC or PCV, respectively)
■ patient can breathe spontaneously (these breaths may be pressure supported) between controlled breaths
• pressuresupportventilation(PSV)
■ patient initiates all breaths and the ventilator supports each breath with a pre-set inspiratory
pressure
■ useful for weaning off ventilator
• high-frequencyoscillatoryventilation(HFOV)
■ high breathing rate (up to 900 breaths/min in an adult), very low tidal volumes ■ used commonly in neonatal and pediatric respiratory failure
■ occasionally used in adults when conventional mechanical ventilation is failing
• non-invasivepositivepressureventilation(NPPV)
■ achieved without intubation by using a nasal or face mask
■ BiPAP: increased pressure (like PSV) on inspiration and lower constant pressure on expiration (i.e.
PEEP)
■ CPAP: delivers constant pressure on both inspiration and expiration
Monitoring Ventilatory Therapy
• Pulse oximetry, end-tidal CO2 concentration • Regular arterial blood gases
• Assess tolerance regularly
Patients who develop a pneumothorax while on mechanical ventilation require a chest tube
Causes of Intraoperative Hypoxemia Inadequate oxygen supply
e.g. breathing system disconnection, obstructed or malpositioned ETT, leaks in the anesthetic machine, loss of oxygen supply Hypoventilation
Ventilation-perfusion inequalities e.g. atelectasis, pneumonia, pulmonary edema, pneumothorax
Reduction in oxygen carrying capacity e.g. anemia, carbon monoxide poisoning, methemoglobinemia, hemoglobinopathy Leftward shift of the hemoglobin-oxygen saturation curve
e.g. hypothermia, decreased 2,3-BPG, alkalosis, hypocarbia, carbon monoxide poisoning
Right-to-left cardiac shunt
A Comparison of Four Methods of Weaning Patients from Mechanical Ventilation
NEJM 1995;332:345-350
Study: Prospective, randomized, multicentre trial. Participants: 130 of 546 patients who received mechanical ventilation and were considered ready for weaning but had respiratory distress during a 2 h trial of spontaneous breathing.
Intervention: One of four weaning techniques following standardized protocol.
Outcome: Median duration of weaning.
Results: The median duration of weaning for intermittent mandatory ventilation, pressure- support ventilation, intermittent (multiple) trials
of spontaneous breathing, and once-daily trial of spontaneous breathing was 5 d, 4 d, and 3 d, respectively. The rate of successful weaning
was higher with once-daily trial of spontaneous breathing than with intermittent mandatory ventilation (rate ratio 2.83; 95% CI 1.36-5.89; p<0.006) or pressure-support ventilation (ratio 2.05; 95% CI 1.04-4.04; p<0.04). There was no significant difference in the rate of success between once-daily trials and multiple trials of spontaneous breathing.
Conclusions: Once-daily or multiple trials of spontaneous breathing led to extubation more quickly than intermittent mandatory or pressure- support ventilation.
Table 3. Causes of Abnormal End Tidal CO2 Levels
Hypocapnea (Decreased CO2)
Hyperventilation
Hypothermia (decreased metabolic rate)
Decreased pulmonary blood flow (decreased cardiac output)
Technical issues
Incorrect placement of sampling catheter Inadequate sampling volume
V/Q mismatch
Pulmonary thromboembolism Incipient pulmonary edema Air embolism
Hypercapnea (Increased CO2)
Hypoventilation
Hyperthermia and other hypermetabolic states
Improved pulmonary blood flow after resuscitation or hypotension
Technical issues
Water in capnography device Anesthetic breathing circuit error
Inadequate fresh gas flow Rebreathing
Exhausted soda lime
Faulty circuit absorber valves
Low bicarbonate