Andrew Stephen, MD
Background
Respiratory failure or compromise is one of the most common reasons for a surgical patient to need ICU care. Etiologies are varied and include blunt chest trauma, altered mental status, TBI, recent abdominal surgery, cardiothoracic surgery, sepsis, medical comorbidities and shock. A significant percentage of ICU resources, estimated close to 40% are consumed by patients with respiratory failure.
Type 1 Respiratory failure – Hypoxemia due to impaired gas exchange. Pneumonia, contusions, PE could be causes.
Type 2 Respiratory failure – Hypercapnia due to inadequate ventilation. Far more common in surgical patients due to splinting, atelectasis, poor effort, obesity, oversedation, effusions. Hypoventilation can lead to hypoxemia as well.
We have a significant number of elderly patients at RIH. The combination of rib fractures or recent abdominal/chest surgery coupled with a weak chest wall and respiratory muscle fatigue is the most common issue that leads to decline.
Patient assessment
The two most useful tools to determine if a patient is declining and may need intubation:
-General appearance – look for distress, diaphoresis, panic, gasping respirations,
use of accessory muscles of breathing
-Vital signs – tachycardia/hypertension are signs of a high sympathetic state, and worsening tachypnea is probably the most useful
If the patient appears to need intubation call the SICU or in house trauma attending and state your plan, how urgent the situation is, the patient’s airway history, physical exam signs that it may be a difficult airway, and whether anesthesia help is needed.
ABG and CXR are much less useful. ABG with normal to high normal PaCO2 can be falsely reassuring as the patient may be compensating their minute ventilation by increasing their respiratory rate(RR) which can be hard to sustain. The converse can also be true when using BiPAP to stave off intubation. A patient may start a BiPAP trial with a PaCO2 of 50 and RR of 34. After 4 hours the ABG reveals again a PaCO2 of 55. The RR is now 14 and the patient is more alert and more comfortable. BiPAP has been effective.
Once the patient is intubated
The patient will usually be on pressure control ventilation(PCV) initially. In this mode tidal volume becomes the dependent variable and goal is 6 mL per kg based on ideal body weight. FiO2 should be weaned first gradually to 0.4 then PEEP weaned. Daily efforts should be made to do pressure support(PSV) trials so that the patient can be weaned to extubation. PSV conditions the patient and results in more even distribution of air flow. Contraindications to PSV trials are few but include:
-hemodynamic instability
-unstable oxygenation
-chemical paralysis
-ICP concerns
All residents in the ICU should read the landmark low tidal volume and plateau pressure trial from ARDSnet: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. Acute Respiratory Distress Syndrome Network, Brower RG, Matthay MA, Morris A, Schoenfeld D, Thompson BT, Wheeler A. N Engl J Med. 2000 May 4;342(18):1301-8.
Special considerations, questions
1. What should a patient’s oxygen saturation be for me to wean FiO2 and PEEP? We still don’t know the answer to this but likely a saturation in the low 90’s is ok. A JAMA RCT from 2016 found lower mortality with a goal saturation of 94-98% compared to 97-100% in a mixed MICU-SICU.
Effect of Conservative vs Conventional Oxygen Therapy on Mortality Among Patients in an Intensive Care Unit: The Oxygen-ICU Randomized Clinical Trial.Girardis M, Busani S, Damiani E, Donati A, Rinaldi L, Marudi A, Morelli A, Antonelli M, Singer M. JAMA. 2016 Oct 18;316(15):1583-1589.
Increasing PEEP to move saturation to the high 90’s may result in hemodynamic issues, increased fluid administration, new organ dysfunction. High vs low PEEP strategies have been reviewed and can be found in the literature.
2. What are some rescue modes of mechanical ventilation for ARDS?
High frequency percussive ventilation(HFPV or VDR) and airway pressure release ventilation(APRV) are two common modes we will use but are beyond the scope of the handbook.
Some mortality benefit has been shown in some trials of burn patients but none in ARDS patients. It can improve oxygenation in ARDS patients. Brief VDR commentary, review by David Harrington: Volumetric diffusive ventilator. Harrington D. J Burn Care Res. 2009 Jan-Feb;30(1):175-6.
Pulmcrit provides a nice review of techniques and evidence for APRV: https://emcrit.org/pulmcrit/aprv/