Grand Rounds Recap 9.29.21
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Management of Hypoxemic Respiratory Failure with Dr. Susan Wilcox
Ventilation/Perfusion Matching
Normal physiology: capillary blood is oxygenated as it passes the alveoli
Shunt physiology: Some blood participates in normal gas exchange, but a portion of the blood does not, due to a variety of reasons.
PFO is, anatomically, the most pure type of shunt as deoxygenated blood passes the lungs entirely
Physiologic shunts are more common in the emergency department: pulmonary edema, pneumonia, atelectasis
Derecruitment contributes to shunt. Decompensation after intubation in pulmonary edema is a good representation of this. The patient who was previously tripoding is now flat and increased density of pulmonary edema causes collapse of previously oxygenated alveoli. The weight of the heart and cephalad pressure of abdominal contents can also contribute to derecruitment.
Minute ventilation = tidal volume x breaths per minute. Essentially this represents the surface area available for exchange over time. This is most important in patients with obstructive disease and profound acidosis, where carbon dioxide exchange is essential.
Pulmonary pressures are a function of both resistance and compliance.
Resistance is represented by the peak inspiratory pressure. Elevation of PIP is due to decreased flow. Tracheal stenosis and bad asthma are two conditions that contribute to this. A difference in the peak inspiratory pressure and plateau pressure greater than 5 cmH2O is suggestive of a resistance problem.
Compliance is represented by plateau pressure. Compliance can be increased due to stiffness of the chest wall, truncal obesity, chest wall burns, etc. You can check plateau pressure with an inspiratory hold. This is the pressure the alveoli see.
When setting up a ventilator: You can set pressure or volume, and will get the other back as a dependent variable. This relationship is a function of compliance. (C= delta volume/delta pressure)
Air trapping: incomplete exhalation after each breath. This causes progressive overdistention of the lungs. You can quantify air trapping: An expiratory hold will tell you the total peep. Intrinsic PEEP = total PEEP - set PEEP. Intrinsic PEEP less than 5 cmH20 is generally acceptable, but this becomes dangerous and intrinsic PEEP surpases 10 cmH20.
Obstructive Disease:
High intrinsic peep causes distention of the lungs -> increased intrathoracic pressures and poor cardiac return that can lead to arrest.
Decreasing the respiratory rate can help with air trapping by allowing a greater expiratory time, even with the same I:E ratio.
Bronchodilators can also help to decrease auto-peep by improving flow.
Acute Respiratory Distress Syndrome:
Berlin criteria are used to diagnose ARDS. A deep dive into ARDS by Dr. Chris Shaw can be found here.
ABG in the ED is very useful here to quantify the severity of disease.
Note: most patients do not die from hypoxemia alone, but ARDS severity correlates with risk of death from other causes.
ARDS in the era of COVID:
LOCO2 Trial investigated patients who were treated with conservative oxygen (88-92%) vs liberal (>96%). Trial was stopped early because of the risk of harm in the hypoxemic group.
Though COVID patients may not have significant symptoms with hypoxemia, there is evidence that end organ ischemia occurs.
Lung Protective Ventilation in ARDS
Tidal volume of 6 mL/kg predicted body weight. High tidal volume ventilation causes direct lung injury with damage to alveoli, edema, and hemorrhage. Pulmonary injury worsens the inflammatory response, increasing risk of death.
Plateau pressure <30 cmH20 prevents lung injury
Driving pressure <15 cmH20. This is the pressure pushing air into the lungs (difference between plateau pressure and set PEEP).
What can we do to optimize these patients?
PEEP keeps alveoli open and can prevent atelectasis to improve compliance. This will also improve oxygenation.
A volume/pressure curve can help determine the ideal PEEP.
Decreasing tidal volume improves plateau pressure, and we can facilitate this by allowing some hypercapnia (pH > 7.2), but increasing respiratory rate will compensate for this.
Don’t just look at the vent settings. Look at what your patients are actually getting.
Neuromuscular Blockade
Current data shows no mortality benefit by giving patients cisatracurium in early ARDS, but there does not appear to be any associated harm.
In patients who are dyssynchronous, this can be helpful to prevent lung injury associated with large tidal volumes.
Recruitment maneuver
Goal is to recruit atelectatic alveoli to improve shunt in persistent hypoxemia
PEEP is increased in stepwise increments of 3 cmH2O while looking at the patient. Then set PEEP to a higher level than at the start to prevent derecruitment.
Risks: Good alveoli can become overdistended, collapsing the capillary and causing a shunt. Increased intrathoracic pressure can decrease venous return to the heart and worsen hemodynamically unstable patients.
Prone positioning:
Prone positioning improves recruitment of the posterior lung and removes the weight of the heart.
PROSEVA study showed decrease mortality with proning
Inhaled pulmonary vasodilators:
Inhaled nitric oxide and epoprostenol
These medications improve V/Q matching by specifically dilating ventilated capillaries and decrease RV stress by improving pressures in the pulmonary vasculature. There is no evidence of a mortality benefit, but this can be used as a bridge to other therapies such as ECMO.
Inhaled vasodilators have little systemic effects and should not decrease MAP.
Minimize FiO2:
Hyperoxia has been linked to worse outcomes and increased lung injury.
After optimization, as noted above, reassess and decrease FiO2 as able.
Case-based Application:
Case 1: 20 y/o M with GSW at the left costal margin. He was intubated in the field. HR 120 and BP 82/65. There is color change on the CO2 detector. You are having difficulty auscultating breath sounds due to commotion in the room. He has peak pressures of 45 cmH2O.
The ventilator can be a diagnostic tool. High pressures may suggest tension PTX or mainstem intubation.
An expired tidal volume lower than the delivered tidal volume suggests that there may be a tension pneumothorax with leak into the chest or a cuff leak.
Case 2: 60 y/o M with diastolic dysfunction and obesity presents 4 days after a hernia repair with fever, night sweats, and cough. Vitals: 83% on RA, HR 105, temp 101.6, BP is normal. CXR demonstrates bilateral infiltrates. Non-invasive maneuvers are unsuccessful, and he is ultimately intubated, but he is now dyssynchronous on the ventilator.
HFNC can be useful in hypoxia with diastolic dysfunction to avoid worsening cardiac output from increased intrathoracic pressures.
Setting a low tidal volume is not always sufficient. You have to look at how they are actually breathing
Neuromuscular blockade can be useful for dyssynchrony, and patients who are overbreathing the ventilator.
Case 3: 50 y/o M with 2-3 days of progressive shortness of breath. Saturating is in the mid-70s in triage and improves to low 80s on NRB. He is started started on BiPAP and ultimately intubated. His saturation remains in the mid-80s. His CXR demonstrates dense opacification of the right middle and lower lobes with mild rightward tracheal deviation.
Dense opacification with tracheal deviation suggests lobar collapse.
Severe pneumonia, obstruction, and post-obstructive pneumonia can cause significant shunt and persistent hypoxia.
Patients with obstruction should be positioned with the non-obstructed lung down to facilitate V/Q matching.
Inhaled pulmonary vasodilators can also help improve V/Q matching by selectively vasodilating the vasculature of the ventilated lung
Bronchoscopy is helpful for evaluating and removing a mucous plug.
There are diminishing returns on increases in respiratory rate. Even in normal lungs, a rate > 30 will likely cause air trapping.
Case 4: 50 y/o F with history of COPD and AUD was unresponsive at home. For EMS she was hypotensive, unresponsive, taking 8 shallow breaths per minutes with sats in the 80s on RA. They have been bagging her and she arrives with saturations in the 90s. After a bolus her HR is 117 and her BP is 90/55.
In severe COPD the lungs have very high compliance, allowing overdistention to occur easily. COPD also causes alveolar inflammation that leads to high airway pressures. Combined, these cause extremely high compliance with high resistance that make air trapping and intrinsic PEEP likely.
Eventually the air trapping can lead to volutrauma and bleb rupture as well as increased intrathoracic pressures and decreased venous return to the right heart.
In severe instances of overdistention, disconnecting the vent and pressing on the chest to encourage exhalation can be helpful.
Global and Resource-Limited Ultrasound Considerations with Dr. Patricia Henwood
Global Ultrasound has a Transformative Impact:
Ultrasound can be used when there is limited access to other imaging modalities
Non-ionizing radiation
Equipment needed is portable and affordable
These can be attached to phones or smart screens, allowing us to bring them with us.
Images can be transmitted worldwide
SonoClipShare can be used to transmit de-identified images for research and educational purposes.
WhatsApp and other similar apps allow collaboration in care and bedside teaching across borders
Remote education is increasing in the era of COVID
Experiences in Ultrasound Education and Implementation in Africa
The emergence of Emergency Medicine in Africa and the availability of ultrasound are synergistically elevating the standard of care. Quality and Safety must be considered when introducing a new technology and developing a self-sustaining educational program:
Recognizing limitations
Operator dependent exams
Speed of sound is fixed
Seek an accurate diagnosis and consider an appropriate differential
Quality assurance programs
Ensure patients and confidentiality are protected
Ensure that there is bidirectional feedback and skills do not degrade over time. We can learn from the experiences and findings of those practicing in a different environment.
Consultant capacity
How much time do you have for collaboration
Back-up plan if equipment fails
Building a program around one device can be problematic, because equipment failure removes a critical diagnostic tool
Local or international repair pathways must be in place for the device chosen
These factors form the basis for the work being done by PURE Ultrasound
Liberian experience as a case study:
Civil war from 1989 to 2003 led to destruction of systems influencing health, including: water, electricity, roads, education, and health systems. Dr. Henwood worked with International Medical Corps during the 2014 Ebola pandemic. Mortality was high with a case fatality ratio of 0.5. No diagnostic testing modalities beyond malaria testing, pregnancy testing, and Ebola PCR were available, leading to largely empiric management. Ultrasound allowed advancement of the standard of care:
Prospective applications and training had to be carried out before the device was taken into the hot zone, because it would not be able to come out.
Ultrasound allowed better evaluation of volume status and guided resuscitation.
Patients were decompensating after resolution of their viral illness. Ultrasound allowed diagnosis of secondary illnesses, such as: paralytic ileus, bowel wall edema, multi-organ failure, and bacterial translocation across the gut with gram negative sepsis. This allowed the transition from empiric treatment to focused treatment with antibiotics that lowered mortality.
Ultrasound in pregnancy helped evaluate for congenital defects as a result of Ebola and dispel the prevailing outlook that pregnant women had a higher mortality.
Lessons learned from the development of emergency medicine ultrasound programs in resource-limited settings can be used at a local level to enhance system thinking and system building at inflection points such as the COVID pandemic or the Afghan refugee crisis.
Ultrasound Screening for Tuberculosis
Tuberculosis is the leading cause of death from a single infectious agent, globally. 1 in 4 people are infected globally with 14 million treated in 2018-2019. Additionally, TB is the leading cause of death in HIV positive adults in resource-limited settings. There is an urgent need for diagnostic modalities for TB as missed cases increase morbidity, mortality, and transmissions.
Focused Assessment with Sonography for HIV-associated TB (FASH) can aid diagnosis in settings iwth a high prevelance of both diseases.
Ongoing research is investigating the performance of ultrasound to diagnose pulmonary TB in adults, against sputum microbiology as the gold standard and other imaging modalities. Subpleural consolidations are being investigated as a diagnostic sign.
