Ennis James, M.D. Wednesday May 8th, 2018

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Presentation transcript:

Ennis James, M.D. Wednesday May 8th, 2018 Hypoxemia Ennis James, M.D. Wednesday May 8th, 2018 Please go to pollev.com/ennisjames849

Objectives Define hypoxemia Differentiate the major pathophysiologic categories of hypoxemia Describe the pathophysiologic mechanisms of hypoxemia Apply pathophysiologic mechanisms to clinical care of patients with hypoxemia (pollev.com/ennisjames849)

A 63yo woman with a h/o DM type 2, GERD, hypertension, and dementia presents with 4 days of cough, fever, and right-sided chest pain. Her temperature is 101◦ F, heart rate is 120bpm, SBP 100mm Hg, SpO2 88% on room air, and RR 34 breaths/minute. She has decreased right basilar lung sounds and right mid-lung crackles. Antibiotics have been ordered and chest x-ray is pending. What is the most appropriate immediate next step in her care? A) Perform an arterial blood gas B) Order an albuterol nebulizer treatment C) Obtain a STAT chest CT angiogram D) Initiate supplemental oxygen via full face mask E) Initiate non-invasive positive pressure ventilation (BiPAP) (pollev.com/ennisjames849)

https://www.polleverywhere.com/multiple_choice_polls/hbDPxQ2wue0mu9p

(pollev.com/ennisjames849) What is hypoxemia? (pollev.com/ennisjames849)

https://www.polleverywhere.com/free_text_polls/q4D99EgGS2dYnVY

But. . . what determines PaO2 and SaO2? What is hypoxemia? = PaO2 SaO2 But. . . what determines PaO2 and SaO2?

What is hypoxemia? Alveolar gas equation PAO2 = FiO2 (PB-PH20) – PaCO2/R On ambient air at sea level PAO2 = 150 – (PaCO2 x 1.25)

Great, so what is normal? Alveoli Arterial blood Mixed venous blood PO2 PCO2 100 95 40 40 46 What’s all this talk about oxygen “tension” and “content”? Tension: the pressure gradient that drives passive diffuse of gas across a membrane (ie, that of the alveolar-capillary type) Content: more about oxygen transport (involves CO and hemoglobin)

PaO2 What is hypoxemia? mild moderate severe (PaO2 60-79) (PaO2 40-59)

Oxygen-hemoglobin curve

“Supplemental” review Delivery method LPM Nasal Cannula 1-6 Simple mask 5-8 Face Tent 10-15 Venturi Mask variable Partial NRB 6-10 NRB HFNC 10-60 FiO2 0.24 - 0.44 (4% rule) 0.35 - 0.55 0.40 0.24 - 0.50 0.50 - 0.70 0.70 – 0.9 1.0 (?PEEP)

How does oxygen get from the environment to the blood? PH2O PH2O Patm PAO2 PAO2 Alveolar-capillary membrane PaO2 PaO2 PaO2

What are causes of hypoxemia? PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2

What are causes of hypoxemia? PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2

What are causes of hypoxemia? V/Q mismatch Diffusion deficit Shunt Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

SUPER-QUICK CLINICAL QUESTION

What is the most appropriate next step in his clinical care? A 45yo homeless man is brought to the ED after having been found down covered in emesis. He is known to have a history of asthma and EtOH abuse. His SpO2 is 90% on room air, and his respiratory rate is 10 breaths/minute. ABG on room air: 7.16 70 60 What is the most appropriate next step in his clinical care? pH PaCO2 PaO2 A) Start antibiotics for aspiration pneumonia B) Give bicarbonate for a metabolic acidosis C) Intubate the patient and initiate invasive ventilation D) Give steroids E) Give inhaled bronchodilators F) Both A & B (Please go to pollev.com/ennisjames849)

https://www.polleverywhere.com/multiple_choice_polls/dDLBTWFWRSYqC6f

Hypoventilation What are causes of hypoxemia? V/Q mismatch Diffusion deficit Shunt Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

What are causes of hypoventilation? (pollev.com/ennisjames849)

https://www.polleverywhere.com/free_text_polls/D6La1c8oOKN5A0x

Alveolar hypoventilation PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2

Sites where pathology may cause hypoventilation Sites at which disease may cause hypoventilation

Physiologic relationships VE = VT x RR = (VA + VD) x RR Alveolar ventilation equation PaCO2 = VA VCO2 x k VE = minute ventilation; VT = tidal volume; VA = alveolar ventilation; VD = dead space; VCO2 = systemic production of carbon dioxide

Anatomic and physiologic dead space ANATOMY(IC) Dead space Normal=150mL Normal VT = ~500mL (150 is VD, 350 VA) Reduce VT to 250mL, VD doesn’t change but VA goes down (350 to 100mL)…so PaCO2 goes up

How does hypoventilation result in hypoxemia? Small breaths  decrease VA Decrease VA  pCO2 goes up… So by definition PAO2 goes down…. PAO2 = FiO2 (PB-PH20) – PaCO2/R

PAO2 = FiO2 (PB-PH20) – PaCO2/R And, as a reminder. . . Alveolar gas equation PAO2 = FiO2 (PB-PH20) – PaCO2/R On RA at sea level PAO2 = 150 – (PaCO2 x 1.25)

Does he have an aspiration pneumonia? A 45yo homeless man is brought to the ED after having been found down covered in emesis. He is known to have a history of asthma and EtOH abuse. His SpO2 is 90% on room air, and his respiratory rate is 10 breaths/minute. ABG on room air: 7.16 70 60 Does he have an aspiration pneumonia? pH PaCO2 PaO2 A) Yes B) No C) I don’t know D) I don’t care E) I need more information F) Both A & B (Please go to pollev.com/ennisjames849)

https://www.polleverywhere.com/multiple_choice_polls/0yvEQXAkxK98aWX

Your answers

TANGENT

Pulmonary acid / base tangent Acidemia (low pH) PaCO2 high or low? PaCO2 is high PaCO2 is low Respiratory Acidosis Metabolic Acidosis

Pulmonary acid / base tangent Respiratory acidoses ACUTE CHRONIC For every 10mm Hg increase in PaCO2 For every 10mm Hg increase in PaCO2 pH decreases by 0.08 pH decreases by 0.03

Back to hypoxemia

What are causes of hypoxemia? V/Q mismatch Diffusion deficit Shunt Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

Shunt What are causes of hypoxemia? V/Q mismatch Diffusion deficit Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

Shunt pathophysiology PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2 Hypoxemia due to shunt does not improve with supplemental O2.

(Please go to pollev.com/ennisjames849) Which of the following diseases causes hypoxemia by shunt pathophysiology? A) Acute COPD exacerbation B) Pulmonary embolism C) Atelectasis D) Acute respiratory distress syndrome E) Acute pulmonary edema F) Obesity hypoventilation syndrome (Please go to pollev.com/ennisjames849)

Your answers

Your answers – take 2

V/Q mismatch What are causes of hypoxemia? V/Q mismatch Diffusion deficit Shunt Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

Most common cause of hypoxemia (encompasses many diagnoses). V/Q mismatch PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2 Most common cause of hypoxemia (encompasses many diagnoses).

Ventilation to Perfusion ratio V/Q relationships Number of lung units Clearly shunting and dead space ventilation are the extremes of ventilation:perfusion mismatch and less extreme forms of mismatch exist. Indeed acute respiratory failure is characterised by increase in the spread of ventilation perfusion ratios as opposed the normal situation where ratios are closely clustered around 1 Diseased Normal Hypoxemia due to V/Q mismatch does improve somewhat with supplemental O2. 1 Ventilation to Perfusion ratio

“Gas exchange” deficits Hypoxemia due to V/Q mismatch does improve somewhat with supplemental O2.

Which of the following diseases causes hypoxemia by V/Q mismatch? A) Heroin overdose B) Pneumothorax C) Atelectasis D) Right lower lobe pneumonia E) Advanced idiopathic pulmonary fibrosis F) Mucus plug (Please go to pollev.com/pathophysiology)

Which of the following diseases causes hypoxemia by V/Q mismatch? A) Heroin overdose B) Pneumothorax C) Atelectasis D) Right lower lobe pneumonia E) Advanced idiopathic pulmonary fibrosis F) Mucus plug (Please go to pollev.com/pathophysiology)

Diffusion Deficit What are causes of hypoxemia? V/Q mismatch Shunt Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

Diffusion deficit Patm PAO2 PAO2 PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 Hypoxemia due to diffusion deficit almost always occurs with exertion (and not at rest.) PaO2

Diffusion deficit Due to a reduction in alveolar surface area (not “increased thickness” of the pulmonary parenchyma)

(Please go to pollev.com/pathophysiology) Which of the following x-rays is most consistent with hypoxemia caused by diffusion deficit? A) Chest x-ray A B) Chest x-ray B C) Chest x-ray C (Please go to pollev.com/pathophysiology)

Chest x-ray A

Chest x-ray B

Chest x-ray C

What are causes of hypoxemia? V/Q mismatch Diffusion deficit Shunt What differentiates these mechanisms? Low PIO2 Hypoventilation Carboxy- or met-hemoglobin =“histotoxic” hypoxemia

What is the “A-a gradient”? PAO2 PaO2 PAO2 – PaO2 = A-a gradient

We can calculate the PAO2 Alveolar gas equation PAO2 = FiO2 (PB-PH20) – PCO2/R On RA at sea level PAO2 = 150 – (PCO2 x 1.25)

What are the definitions of: Quick self-review What are the definitions of: PAO2 SpO2 PaO2 A-a gradient FiO2 V/Q mismatch Shunt Diffusion deficit Hypoventilation Low PIO2

(Please go to pollev.com/pathophysiology) A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles. ABG on room air: 7.30 56 45 He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH PaCO2 PaO2 A) Low PIO2 B) Alveolar hypoventilation C) V/Q mismatch D) Shunt E) Diffusion deficit F) Acute respiratory distress syndrome (Please go to pollev.com/pathophysiology)

(Please go to pollev.com/pathophysiology) A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles. ABG on room air: 7.30 56 45 He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH PaCO2 PaO2 A) Low PIO2 B) Alveolar hypoventilation C) V/Q mismatch D) Shunt E) Diffusion deficit F) Acute respiratory distress syndrome (Please go to pollev.com/pathophysiology)

(Please go to pollev.com/pathophysiology) A 55yo man with a h/o severe COPD presents with 3 days of worsening shortness of breath. Chest x-ray reveals a right lower lobe pneumonia. His respiratory rate is 28 breaths/min and his SpO2 is 80%. He is using accessory muscles. ABG on room air: 7.30 56 45 He is placed on 60% FiO2, and his SpO2 increases to 96%. What is the most likely cause of his acute hypoxemia? pH PaCO2 PaO2 A) Low PIO2 B) Alveolar hypoventilation C) V/Q mismatch D) Shunt E) Diffusion deficit F) Acute respiratory distress syndrome (Please go to pollev.com/pathophysiology)

Methods of supplementing O2 Regular ~6 Lpm Nasal cannula Oximizer ~15 Lpm Ventimask ~50% FiO2 Face mask Non-rebreather ~65% FiO2 Non-invasive 100% FiO2 Positive pressure ventilation 100% FiO2 Endotracheal

(Please go to pollev.com/pathophysiology) After careful review of his clinical circumstances and ABG results, you have determined that his acute hypoxemia is due to V/Q mismatch due to pneumonia (superimposed on chronic alveolar hypoventilation due to COPD.) In addition to supplemental oxygen, what is then next most appropriate step in his clinical care? A) Initiate non-invasive ventilation B) Intubate him and initiate invasive ventilation C) Provide high dose steroids for a COPD exacerbation D) Provide antibiotics for a pneumonia E) Perform a chest CT angiogram to assess for PE F) Provide incentive spirometry (Please go to pollev.com/pathophysiology)

(Please go to pollev.com/pathophysiology) After careful review of his clinical circumstances and ABG results, you have determined that his acute hypoxemia is due to V/Q mismatch due to pneumonia (superimposed on chronic alveolar hypoventilation due to COPD.) In addition to supplemental oxygen, what is then next most appropriate step in his clinical care? A) Initiate non-invasive ventilation B) Intubate him and initiate invasive ventilation C) Provide high dose steroids for a COPD exacerbation D) Provide antibiotics for a pneumonia E) Perform a chest CT angiogram to assess for PE F) Provide incentive spirometry (Please go to pollev.com/pathophysiology)

The patient is placed on full face mask with a FiO2 of 60%. As a reminder, his initial ABG on room air was: 7.30 56 45 And, repeat ABG on 60% FiO2 is: 7.16 72 200 What is the most likely explanation for he acute increase in his PaCO2 after he was placed on 60% FiO2? pH PaCO2 PaO2 pH PaCO2 PaO2 A) Worsening V/Q mismatch B) The patient stopped breathing C) Oxygen induced bronchospasm D) Oxygen toxicity of the lung E) A shift in the oxygen-hemoglobin dissociation curve F) Progressive acute respiratory distress syndrome (Please go to pollev.com/pathophysiology)

The patient is placed on full face mask with a FiO2 of 60%. As a reminder, his initial ABG on room air was: 7.30 56 45 And, repeat ABG on 60% FiO2 is: 7.16 72 200 What is the most likely explanation for he acute increase in his PaCO2 after he was placed on 60% FiO2? pH PaCO2 PaO2 pH PaCO2 PaO2 A) Worsening V/Q mismatch B) The patient stopped breathing C) Oxygen induced bronchospasm D) Oxygen toxicity of the lung E) A shift in the oxygen-hemoglobin dissociation curve F) Progressive acute respiratory distress syndrome (Please go to pollev.com/pathophysiology)

Flow-volume loop

Why does your resident recommend doing this? Based on the patient’s response to delivering a FiO2 of 60%, your resident recommends that you decrease the FiO2 delivered so that the patient’s SpO2 is 90-92%. Why does your resident recommend doing this? A) Lower SpO2s will increase the patient’s drive to breath B) Lower SpO2s will allow you to detect hypoventilation sooner C) Lower SpO2s result in higher PaO2s D) Lower SpO2s decrease oxygen toxicity E) Alveolar hypoventilation is not treated with oxygen F) Bacteria proliferate more readily with higher PAO2s (Please go to pollev.com/pathophysiology)

Why does your resident recommend doing this? Based on the patient’s response to delivering a FiO2 of 60%, your resident recommends that you decrease the FiO2 delivered so that the patient’s SpO2 is 90-92%. Why does your resident recommend doing this? A) Lower SpO2s will increase the patient’s drive to breath B) Lower SpO2s will allow you to detect hypoventilation sooner C) Lower SpO2s result in higher PaO2s D) Lower SpO2s decrease oxygen toxicity E) Alveolar hypoventilation is not treated with oxygen F) Bacteria proliferate more readily with higher PAO2s (Please go to pollev.com/pathophysiology)

Summary – physiology of oxygenation PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2

Summary – pathophysiology of hypoxemia PH2O PH2O Patm PAO2 PAO2 PaO2 PaO2 PaO2

Objectives Define hypoxemia Differentiate the major pathophysiologic categories of hypoxemia Describe the pathophysiologic mechanisms of hypoxemia Apply pathophysiologic mechanisms to clinical care of patients with hypoxemia (pollev.com/pathophysiology)

THANK YOU!