SM_160a-162a: Hypercapnia and Hypoxemia Flashcards
What is hypoxemia?
Decreased PO2 in blood
What is hypoxia?
Decreased PO2 in tissue
Most hypoxia is driven by ________
Most hypoxia is driven by hypoxemia
In clinical practice, problems with oxygenation are typically identified using ________
In clinical practice, problems with oxygenation are typically identified using pulse oximetry
What is SpO2?
Peripheral oxygen saturation
Standard ABG notation is ____ / ____ / ____
Standard ABG notation is pH / PaCO2 / PaO2
Normal ABG notation is ____ / ____ / ____
Normal ABG notation is pH 7.40 / PaCO2 40 mmHg / PaO2 95 mmHg
Point 1 on the oxygen-hemoglobin dissociation curve is _____ and corresponds to a PO2 of ____ mmHg and a Hgb sat of ____ %
Point 1 on the oxygen-hemoglobin dissociation curve is normal and corresponds to a PO2 of 95 mmHg and a Hgb sat of 97 %
Point 2 on the oxygen-hemoglobin dissociation curve is _____ and corresponds to a PO2 of ____ mmHg and a Hgb sat of ____ %
Point 2 on the oxygen-hemoglobin dissociation curve is the knee and corresponds to a PO2 of 60 mmHg and a Hgb sat of 90 %
Point 3 on the oxygen-hemoglobin dissociation curve is _____ and corresponds to a PO2 of ____ mmHg and a Hgb sat of ____ %
Point 3 on the oxygen-hemoglobin dissociation curve is mixed venous and corresponds to a PO2 of 40 mmHg and a Hgb sat of 75 %
Below point 1 on the oxygen-hemoglobin dissociation curve, _____ changes in Hgb sat are associated with _____ changes in PO2
Below point 1 on the oxygen-hemoglobin dissociation curve, small changes in Hgb sat are associated with large changes in PO2
What is MvO2 or SVO2?
Mixed venous Hgb saturation
(mixed because blood contains venous drainage from SVC, IVC, and coronary sinus)
PA catheter measures _____, which is _____ venous Hgb saturation
Triple lumen catheter measures ______, which is _____ venous Hgb saturation
PA catheter measures MvO2 (SVO2), which is mixed venous Hgb saturation
Triple lumen catheter measures ScVO2, which is central venous Hgb saturation
(PA catheter goes farther in than triple lumen catheter)
A 65 year old patient is currently in the MICU with septic shock. He is febrile and has a worsening metabolic acidosis with an ABG of 7.15/30/60. His SpO2 is likely to be ____%
A 65 year old patient is currently in the MICU with septic shock. He is febrile and has a worsening metabolic acidosis with an ABG of 7.15/30/60. His SpO2 is likely to be 85%.
PO2 of 60 mmHg corresponds to Hgb sat of 90%, but this patient is acidemic and febrile -> shifts O2-Hgb dissociation curve to the right -> O2 offloaded to tissues -> Hgb less saturated for a given PaO2 (Bohr effect)
What shifts the oxygen-hemoglobin dissocation curve to the right?
- Increased CO2
- Decreased pH
- Increased temperature
- Increased 2,3-BPG
What are the normal values for the following?
PaO2: ___ mmHg
SpO2: ___ %
PvO2: ___ mmHg
SvO2: ___ %
Normal values
PaO2: 95 mmHg
SpO2: 97 %
PvO2: 40 mmHg
SvO2: 75 %
_____ is the driving force for oxygenating capillary blood coming through alveolar-capillary units
PAO2 is the driving force for oxygenating capillary blood coming through alveolar-capillary units
The more efficient gas exchange is at the alveolar-capillary interface, the ______ the A-a difference
The more efficient gas exchange is at the alveolar-capillary interface, the narrower the A-a difference
O2 taken in = ________ + ________
O2 taken in = O2 consimed (V·O2) + O2 delivered out of lung
What is the equation for PAO2?
PO2 = _____ * _____
PO2 = Patm * FIO2
(PO2 is set by barometric pressure and fractional concentration of oxygen)
Air is ______ and ______ in the upper airway, so PiO2 = _________
Air is warmed and humidified in the upper airway, so PiO2 = (Patm - PH20) * 0.21
(warming and humidifying air in the upper airway adds water vapor)
Respiratory quotient is set by _____ and ______
In a typical American diet, R = ____
Respiratory quotient is set by diet and metabolism
In a typical American diet, R = 0.8
(V·CO2 = 200 mL/min and V·O2 = 250 mL/min)
What is the formula for respiratory quotient?
R = V·CO2 / V·O2
What is the alveolar gas equation?
PAO2 = (Patm - PH2O) * FiO2 - (PaCO2 / R)
What are the normal values for the pressures and saturations?
_____ sets the partial pressure gradient for oxygenating blood
PAO2 sets the partial pressure gradient for oxygenating blood
PiO2 ____ PAO2
PiO2 > PAO2
(have to subtract water vapor pressure for inspired partial pressure of oxygen)
Decreasing PAO2 will _______ PaO2
Decreasing PAO2 will decrease PaO2
What are the causes of hypoxemia?
- Shunt (V·A/Q = 0)
- Decreased V·A/Q (but not 0)
- Diffusion limitation
- Decreased MvO2 (SVO2)
- Decreased Patm
- Decreased FiO2
- Increased PaCO2
- Decreased R (respiratory coefficient)
______, ______, and ______ are causes of hypoxemia that are due to increased A-a difference
Shunt (V·A/Q = 0), decreased V·A/Q (but not 0), and diffusion limitation are causes of hypoxemia that are due to increased A-a difference
______, ______, ______, and ______ are causes of hypoxemia that are due to decreased PAO2 and have a small A-a difference
Decreased Patm, decreased FiO2, increased PaCO2, and decreased R are causes of hypoxemia that are due to decreased PAO2 and have a small A-a difference
Shunt (V·A/Q = 0), decreased V·A/Q (but not 0), and diffusion limitation are causes of hypoxemia resulting from _______
Shunt (V·A/Q = 0), decreased V·A/Q (but not 0), and diffusion limitation are causes of hypoxemia resulting from increased A-a difference
Decreased Patm, decreased FiO2, increased PaCO2, and decreased R are causes of hypoxemia resulting from ______ and that have a ______
Decreased Patm, decreased FiO2, increased PaCO2, and decreased R are causes of hypoxemia resulting from decreased PAO2 and have a small A-a difference
Altitude causes hypoxemia by _____
Altitude causes hypoxemia by decreasing Patm
What is the immediate physiologic response to altitude?
Hyperventilating to drop PCO2 and raise PAO2
Fires and mining cause hypoxemia by ______
Fires and mining cause hypoxemia by decreasing FiO2
Increasing PaCO2 will ______ PAO2
Increasing PaCO2 will decrease PAO2
What caused this patient’s hypoxemia?
This patient is hypoxemic and hypercapnic and the A-a < 10 mmHg, so the patients hypoxemia is reasonably explained by hypercapnic respiratory failure driven by fentanyl overdose
If A-a difference ≤ 10, hypoxemia _____ explained by hypercapnia
If A-a difference ≤ 10, hypoxemia is reasonably explained by hypercapnia
If A-a difference > 10, hypoxemia ____ explained by hypercapnia
If A-a difference > 10, hypoxemia is NOT reasonably explained by hypercapnia
Something else must be causing the hypoxemia
Q· represents perfusion and is analogous to ____ (5L/min)
Q· represents perfusion and is analogous to cardiac output (5L/min)
What are the types of ventilation?
- Minute ventilation (V·E)
- Dead space ventilation (V·D)
- Alveolar ventilation (V·A)
What is the formula for minute ventilation?
V·E = RR * VT
(V·E = 14 breaths/min * 500 mL = 7L/min)
______ is the fact that air stays in bronchi and trachea and does not contribute to ventilation
Anatomic dead space is the fact that air stays in bronchi and trachea and does not contribute to ventilation
What is the formula for anatomic dead space?
V·D = RR * VD
(V·D = 14 breaths/min * 150 mL = 2.1 L/min)
Alveolar dead space is when alveoli are ______ but not ______, so they _____ gas exchange
Alveolar dead space is when alveoli are ventilated but not perfused, so they do not contribute to gas exchange
(emphysema, pulmonary embolism)
Physiologic dead space = ______ + ______
Physiologic dead space = anatomic dead space + alveolar dead space
In health, anatomic dead space _____ physiologic dead space
In health, anatomic dead space = physiologic dead space
(should not have any alveolar dead space)
What is the formula for alveolar ventilation?
V·A = V·E - V·D
(V·A = 7 L/min - 2 L/min = 5 L/min -> because Q· = 5 L/min, V·A/Q· = 1)
Shunts can be ______ or ______
Shunts can be intracardiac (PDA, ASD, AVM) or intrapulmonary (blood, pus, water, atelectasis)
What is the differential for an intrapulmonary shunt?
Blood, pus, water, atelectasis
When circulations are mixed, _____ and _____ are averaged, but _____ is not averaged
When circulations are mixed, Hgb sat and O2 content are averaged, but PO2 is not averaged
In a large shunt, the patient may be ______ to supplemental oxygen and still ______ as a result
In a large shunt, the patient may be unresponsive to supplemental oxygen and still hypoxemic as a result
Shunts are difficult to deal with because the oxygen-hemoglobin dissociation curve is _____ under normal physiologic circumstances
Shunts are difficult to deal with because the oxygen-hemoglobin dissociation curve is flat under normal physiologic circumstances
(oxygen content of Hgb is maximum at maximum Hgb sat)
________ is an example of a “blood” cause of intrapulmonary shunt
Diffuse alveolar hemorrhage is an example of a “blood” cause of intrapulmonary shunt
________ is an example of a “pus” cause of intrapulmonary shunt
Pneumonia is an example of a “pus” cause of intrapulmonary shunt
________ is an example of a “water” cause of intrapulmonary shunt
Pulmonary edema is an example of a “water” cause of intrapulmonary shunt
Contrast high pressure/cardiogenic and low pressure/non-cardiogenic pulmonary edema
- High pressure/cardiogenic: increased microvascular hydrostatic pressure, vascular endothelium intact, low protein
- Low pressure/non-cardiogenic: vascular endothelium damaged, high protein
Reflection coefficient in the Starling equation represents _______
Reflection coefficient in the Starling equation represents how effective the microvascular endothelium is at holding fluid in capillary vessels
ARDS is a type of ______ pulmonary edema
ARDS is a type of low pressure/non-cardiogenic pulmonary edema
What are the diagnostic criteria for ARDS?
On room air, use _____ to look for a primary lung problem
On room air, use A-a difference to look for a primary lung problem
(higher is worse)
On supplemental oxygen, use ______ to look for a primary lung problem
On supplemental oxygen, use PaO2/FiO2 ratio to look for a primary lung problem
(also called P-F ratio, <400 is abnormal)
(lower is worse)
What would you expect to see on an X-ray of a patient with ARDS?
Diffuse white matter
What is the pathobiology of ARDS?
- Injury to alveolar-capillary barrier
- Influx of protein-rich edema fluid into alveolar space
(reflection coefficient drops to nearly zero -> high-protein pulmonary edema)
________ is the pathologic hallmark of ARDS
Diffuse alveolar damage (DAD) is the pathologic hallmark of ARDS
Atelectasis means ______
Atelectasis means collapse of the alveolar unit
(occurs w/ endobronchial cancer or in supine ventilated patients, alveoli vasoconstrict in response to hypoxia)
A cause of hypoxemia, decreased V·A/Q· occurs when V·A ______ or Q· ______
A cause of hypoxemia, decreased V·A/Q· occurs when V·A decreases (chronic bronchitis, asthma) or Q· increases (pulmonary embolism)
A cause of hypoxemia, diffusion impairment occurs when t _____ or A ______
A cause of hypoxemia, diffusion impairment occurs when t increases (interstitial lung disease) or A decreases (emphysema)
V·gas = (A/t) * D * ∆P
Decreased MvO2 (SvO2) is only relevant in modulated the causes of hypoxemia that ______ because healthy alveoli maintain normal PAO2
Decreased MvO2 (SvO2) is only relevant in modulated the causes of hypoxemia that increase A-a difference because healthy alveoli maintain normal PAO2
(shunt, decreased but not zero V·A/Q, and diffusion limitation increase A-a difference)
What is the Fick equation?
CO = V·O2 / (CaO2 - CvO2)
What is the formula for CaO2 or CvO2?
CaO2 or CvO2 = (1.34 mL O2/g * Hgb * % sat) + (0.003 * PaO2)
What is the formula for SvO2?
SvO2 = CO * [(1.34 mL O2/g * Hgb * % sat) + (0.003 * PaO2)] - V·O2
_______, _______, _______, and _______ are causes of low SvO2
Low cardiac output, anemia, hypoxemia, and increased oxygen consumption are causes of low SvO2
(less O2 sent out, so less comes back)
What are the causes of low SvO2?
- Low cardiac output
- Anemia
- Hypoxemia
- Increased oxygen consumption
Low SvO2 is important for the following causes of hypoxia: ______, _______, and _______
Low SvO2 is important for the following causes of hypoxia: shunt (V·A/Q· = 0), decreased V·A/Q· (but not zero), and diffusion impairment
What is the formula for PaCO2?
PaCO2 = (V·CO2 * 863 mmHg) / V·A
PACO2 and PaCO2 are ______, while PAO2 and PaO2 are ______
PACO2 and PaCO2 are interchangeable, while PAO2 and PaO2 are not interchangeable
PaCO2 and V·A are _____ related
PaCO2 and V·A are inversely related
What is the alveolar ventilation equation for CO2?
PaCO2 = (V·CO2 * 863 mmHg) / [RR * VT (1 - VD/VT)
Problems with ______, ______, and ______ can cause abnormal PaCO2
Problems with CO2 production (V·CO2), minute ventilation (RR * VT), and ratio of dead space to tidal volume (VD/VT) can cause abnormal PaCO2
A 75 year old patient with severe emphysema presents to the ED with 3 days of fever and cough. He is visibly tachypneic with a RR of 30 breaths/min. Temperature is 102.5 F. What is true regarding PaCO2?
Despite the high RR, PaCO2 is likely high
(cannot assume tachypneic patient is hypocapnic)
RR ___ PaCO2
RR ≠ PaCO2
You are caring for a mechanically ventilated and deeply sedated patient with ARDS in the MICU. The set RR and VT on the ventilator are 28 breaths/minute and 350 mL respectively, On the morning rounds, you note the PaCO2 has risen from 50 to 65 mmHg since yesterday evening. The ventilator settings and vital settings are unchanged. What do you do?
Evaluate for pulmonary edema
Dead space to tidal volume ratio has changed acutely resulting from a pulmonary embolism, which corresponds to ventilated but not perfused (V/Q = infinity)
_____, _____, and _____ cause increased PaCO2
Increased V·CO2, decreased V·E, and increased VD/VT cause increased PaCO2
What raises V·CO2 and subsequently increases PaCO2?
- Fever
- Seizures
- Over-feeding
- Hypermetabolic states such as hyperthyroidism
What decreases V·E and subsequently increases PaCO2?
- Will not breathe: head injuries, stroke/ICH, CNS infections, CNS depressant drugs (opioids, benzodiazepines, barbiturates), obesity leading to hypoventilation, hypothyroidism
- Cannot breathe: neuromuscular disease (ALS, myasthenia gravis, Guillan-Barre, polio, muscular dystrophy), chest wall deformities, obstructive and restrictive lung disease (severe) + respiratory muscle fatigue
What increases VD/VT and subsequently increases PaCO2?
- Pulmonary embolism
- Emphysema
- Pulmonary hyperinflation
Hypercapnia is often caused by ______ coupled with _______
Hypercapnia is often caused by impaired strength coupled with increased elastic or resistive load on respiratory system
For every ___ mmHg change in PaCO2, pH changes by ___ in the _____ direction
For every 10 mmHg change in PaCO2, pH changes by 0.08 in the opposite direction
Respiratory alkalosis can be caused by ____ V·E
Respiratory alkalosis can be caused by increased V·E
- Drugs (cocaine, meth)
- Fever
- Sepsis
- Pain/anxiety
- Thyrotoxicosis
- Mild respiratory disease: mild asthma, pneumonia, PE, pulmonary edema
- Pregnancy
- Cirrhosis/liver disease
- Salicylate toxicity
A 21 year-old medical student presents to the ED with acute anxiety and tingling in her hands. Her exam and CXR are unrevealing. SpO2 on room air is 97%. Room air ABG is 7.56/20/95. She has ______ and [further / no further] evaluation is needed.
She has acute respiratory alkalosis and no further evaluation is needed
Low PaCO2, high pH so respiratory alkalosis
