Respiratory Physiology and Critical Care Flashcards
What is the most common cause of cyanide toxicity? What is the treatment?
Cyanide toxicity is often due to smoke inhalation during home or commercial fires.
First-line treatment is with hydroxocobalamin
What is the equation for Pulmonary Vascular Resistance?
PVR = (mean PA pressure - wedge pressure) / (cardiac output x 80)
Normal PVR is 50 to 150
Describe functional residual capacity
FRC = Expiratory reserve volume + Residual volume
Decreases with pregnancy, ascites, neonates, general anesthesia, obesity, and supine position
Increased with PEEP
During apnea, how fast will PaCO2 rise?
6 mm Hg during the first minute and then 3 to 4 mm Hg each minute after
What is the equation for BMI?
kg/m^2
Define anatomic dead space, alveolar (functional) dead space, and physiologic dead space
Anatomic dead space - volume of gas in the conducting airways of the lungs
Alveolar dead space - volume of gas in ventilated, but not perfumed alveoli
Physiologic dead space - sum of anatomic and alveolar dead space
What is the normal O2 requirement in adults?
3 to 4 mL/kg/min
250 mL/min in typical adult
What is the equation for O2 Content?
(1.39 x Hgb x Sat) + (0.003 x PaO2)
What is the Bohr dead space equation?
Dead space ratio (VD/VT) = (PaCO2 - EtCO2) / PaCO2
What is the mechanism for the compensatory shift of the oxyhemoglobin dissociation curve in response to chronic acid base abnormalities?
Altered metabolism of erythrocyte-2,3-DPG
What is the P50 of adult hemoglobin?
27 mm Hg
What is mixed venous O2 saturation (SvO2)? What is the equation? What would cause a decreased SvO2? An increased SvO2?
Mixed venous oxygen saturation is the percentage of oxygen bound to hemoglobin in the blood that returns to the right side of the heart
- Measured from a pulmonary artery catheter and includes all of the venous blood returning from the SVC, IVC, and coronoary sinus
SvO2 = SaO2 - VO2/(13.9 x CO x Hgb)
SvO2 is decreased by:
- Increased O2 consumption (hyperthermia, pain)
- Decreased CO (MI, hypovolemia)
- Decreased hemoglobin
- Decreased SaO2
SvO2 is increased by:
- Increased hemoglobin (transfusion)
- Increased SaO2
- Decreased O2 consumption (cyanide, sepsis, CO poisoning)
- Increased CO (sepsis, L to R shunt)
A change in PCO2 of 10 mm Hg will alter the pH by what amount?
~0.1 pH units
What is normal vital capacity in an adult?
60 to 70 mL/kg
About 5L in the average adult
How do you calculate the amount of bicarbonate that needs to be given to correct acidosis?
Total body weight (kg) x base deficit x 0.2
When administering sodium bicarbonate, 1/2 calculated dose is given and measurements are repeated
What is the magnitude of compensatory reaction to respiratory alkalosis?
Maximum decrease in [HCO3-] of 5 mEq/L for 10 mm Hg decrease in PCO2 under 40 mm Hg
What is the P50 of sickle cell hemoglobin?
31 mm Hg (decreased affinity for oxygen)
List some conditions that cause a rightward shift of the oxy-hemoglobin dissociation curve
Results in decreased affinity, increased unloading of oxygen
Acidosis Hyperthermia Increased 2,3-DPG Pregnancy Abnormal hemoglobins (sickle, thalassemia) Volatile anesthetics
List some conditions that cause a leftward shift of the oxy-hemoglobin dissociation curve
Results in increased affinity, decreased unloading of oxygen
Alkalosis
Hypothermia
Different hemoglobin species
Decreased 2,3-DPG
List the O2 saturation at specific PaO2 values
PaO2 of 27 mm Hg = 50% sat
PaO2 of 40 mm Hg = 75% sat
PaO2 of 60 mm Hg = 90% sat
PaO2 of 100 mm Hg = 98% sat
What is a general estimation of shunt fraction?
A-a gradient / 20
In a healthy, resting adult, what % of total body O2 consumption is due to the work of breathing?
1 to 3%
Describe Ventilation/Perfusion ratios in the lungs
Because alveoli in the dependent portion of the lungs are more compliant than alveoli in the non-dependent lung, alveolar ventilation increases from top to bottom. Pulmonary blood flow also increases from top to bottom, but more so than alveolar ventilation,
Ultimately, ventilation/perfusion ratios are higher in the non-dependent areas of the lung and low in the dependent areas of the lung.
Thus, PaO2 and pH are higher in the apex and PCO2 is higher at the base
How much anatomic dead space does an average adult have?
2 mL/kg
In an average adult, dead space is 150 mL
A change of 10 mm Hg of PCO2 results in what magnitude of change in plasma [K+]?
0.5 mEq/L in the same direction
A change in [HCO3-] of 10 mEq/L will alter the pH by what amount?
0.15 pH units
Describe the negative effects of acidosis
CNS depression
Increased ICP
Cardiovascular depression
Cardiac dysrhytmias
Vasodilation
Hypovolemia
Pulmonary HTN
Hyperkalemia
Describe Organophosphate poisoning
Organophosphates bind with acetylcholinesterase and produce signs of excess parasympathetic activity:
- Diarrhea
- Urination
- Miosis (pinpoint)
- Bronchocontriction
- Emesis
- Lacrimation
- Salivation
Treatment is with atropine and pralidoxime
How does recombinant human activated protein C (Xigris) work? What is it used for?
APC is indicated for the treatment of septic shock
It works via inhibition of factors Va and VIIIa, reducing inflammation and microthrombi. It also blocks production of TNF.
Major side effect is hemorrhage.
Describe transfusion related lung injury (TRALI)
A serious potential complication of transfusing any product containing plasma, usually diagnosed 1 to 2 hours later
- leading cause of transfusion-related mortality
Key features: Wide A-a gradient Non-cardiogenic pulmonary edema Leukopenia (2/2 lung sequestration) Fever Hypotension
Describe the difference between dead space and shunt
Dead space - ventilation without perfusion
Leads to elevated PCO2
Shunt - perfusion without ventilation
Leads to decreased PO2 and widened A-a gradient
What are some causes of dead space?
PE COPD ARDS Fibrosis Shock
Define closing capacity
= closing volume + residual volume
The volume at which small airways begin to close
Increases with age, bronchitis, LV failure, surgery, and smoking
At 44 y/o, CC = FRC when supine
At 66 y/o, CC = FRC when standing
Describe Flow-Volume Loops
Expiratory above, Inspiratory below
TLC to RV (left to right)
Fixed lesions: reduction in both inspiratory and expiratory components
Extrathoracic lesions: reduce inspiratory component
Intrathoracic lesions: reduce expiratory component
List the lung volumes and capacities
Tidal volume (TV) - volume of air inhaled/exhaled during normal breathing
Inspiratory reserve volume (IRV) - maximum volume of gas that can be inhaled after normal inspiration
Expiratory reserve volume (ERV) - maximum volume of gas that can be exhaled after normal expiration
Residual volume (RV) - volume of gas remaining in lungs after a forced exhalation
Vital capacity (VC) = IRV + TV + ERV - maximum volume of gas that can be exhaled after maximum inhalation
Inspiratory capacity (IC) = IRV + TV - maximum amount of gas that can be inhaled after normal expiration
Functional residual volume (FRC) = ERV + RV - remaining lung volume at the end of normal expiration
Total lung capacity (TLC) = IRV + TV + ERV + RV - lung volume after maximum inspiration
How much deoxyhemoglobin is required to appear cyanotic?
At least 5g deoxyhemoglobin
How does carbon monoxide exert its toxic effects? What are the symptoms? What will an ABG look like? What is the treatment?
Carbon monoxide effects:
- Impaired mitochondrial oxidative phosphorylation
- Impaired muscle oxygen utilization from impaired myoglobin function
- Functional anemia and leftward shift of oxygen-Hb curve
- Vasodilation 2/2 release of NO
Initial symptoms include headache, nausea, dizziness, and vomiting
- Cyanosis is NOT common
An ABG will show a metabolic acidosis with a normal PaO2 and a falsely elevated calculated SaO2 and measured SpO2
- PaO2 is not reliable in setting of carboxyhemoglobinemia since it is only a measure of dissolved oxygen and not bound oxygen
Treatment is O2
- Hyperbaric O2 only indicated for coma, cardiac ischemia, or levels over 40%
What is Diffusing Capacity of the Lungs (DLCO)?
A measure of the gas exchange capacity of the lungs
Influenced by the volume of blood in the pulmonary circulation
Decreased by:
- Anemia
- Hypovolemia
- Emphysema
- Pulmonary HTN
- Pulmonary edema
- Cystic fibrosis
Increased by:
- Supine position
- Exercise
- Obesity
- Left to right shunt
- Polycythemia
- Asthma
How will an air bubble effect an ABG?
It will result in elevated PO2 and decreased PCO2
What is the equation for the A-a Gradient?
A-a gradient = PAO2 - PaO2
PAO2 = FiO2(713) - PaCO2/0.8
Normal A-a gradient = age/4 + 4
How is ARDS diagnosed?What is the treatment?
Characterized by acute onset, bilateral infiltrates on chest X-ray, and hypoxemia (PaO2/FIO2 ratio less than 300)
Treatment is mainly supportive, while underlying cause is identified:
- Intubation and ventilation
- Low tidal volumes (permissive hypercapnia)
- Low PIP
- High FiO2
- High PEEP
Conservative fluid management is advised
Long-term antibiotics are NOT recommended
What are some causes of elevated peak pressure? What about plateau pressure?
Elevated peak pressures is indicative of high airway resistance:
- Bronchospasm
- Kinked ET tube
- Mucus plug
- Foreign body
Elevated plateau pressure is indicative of an issue with system compliance:
- Poor positioning
- Pulmonary fibrosis
- Pneumothorax
- Obesity
- Insufflation
What will the pulse oximetry be in a patient with methemoglobinemia? How is the PaO2 affected? What is the treatment?
Pulse oximetry will read ~85% regardless of true oxygen saturation
- need co-oximetry for proper analysis
PaO2 will be normal or unchanged
Treatment is with methylene blue
- Patients with G6PD can be treated with ascorbic acid
What is the equation for oxygen delivery (DO2)?
= CO x CaO2
= [HR x SV] x [(SaO2)(Hgb x 1.34) + (0.003 x PaO2)]
How does positive pressure ventilation cause oliguria?
Decreased preload 2/2 increased intrathoracic pressure
- decreased venous return
- increased CVP
- increased release of ADH from posterior pituitary
Increased sympathetic nervous system stimulation
- activation of renin-angiotensin system
Release of inflammatory cytokines
How can antibiotics used to treat sepsis potentially worsen hypotension?
If sepsis is due to gram-negative bacteria, antibiotics may lead to cell wall lysis and release of lipopolysacharide (LPS)
LPS induces immune response, release of cytokines, and production of NO, leading to vasodilation
What is refeeding syndrome? What is the mechanism behind it? What are the manifestations?
Hypophosphatemia seen during refeeding in alcoholic, malnourished, or debilitated patients
Caused by glucose loading
- increased secretion of insulin leads to cellular uptake of glucose and phosphate
Can result in AMS, hemolytic anemia, and arrythmias
- can be a cause of failed weaning from ventilator
What drug can be given to intubated patients to reduce the incidence of ventilator associated pneumonia
Sucralfate
H2-blockers or PPIs can actually increase the risk as they reduce gastric acidity, thus increasing bacterial growth
What is the RSBI? How is it used?
The Rapid Shallow Breathing Index is a test used in the weaning of mechanical ventilation in ICUs
RSBI = (respiratory frequency)/(tidal volume)
An RSBI less than 65 is considered an indication of weaning readiness
What is respiratory quotient? How does diet affect it?
RQ is the amount of CO2 produced per unit of oxygen consumed from specific energy substrates
Carbohydrates have a Q of 1
Proteins have a Q of 0.8
Lipids have a Q of 0.7
Percutaneous tracheostomy has many advantages over the open approach. However, what are the absolute contraindications of the percutaneous approach?
- Active infection at the site of trach
- Uncontrolled bleeding disorder
- Unstable cardiopulmonary status
- Patient unable to stay still
- Abnormal tracheolaryngeal anatomy
