Chapter 4-Ventilation
VD (V dot , subscript D). What is is equal to
Dead space ventilation is equal to alveolar dead space ventilation plus anatomical space space ventilation
Hyperpnea
Deep breathing
If alveolar minute ventilation is decreased by half PACO2/PaCO2...
Doubles
The lungs of a patient in the intensive care unit are mechanically ventilated with the following settings: f= 12/min VT = 500 mL You also have the following information about your patient: VD = 150 mL PaCO2 =40 mm Hg Approximately 1 hour later, the patient seems to be agitated and is triggering the ventilator at a rate of 24/min (VT is still 500 mL). Estimate the patient's new PaCO2 .
First find VA in the first scenario V˙ A = V˙ E −VD V˙ A = (500 mL × 12)−(150 mL × 12) V˙ A = 6000 mL/min−1800 mL/min V˙ A = 4200 mL/min Second find VA in the second scenario V˙ A = V˙ E −VD V˙ A = (500 × 24)−(150 × 24) V˙ A = 12, 000 mL/min−3600 mL/min V˙ A = 8400 mL/min Thirdly, since alveolar ventilation and PaCO2 are inversely related a double in alveolar ventilation will by a reduction by half of PaCO2. new PaCO2 is equal to 20 mm Hg
FACO2 (subscript A)
Fractional concentration of CO2 in the alveoli
From where to where is considered anatomical dead space
From the mouth or nose to the terminal bronchioles
External respiration
Gas exchange that occurs between blood and alveolar air
Describe deadspace in respect to ventilation and perfusion
Ventilation without perfusion
VA (subscript A)
Volume of alveolar space
VDANAT (Subscript DANAT)
Volume of anatomical dead space
Dead space ventilation is synonymous to...
Wasted ventilation
Why is the PO2 of tracheal gas less than the PO2 of inspired atmospheric gas?
Water vapor pressure (PH2O) accounts for more of the total pressure in tracheal gas compared with atmospheric gas; this leaves less total pressure available for the remaining atmospheric gases, and their pressures (including the PO2) must therefore decrease; that is, in the trachea PO2 = FIO2 (PB - PH2O) rather than PO2 = FIO2 (PB).
What is alveolar volume of a 150lb man with a tidal volume of 500mL?
We know the volume of each breath is divided , 1/3 to physiological dead space and 2/3 to alveolar volume so we can use 0.33 (1/3) as the VD/VT ratio for the following formula VA=VT(1-VD/VT) VA =500(1−0.33) VA =500(0.67) VA = 335 mL
Hyperventilation
When alveolar and blood PCO2 decrease
Hypoventilation
When alveolar and blood PCO2 increase
What effect does increasing the tidal volume have on VDanat and VA?
dead space volume (VD) in a single breath is fixed, determined by the dimensions of the conducting airways. Therefore, increasing the tidal volume increases the single-breath alveolar volume (VA) but not the single-breath VD.
What are the compartments respiratory systems?
i: ambient environment=FIO2. Think about the fact not all people are breathing regular atmospheric air ii: conducting airways iii: alveoli iv: pulmonary capillaries v: RBC in 4 vi: HB, hemoglobin
Deep breaths do what to the VD/VT, physiological dead space over tidal volume? Why?
this decrease the ratio because VD does not change, but VT gets bigger, resulting in a lower number once divided. Alveolar ventilation increases because as tidal volume gets bigger, (dead space utilizes the same amount or air) so more air reaches the functional alveoli
VD/VT(physiological dead space over tidal volume) is equal to (2 equation...
(PaCO2-PECO2)/PaCO2 (partial pressure of carbon dioxide in the arterial blood minus the mean carbon dioxide pressure of mixed expired gas) divided by the partial pressure of carbon dioxide in the arterial blood
What is the Bohr dead space equation?
(VD/VT)=((PaCO2-PECO2)/PaCO2)
What pressure does oxygen in the lungs exert?
47 mmHg
Normal minute ventilation rate or volume
5-8 L/min
What percentage of minute ventilation is alveolar ventilation?
60%-70%
In what 5 scenarios does anatomical dead space change?
1. Endotracheal tube bypasses upper airway's deadspace 2. Surgery removes it 3.Deep inspiration increases it slightly 4.Drugs that relax smooth airways muscle increase it slightly 5. Diseases that cause hyperinflation increase it
1.What is minute alveolar ventilation if tidal volume is 8000mL, VDANAT is 150mL, respiratory rate of 16 ? 2.What is minute alveolar ventilation if tidal volume is 8000mL, VDANAT is 150mL, respiratory rate of 32 ? 3.What is minute alveolar ventilation if tidal volume is 8000mL, VDANAT is 150mL, respiratory rate of 8 ? What do these scenarios tells us about the patient if the CO2 is kept normal?
1. Minute ventilation of physiological dead space= VDANAT x f =16 x 150mL=2400mL Minute alveloar ventilation= Minute ventilation - Physiological dead space ventilation =8000mL-2400mL=5600, normal ventilation 2. Minute ventilation of physiological dead space= VDANAT x f =32 x 150mL=4800mL Minute alveloar ventilation= Minute ventilation - Physiological dead space ventilation =8000mL-4800mL=3200, respiratory distress and possible ventilatory failure 3. Minute ventilation of physiological dead space= VDANAT x f =16 x 150mL=1200mL Minute alveloar ventilation= Minute ventilation - Physiological dead space ventilation =8000mL-1200mL=6800, the most efficient ventilatory pattern in terms of the fraction of V˙ E received by alveoli.
Alveolar volume is equal to...(2 formulas)
1. Tidal volume minus physiological dead space volume 2. Tidal volume times (1-(physiological dead space volume/tidal volume))
Tidal volume is equal to...
1.Alveolar volume plus physiological dead space volume
VDANAT is equal to how much of the tidal volume?
1/3 of it
In normal adults, how is the anatomical dead space measured?
1mL/lb of ideal body weight
What percentage of minute ventilation is phsiological dead space venitlation?
30%-40%
Capnometer
A device used in the clinical setting to analyze exhaled CO2. These instruments respond instantaneously to PCO2 changes.
VA (V dot , subscript A)
Alveolar ventilation. the amount of gas entering or leaving the alveoli per minute
A 55-year-old woman is admitted to the hospital in obvious respiratory distress. Her increased rate and depth of breathing have increased her minute ventilation to 15 L per minute. Arterial blood gases show a normal PaCO2 of 40 mm Hg. It seems odd that this woman, with a minute ventilation this great, has a normal PaCO2 . What is the explanation for this?
A normal PaCO2 associated with high minute ventilation indicates that much of this patient's ventilation is not in contact with blood flow. Dead space is the term used to describe alveoli that have normal ventilation but no blood flow (perfusion) through their capillaries. Any factor that decreases perfusion increases alveolar dead space. Increased alveolar dead space decreases alveolar ventilation if minute ventilation stays the same. Pulmonary embolism (obstruction of pulmonary vessels by blood clots) and shock (decreased cardiac output and low perfusion) are conditions that cause increased alveolar dead space. V D/VT increases as a larger percentage of the VT becomes dead space. In conditions producing dead space, the body tries to maintain a normal PaCO2 by increasing the minute ventilation. This increased minute ventilation does not mean alveolar ventilation increases because much of the minute ventilation is directed toward dead space units. A physiological consequence of increased dead space is the increased work of breathing required to maintain a normal PaCO2 . Patients with obstructive lung disease have impaired ventilatory capacity and may be unable to accommodate the increased demand for minute ventilation created by conditions that produce dead space. Increased dead space may precipitate ventilatory failure in these patients.
The Fowler technique
A way to measure anatomical dead space. 1. A person exhales to residual volume 2. Inhales 100% oxygen 3. Exhales to residual volume 4. N2 analyzer measure N2% at the mouth 5. Beginning of expiration 0% N2 6. Alveolar gas moves through c. airways in conical fashion causing a mix of alveolar and dead space air. 7. therotical basis of air is conducting airways released first 0% N2, then alveolar released second (Sharp increase in N2%). The volume expired up to that theoretical point of that transition is the anatomical dead space volume.
You are called to assess a patient who seems to be hyperventilating. The patient's respiratory rate is 30 breaths per minute. You measure the patient's arterial blood gases and find the PaCO2 is normal. Is this patient hyperventilating?
Although a rapid respiratory rate is often associated with hyperventilation, an increased rate alone is not diagnostic of hyperventilation. The relationship between carbon dioxide production and carbon dioxide elimination determines the ventilatory status. The presence of hyperventilation can be verified only by measuring PaCO2 . In this situation, the normal PaCO2 value indicates a normal relationship between carbon dioxide production and alveolar ventilation. Although this patient is not clinically hyperventilating, the respiratory pattern is obviously abnormal. Breathing this rapidly without a decreased PaCO2 generally means the tidal volumes are extremely small. Such rapid, shallow breathing often signals increased ventilatory work and potential ventilatory failure.
VDA (subscript DA). What does it represent?
Alveolar dead space volume. Volume contained in nonperfused alveoli. it represents decreased surface area for gas exchange and an increase in total wasted ventilation. Any presence of this is abormal
PETCO2 (subscript ET) what is it equal to?
Alveolar gas composition it is equal to PACO2
Minute ventilation is equal to...
Alveolar ventilation plus anatomical dead space ventilation
(VA, V dot, Subscript A)
Alveolar ventilation. The part of the minute ventilation that ventilates the alveoli
If VT is 600 mL, f equals 10 breaths per minute, PaCO2 equals 40 mm Hg, and V˙CO2 equals 200 mL per minute, what is the V˙D?
Alveolar ventilation= (Carbon dioxide exhaled from the lungs in one minute x 0.863)/PaCO2 After calculating alveolar ventilation, subtract it from minute ventilation. Remember, minute physiological dead space ventilation equals minute ventilation minus alveolar ventilation. Calculate alveolar ventilation as follows: = (200 mL/min x 0.863)/40 mm Hg = 4.315 L/min, or 4315 mL/min Minute ventilation= f x VT = 10/min x 600 mL = 6000 mL/min Physiological dead space = 6000 mL/min - 4315 mL/min = 1685 mL/min
If dead space ventilation increases but V˙ E stays the same, how is V˙ A affected? How does this affect gas exchange between blood and air?
Alveolar ventilation=minute ventilation - physiological dead space ventilation ; thus, an increase in physiological dead space ventilation with a constant minute ventilation causes alveolar ventilation to decrease. Because only alveolar ventilation affects gas exchange, its decrease causes alveolar PCO2 to rise and alveolar PO2 to decrease.
It is assumed that all CO2 in mixed expired gas comes from where?
Alveoli
(VDANAT, V dot, Subscript DANAT)
Anatomical dead space ventilation. The part of the minute ventilation that ventilates the conducting airways,
A mechanical ventilator is delivering 10 breaths per minute to your patient. The VT is 700 mL, and the patient's estimated V Danat is 160 mL. Arterial blood gas results reveal a PaCO2 of 40 mm Hg. What happens to V˙ A and PaCO2 if the respiratory rate is increased to 16 breaths per minute but the V˙ E remains constant?
Initial V˙E and V˙A are calculated as follows: V˙ E =700 × 10 =7000 mL/min V˙A =(700−160) × 10 = 5400 mL/min With a V˙A of 5400 mL/min, PaCO2 is normal at 40 mm Hg. If V˙ E stays the same while you increase the respiratory rate to 16 breaths per minute, VT must decrease (i.e., the ventilatory pattern is changed to a rapid, shallow pattern). This is shown as follows: 7000 = VT × 16 VT = 7000 / 16 = 437.5 mL Because V Danat remains the same at 160 mL, V˙A must decrease. This is shown as follows: V˙A = (437.5−160) × 16 = 4440 mL/min After the change in respiratory rate, arterial blood gases reveal a PaCO2 increase from 40 mm Hg to 49 mm Hg (see Clinical Focus 4-4). CO2 production remained the same, and the reduced V˙ A resulted in a decreased removal of CO2 from the body, creating a state of hypoventilation.
If alveolar minute ventilation is increased by half PACO2/PaCO2...
Is reduced by half
Rapid breathing does what to VDANAT? Why?
It increases it because it simply increases the number of time VT goes through it
PECO2 is always less or more than PACO2? Why or why not?
It is always less because PECO2 represents CO2 in mixed exhaled air, VDANAT (0 mmHg of CO2 + 40 mm Hg, dilution) while PACO2 is CO2 in the pure alveolar air (40 mm Hg)
Endotracheal intubation does what to VDANAT? Why?
It reduces it because the tube removes the upper airways and decreases the diameter of the lower airways.
A high VD/VT siginifies what?
It signifies that much of the V˙ E is wasted in ventilating nonperfused alveoli, requiring high-energy expenditure to accomplish a relatively small amount of V˙ A.
PECO2 (subscript E line on top)
Mean carbon dioxide pressure of mixed expired gas (dead space plus alveolar)
(VE, V dot, subscript E)
Minute ventilation. The volume of air entering or leaving the lungs in each minute. tidal volume (VT, subscript T) x frequency (f) per minute.
Can hypo or hyperventilation be detemined just by alveolar ventilation? Why?
No, one must look at PaCO2 values to diagnose hypo or hyperventilation
The degree to which VDANAT dilutes CO2 in mixed exhaled gas is reflected by...
PACO2-PECO2
What are the values of CO2, O2, N2, and H20 of the dead space at the end of inspiration?
PCO2 =0 mm Hg PO2=140 mm Hg PN2=564 mm Hg PH2O= 47 mmHg
What are the values of CO2, O2, N2, and H20 of the dead space at the end of expiration?
PCO2=40 mm Hg PO2=100 mm Hg PN2=573 mm Hg PH2O=47 mm Hg
Hypercapnia
PaCO2 that is above normal
Hypocapnia
PaCO2 that is below normal
Describe shunting in respect to ventilation and perfusion
Perfusion without ventilation
VD (subscript D). What is it equal to?
Physiological dead space is equal to the sum of alveolar dead space plus anatomical dead space
Tachypnea
Rapid breathing
Capnography
Refers to the PCO2 changes of exhaled tidal volumes graphically displayed as a waveform (capnogram).
Refractory
Resistant to a process or stimulus
Hypopnea
Shallow breathing
Bradypnea
Slow breathing
VCO2 (V dot)
The amount of CO2 the lungs exhale every minute
Describe the relationship of Alveolar ventilation with PACO2/PaCO2
They are inversely proportional
How much of the inspiratory breath is reinspired exhaled air?
The first 1/3
How much of the inspiratory breath remains in the conducting airways?
The last 1/3
A patient's total V˙ E is 18 L per minute (normal 5 to 8 L per minute), but the PaCO2 is normal at 40 mm Hg. What is the physiological explanation for this situation?
The only component of minute ventilation that directly affects the PaCO2 is alveolar ventilation, because only alveolar ventilation is in contact with blood. Because the PaCO2 is normal, alveolar ventilation must be appropriate. However, this is not consistent with the greatly elevated minute ventilation of 18 L/min. This high minute ventilation coupled with a normal PaCO2 means that a large proportion of the minute ventilation must be ventilating alveoli that have lost their blood flow. This constitutes alveolar dead space, as might occur in pulmonary embolus or a severe drop in blood pressure (shock). Increased dead space explains why minute ventilation must increase so much to maintain a normal PaCO2. High alveoelar ventilation with a normal PaCO2 is the hallmark of increased dead space. If all the lung was working effectively you wouldn't have to breath so hard to get rid of CO2, but if a part shuts down, the good part would have to receive ventilation more often to get rid of the same amount of CO2 as it would normally.
Dalton's Law
The pressure of each gas that makes up air exerts a partial pressure proportional to its fractional concentration in air
Ventilation
The process of moving each breath, tidal volume (VT, capital T subscript), into and out of the lungs
Shallow, low tidal volume do what to the VD/VT, physiological dead space volume over the tidal volume? Why?What is the effect on alveolar ventilation?
This increase the ratio because VD does not change, but VT gets smaller resulting in higher number once divided. Alveolar ventilation decreases because as tidal volume gets smaller (dead space utilizes the same amount or air) so less air reaches the functional alveoli
PaCO2 -PECO2 is proportional to... What does it account for? Is this clinically significant? Why or why not?
VD (accounts for alveolar dead space). It is clinically significant because changes in blood flow commonly occur in critically ill patients
VD (V dot subscript D) is equal to...
VD(subscript D) times frequency
PACO2 -PECO2 is proportional to... What does it account for? Is this clinically significant? Why or why not?
VDANAT (accounts for only anatomical dead space does not account for alveolar dead space). it is not clinically significant because conducting airways are not subject to change
