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USMLE Step 1 > Physiology > Flashcards

Physiology Flashcards

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USMLE® Step 1 flashcards
1
Q

Factors affecting systolic pressure

A

Stroke volume (The main factor)
Ventricular contractility
In chronic conditions, a decrease in the compliance of the systemic arteries (age-related arteriosclerosis)

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2
Q

Factors affecting diastolic pressure

A 
Total peripheral resistance
Heart rate (directly related)
Stroke volume (not a major factor)
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3
Q

Factors affecting Pulse Pressure

A

Increase in stroke volume

Decrease in vessel compliance (systolic increases and diastolic decreases)

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4
Q

Mean Pressure Formula

A

Diastolic + 1/3 Pulse Pressure
Or
2/3 Diastolic + 1/3 Systolic

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5
Q

CO Formula

A

CO = MAP/TPR

Or

MAP = CO x TPR

But if venous or RAP is severely increased, it must be taken into account when estimating TPR

TPR = (MAP - RAP)/CO

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6
Q

O2 consumption (Fick Principle)

A

VO2 = CO x (CaO2 - CvO2)

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7
Q

O2 Delivery

A

O2 Delivery = Q x CaO2

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8
Q

Q (Flow) Formula

A

Q = Oxygen consumption/[O2]pv - [O2]pa

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9
Q

What does a fall in PvO2 or SvO2 mean?

A

Indicates the patient’s O2 consumption increased and/or there was a fall in Q (flow) or CaO2 or both

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10
Q

Intrinsic or auto regulation of blood flow

A

Metabolic mechanism

Tissue vasodilatory metabolite products such as Adenosine, CO2, H+ and K+

Myogenic mechanism

Increased perfusing pressure causes stretch of the arteriolar wall and the surrounding smooth muscle, causing contraction, arteriole radius decreases and Q doesn’t increase

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11
Q

Alveolar ventilation Formula

A

Va = (Vt - Vd)f

Where Vt (Tidal volume)
Vd (dead space)
f (Respiration rate)

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12
Q

Units of pressure (Lung mechanics)

A

1 cm H2O = 0.74 mmHg

1 mm Hg = 1.36 cm H2O

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13
Q

Lung Compliance Formula

A

Compliance = Change in lung volume (tidal volume) / Change in surrounding Pressure

Examen
Vt = 0.6 L
IPP before inspiration -5 cm H2O
IPP after inspiration -8 cm H2O

Compliance = 0.6/3 = 0.200 L/cm H2O

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14
Q

Partial Pressure of a Gas in ambient air

A

Pgas = Fgas x Patm

Pgas = partial Pressure of a gas 
Fgas = concentration of a gas
Patm = atmospheric pressure (760 mmHg)
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15
Q

Partial Pressure of a gas in inspired air

A

PIgas = Fgas (Patm - PH2O)

PIgas = partial pressure of inspired gas
Fgas = concentration of the gas
Patm = atmospheric pressure
PH2O = partial pressure of H2O vapor (47 mmHg)
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16
Q

Pulmonary capillary gases

A 
PAO2 = 100 mmHg
PACO2 = 40 mmHg
PaO2 = 95 mmHg
PaCO2 = 40 mmHg
PvO2 = 40 mmHg
PvCO2 = 47 mmHg
17
Q

Factor affecting alveolar PCO2

A

Alveolar ventilation (Inverse relationship) = If VA increases, PACO2 decreases and viceversa

Metabolic rate = Direct relationship

18
Q

Factors affecting Alveolar PO2

A

Patm = An increase, increases PAO2 and a decrease (high altitude) decreases PAO2

FiO2 = an increase increases PAO2 (normally 0.21)

PACO2 = An increase, decreases PAO2 and a decrease, increases PAO2

RQ (Respiratory exchange ratio) = normally 0.8

Formula
PAO2 = (Patm - 47)FiO2 - PACO2/RQ

19
Q

Rate of Gas difusión

A

Vgas = A/T x D x (P1-P2)

A = surface area for exchange (decrease in emphysema, increase in exercise)
T = Thickness of the membranes (increase in fibrosis and restrictive diseases) 
D = Diffusion constant = Solubility
P1-P2 = gradient across the membranes (e.g gradient for O2 = 100-40 = 60 mmHg)
20
Q

Hemoglobin O2 Content

A

Each gram of Hb combine with 1.34 mL of O2
For example, if the Hb is 15 g/100 mL (15g%), then the maximal amount of O2 per 100 mL (100% saturation) in combination with Hb is

1.34(Hb) = 1.34(15) = 20 mL O2/100 mL blood = 20 vol%

21
Q

Oxygen - Hb dissociation curves

Shifting the curve to the right

A

Increased CO2
Increased hydrogen ion (Acidosis)
Increased temperature
Increased 2-3 biphosphoglycerate

Reduced affinity of the Hb molecule for oxygen
Easier for tissues to extract oxygen
Strep part of curve, O2 content decreases
P50 increased (PO2 required for 50% saturation)

22
Q

Oxygen - Hb dissociation curves

Shift the curve to the left

A

Decrease temperature
Decrease PCO2
Decrease 2-3 biphosphoglycerate
Decrease hydrogen ion (Alkalosis)

More difficult for tissues to extract oxygen
Steep part of curve, O2 content increased
P50 decreases

23
Q

Carbon monoxide and Hemoglobin

A

CO ha a greater affinity for Hb than does oxygen (240 times greater)
O2-Hb dissociation curve is shifted to the left
HbO2 content is reduced

24
Q

Carbamino compounds

A

When Carbon Dioxide reacts with terminal amine groups of proteins (hemoglobin). About 5% of the total CO2 is carried as carbamino compounds

25
Q

Causes of Hypoxemia - Hypoventilation

A

No increase in the A-a oxygen gradient
Supplemental oxygen can relieve the hypoxemia
End tidal air still reflects the systemic arterial compartment
The problem is not within the lung itself
Equal decrease in PO2 in all 3 compartments (PAO2, pulmonary end capillary PO2 and PaO2)

26
Q

Causes of hypoxemia - Diffusion impairment

A

Increase in A-a oxygen gradient
Supplemental oxygen can relieve the hypoxemia
End tidal air does not reflect the arterial values
Decrease in DLCO
PAO2 is OK
Pulmonary end capillary Pressure < PAO2
PaO2< PAO2

27
Q

Causes of hypoxemia - Low VA/Q units

A

Increased A-a oxygen gradient because PAO2 is normal in areas that don’t have low VA/Q
Supplemental oxygen corrects the hypoxemia because regions still have some ventilation, just much lower than normal
End tidal air does not reflect the arterial values
Low VA/Q creates alveolar and end pulmonary capillary blood gases that are approaching venous gases
Examples (status astmaticus, cystic fibrosis, anaphylaxis, partial occlusion of an airway)

28
Q

Causes of hypoxemia - Intrapulmonary shunt (right to left)

A

Increase in A-a gradient
Supplemental oxygen ineffective at returning PaO2 to normal
End tidal air does not reflect the arterial values
Examples (at electric lung regions in pneumothorax and ARDS, complete occlusion of an airway, right to left shunts created by heart defects)

USMLE Step 1 (3 decks)

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