Lecture 18 - Respiratory Acid-base Disturbances

Question 1

Importance of H+ concentrations

Answer 1

• kept within narrowband
• normal: 40 nmol/L
• range: 35.5 - 44/7
• pH: 7.35 - 7.45
• normal pH:7.4
• pH is alinear: compresses higher concentration

Question 2

Hendesron-hasselbach equation

Answer 2

• HCO3- is the major buffer system in blood and extracellular fluid
• pH = pK + log10 (HCO3/H2CO3)
• H2CO3 is dissolved CO2
• H2CO3 = 0.03 x PCO2 (so the concentration of the buffer is directly proportional to the partial pressure of CO2, which is controlled by the lungs)
• HCO3 is controlled by kidney, while H2CO3 is controlled by lungs)
• pK = 6.1
• at normal pH, HCO3- is 20times H2CO3

Question 3

Normal blood gas values

• pH
• PO2
• O2 Sats
• PCO2
• HCO3-
• BE

Answer 3

• pH: 7.40 (7.35-7.45)
• pO2: 80-100 mmHg
• O2 Sats (95% +)
• PCO2: 35-45 mmHg (37-42)
• HCO3-: 22-26
• BE: +2 to -2 mmol/L

Question 4

Blood gas electrodes

Answer 4

• separate electrodes measure PO2, PCO2 and pH
• HCO3- and BE are derived
• oxygen sat is measured
• electrode jacketed to 37degrees
• partial pressure of a gas in a liquid id the pressure that would exist in a gas phase in equilibrium with the liquid

Question 5

BAse excess

Answer 5

• need steady state for interpretation
• base excess is a calculation of base required to bring the measured pH back to 7.4 AFTER CORRECTING THE PCO2 to 40 mmHg
• negative base excess means there is excess acid in the blood sample after the above correction
• BE is a measure of the metabolic component

Question 6

Arterial blood gas

Answer 6

• crucial for the management of all acute respiratory disorder and many chronic ones
• FiO2 = 150 mmHg
• importance of steady state for interpretation

Question 7

Scheme for interpreting ABG

Answer 7

1) is pH acid or alkaline (7.4)
2) check if disturbance is primary respiratory or acidotic
- if PaCO2 high: respiratory acidosis
- if BE negative: metabolic acidosis
- if BE positiive: metabolic alkalosis
- if PaCO2 low: respiratory alkalosis
3) check if there is compensation: partial or complete (if pH is in normal range)
4) check if hypoxemia is present (PaO2

Question 8

A-a grad

Answer 8

• check is PaO2 matches paCO2
• PaO2 = 150 - (PaCO2 x 1.25)
• interpretation requires a steady state
• raised PaCO2 defines hypoventilation
• if not equal - V/Q mismatch - lung disease
• A-a grad should not be more than 15

Question 9

PaO2 = 44mmHg
Pa CO2 = 67 mmHg
PH = 7.35
BE = + 10

Answer 9

• Primary respiratory acidosis (because both pH and CO2 are in the acid direction)
• complete metabolic compensation
• A-a gradient: 22(wide)
• VQ mismatch Lung disease present
• possible chronic obstructive lung disease with compensated resp acidosis

Question 10

PaO2 = 82 mmHg
PaCO2 = 19 mmHg
PH = 7.38
BE = -13 mmol/L

Answer 10

• metabolic acidosis with respiratory compensation
• Aa gradient = 44 (wide)
• VQ mismatch + lung disease
• sepsis and lactic acidosis from pneumonia
• diabetic acidosis

Question 11

PaO2 = 56 mmHg
PaCO2 = 44 mmHg
PH = 7.50
BE = + 10 mmol/L

Answer 11

• uncompensated metabolic alkalosis
• A-a gradient = 39 (wide)
• VQ mismatch + Lung disease
• corticosteroid therapy, Diuretic therapy, vomiting…
• metabolic alkalosis is rarely compensated because brain also responds to raised paCO2 and high pH is usually well tolerated

Question 12

PaO2 = 117
PaCO2 = 21
PH = 7.55
BE = -4mmol/L

Answer 12

• Respiratory alkalosis
• uncompensated, small BE due to blood electrolyte shifts
• respiratory alkalosis does not usually go long enough for renal compensation (takes 24-48 hr)
• Aa gradient = 7 : normal - no lung disease
• hyperventilation related to anxiety

Question 13

PaO2 = 68 mmHg
PaCO2 = 41 mmHg
- pH = 7.49
BE = +7

Answer 13

• uncompensated metabolic alkalosis (again, brain cant compensate because responds to elevated CO2)
• Aa gradient = 30 (wide)
• lung disease

Question 14

PaO2 = 64 mmHg
PaCO2 = 63 mmHg
PH = 7.22
BE = -2

Answer 14

• uncompensated respiratory acidosis
• Aa gradient = 7
• no lung disease, acute hypoventilation most likely due to reduced consciousness or drugs.
• hypoventilation is acute and not chronic because renal system didnt have time to compensate

Question 15

PaO2 = 56 mmHg
PaCO2 = 59mmHg
PH = 7.05
BE = -13

Answer 15

• severe combined respiratory and metabolic acidosis
• A-a gradient = 20
• most likely shock with lung disease
• ruptured spleen, head and lung injuries from MVA

Question 16

PaO2 = 48 mmHg
PaCO2 = 58mmHg
pH = 7.42
BE = +12 mmol/L

Answer 16

• the sheme would suggest metabolic alkalosis with respiratory compensation
• but remember metabolic alkalosis is not usually compensated and base excess is a calculation after correcting the PCO2, which it is not
• actually is compensated respiratory acidosis with metabolic compensation but with acute hyperventilation bringing down the PaCO2
• wide AA gradient

Question 17

Important messages

Answer 17

• pH of blood and ECF kept in tight boungs
• lungs are rapid adjusters of pH while kidneys take 24-48hours to compensate
• importance of steady state to interpret blood gases (A-a gradient(
• ## respiratory and metabolic acidoses tend to be compensated while alkaloses tend not to because either kidneys are too slow to correct resp alkalosis or the central chemoreceptors prevent pCO2 from increasing in correcting metabolic alkalosis