7 - Ventilatory Tests & ABGs

Henderson-Hasselbalch equation

pH = pK + log(base ÷ acid)

pK = 6.10

base = [HCO₃^-]

acid = [CO₂] × 0.03

normal range for pH

7.35-7.45

0.3 change in pH = __

2-fold change in [H⁺]

normal range for P_aO₂

80-100 mmHg

causes of decreased P_aO₂

• V/Q mismatch

• shunt

• alveolar hypoventilation

• diffusion defect

normal range for P_aCO₂

35-45 mmHg

base excess

difference between actual and expected buffer capacity

temperature correction

ABGs reported at 37°C

fever

• ↓PO₂ & ↓PCO₂

• ↑pH

hypothermic

• ↑PO₂ & ↑PCO₂

• ↓pH

normal range for P_vO₂

37-43 mmHg

normal range for P_cO₂

60-80 mmHg

average respiratory rate (f) for adults

10-20 b/min

average minute ventilation (V_E) for adults

5-10 L/min

average tidal volume (V_T) for adults

400-700 mL

cause of decreased V_T

pulmonary disease, esp. restrictive

cause of low V_T and f

alveolar hypoventilation

dead space

ventilation without perfusion

formula for alveolar ventilation (V_A)

V_A = V_E − V_D

V_A = alveolar ventilation

V_E = minute ventilation

V_D = dead space ventilation

formula for physiological dead space (V_D)

V_D = V_Danat + V_DA

V_D = physiological dead space

V_Danat = anatomical dead space

V_DA = alveolar dead space

formula for dead space-tidal volume ratio (V_D/V_T)

V_D / V_T = (P_aCO₂ − P_ECO₂) ÷ P_aCO₂

P_aCO₂ = pressure of arterial CO₂

P_ECO₂ = pressure of expired CO₂

normal V_Danat

1 mL/lb IBW

normal range for V_D/V_T

0.2-0.4 (20-40%)

causes of increased V_D/V_T

• lung disease

• airway obstruction

• pulmonary embolism

• pulmonary hypertension (PHTN)

• low cardiac output (PPV)

• body position

• tubing (circuit)

causes of decreased V_D/V_T

• exercise

• high-flow nasal O₂ therapy

• PEEP

• reduced tubing

formula for C_aO₂

C_aO₂ = (1.34 × Hb × S_aO₂) + (P_aO₂ × 0.003)

S_aO₂ = O₂ saturation in arterial blood

average C_aO₂

20 mL/dL

formula for C_vO₂

C_vO₂ = (1.34 × Hb × S_vO₂) + (P_vO₂ × 0.003)

S_vO₂ = O₂ saturation in mixed venous blood

P_vO₂ = pressure of O₂ in mixed venous blood

average C_vO₂

15 mL/dL

(can draw from PAC [pulmonary artery catheter] line)

normal C_(a-v)O₂

5 mL/dL

(inverse of cardiac output)

shunt

perfusion without ventilation

• can be pulmonary or cardiac

formula for shunt (Q_S/Q_T)

Q_S/Q_T = (C_cO₂ − C_aO₂) ÷ (C_cO₂ − C_vO₂)

[C_cO₂ − C_aO₂] = shunted blood

[C_cO₂ − C_vO₂] = total perfusion

formula for C_cO₂

C_cO₂ = (1.34 × Hb) + (P_AO₂ × 0.003)

P_AO₂ = pressure of alveolar O₂

normal shunt

~5%

(30%: large)

causes of shunt

pulmonary

• acutely reduced ventilation

vascular

• portal HTN

• cardiac defects

other methods for estimating shunt

• P_(A-a)O₂

• P_aO₂/P_AO₂ (a/A)

• P_aO₂/F_iO₂ (P/F)

formula for P_AO₂

P_AO₂ = [F_iO₂ × (P_B − PH₂O)] − (P_aCO₂ × 1.25)

F_iO₂ = fraction of inspired O₂

P_B = barometric pressure (normal: 760 mmHg)

PH₂O = pressure of water (47 mmHg)

normal P_AO₂

~100 mmHg

(inverse of P_aCO₂)

normal alveolar-arterial O₂ tension difference (P_(A-a)O₂)

5-10 mmHg (<20 mmHg)

causes of increased P_(A-a)O₂

• poor alveolar gas exchange

  • edema

  • inflammation

  • fibrosis

  • diffusion defects

• V/Q mismatch

  • bronchoconstriction

  • structural issues in airways

  • hypoventilation

• changes in F_iO₂

ranges for P_aO₂/P_AO₂ (a/A ratio)

normal: 0.8-0.9

• <0.6 = shunt, V/Q, diffusion defect

• <0.35 = weaning failure

• <0.15 = refractory hypoxemia

ranges for P_aO₂/F_iO₂ (P/F ratio)

normal: ~450

• 300-200 = mild ARDS

• 200-100 = moderate ARDS

• <100 = severe ARDS

values for oxyhemoglobin dissociation curve

SaO2 (%)	PaO2 (mmHg)
90	60**
75	40
60	30
50	27

** = O₂ not added until <60 mmHg (moderate hypoxemia)

causes of hypoxemia

• V/Q mismatch (shunt/dead space)

• hypoventilation

• diffusion defect

• decreased P_iO₂ (pressure of inspired O₂)

types of hypoxia

• hypoxemic

• stagnant/circulatory

  • HF, blood/fluid loss

• anemic

  • reduced Hb, metHb, COHb

• histotoxic

  • cyanide/alcohol poisoning

signs and symptoms of hypoxemia

• increased ventilation (↑RR and ↑HR)

• increased PVR (pulmonary vascular resistance)

• increased RBC production

• muscle fatigue

• glucose intolerance

signs and symptoms of hypoxia

• disruption of normal cell function

• formation of lactic acid

• metabolic acidosis

• slowing of CNS function

• end organ dysfunction (brain, heart, kidneys, liver)

ABG estimates of ventilator changes

volume needed = (current volume × P_aCO₂) ÷ desired P_aCO₂

F_iO₂ needed = (current F_iO₂ × desired P_aO₂) ÷ current P_aO₂

hemoximetry / CO-oximetry (CO-ox)

measurement of Hb & derivatives:

• oxyhemoglobin (O₂Hb)

• carboxyhemoglobin (COHb)

• methemoglobin (metHb)

• reduced hemoglobin (rHb)

uses different analyzer than that for ABG

oximetry (pulse-ox) (S_pO₂)

measurement of estimation of S_aO₂

• light absorption at 2 wavelengths (spectrophotometer)

• ±2% accuracy (when >90%)

• some can measure COHb and metHb (multi-wavelength)

• transcutaneous monitor (heats skin)

end-tidal capnography (ET_CO2)

measurement of expired CO₂ from gas sample

• techniques

  • infrared analyzer

  • mass spectrometer

  • colorimetric

pressure of end-tidal capnography (PET_CO2)

measurement of P_aCO₂ and changes in V_D

• P_aCO₂ − PET_CO2

pre-analytical errors

• air bubble in sample

• room air at sea level: P_aO₂ 150 mmHg, P_aCO₂ 0 mmHg

• lowers CO₂ and raises pH

• can increase/decrease O₂

pre-analytical errors

• inadvertent venous sample

picture doesn’t match patient

• arterial blood → bright red

• venous blood → dark red

pre-analytical errors

• excessive heparin

• __ decreases pH to 7.0

• dilutes sample

  • affects P_aO₂ and P_aCO₂

pre-analytical error

• metabolic effect

if sample is not iced within 15 minutes:

• high CO₂ → low pH

• low O₂

blood gas analyzers

• pH electrode

Sanz electrode

• measures voltage difference between reference and measuring electrode across pH-sensitive glass membrane

PCO₂ electrode

Severinghaus electrode

• adaptation of Sanz

• based on dissociation formula:

  • CO₂ + H₂O ↔ HCO₃^- + H⁺

• change in H is proportional to change in PCO₂

PO₂ electrode

Clark electrode

• polargraphic

• measures flow of electrons

• partial pressure of __

lab analyzers

combined gas analyzers

• micropressor calculations

  • HCO₃, total CO₂, base excess

• some have co-oximeter (hemoximeter)

point-of-care (POC) analyzers

analyzers that can perform tests at the bedside

• microelectrodes, etc.

• disposable cartridges

• example: I-Stat®

transcutaneous gas analyzers

non-invasive gas analyzer

• heats skin (40-45°C)

• uses Clarke (O₂) and Severinghaus (CO₂) electrodes

• limits

  • need for heated electrode

  • risk for burns; need to rotate site

pulmonary gas analyzers

• O₂

measurement of O₂ via electrochemistry

• 2-point calibration using 21% and 100%

• galvonic (measures pressure and O₂%)

• Clarke electrode

pulmonary gas analyzers

• helium (He)

measurement of helium via thermal conductivity

• Wheatstone Bridge (circuit that measures resistance)

• H₂O and CO₂ must be removed

• very stable; little moving parts

pulmonary gas analyzers

• infrared spectroscopy

measurement of CO and CO₂ via absorption of light

• affected by water vapor and N₂O

pulmonary gas analyzers

• mass spectrometry

measurement of N₂ and N₂O via magnetic field

• can be cumbersome and expensive

pulmonary gas analyzers

• emission spectrometry

measurement of N₂ using Geisler tube analyzer

pulmonary gas analyzers

• gas chromatography

separation of gas into its components via thermal conductivity

• used for D_LCO

pulmonary gas analyzers

• chemoluminescence

measurement of NO and N₂O via photoemission

• mixed with ozone

• NO₂ gives off light while degrading

pulmonary gas analyzers

• gas conditioning

removal of unwanted substances in gas

• CO₂ (via barium/sodium hydroxide)

• H₂O vapor (via anhydrous calcium sulfate or silica gel)

if not removed, can dilute gas analysis