Acute Respiratory Failure

Michael Donahoe MDDivision of Pulmonary, Allergy, and Critical Care Medicine

2015

Acute Respiratory Failure

A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual

Type Pa02 PaCO2 PA02–Pa 02

Hypoxemic (Type I) ⇓ ⇓ − NL ⇑

Hypercapnic (Type II) ⇓ ⇑ NLMixed

(Type III) ⇓ ⇑ ⇑

• Abnormal V/Q or shunt physiology

• Disorders of lung parenchyma

• Airway

• Alveoli / Interstitium

• Pulmonary Vascular

ARF - Hypoxemic

(A-a) GradientA comparison of the calculated alveolar oxygen partial pressure (Big A) to the arterial oxygen partial pressure (Little A)

(PAO2) = FIO2(PATM- PH2O) - PaCO2 / RQ

Normal Gradient Estimate = (Age/4) + 4

(PAO2) = 0.21 (760-47) - 40 / 0.8 = 100ROOM AIR

Oxygenation Measures

PARAMETER CALCULATION COMMENT

A-a gradient PAO2 - PaO2

Requires the use of the alveolar gas equation. The A-a gradient is unaffected by changes in the PaCO2. Normal value age dependent. Varies with inspired oxygen concentration.

a/A ratio PaO2 / PAO2 Requires calculation using the alveolar gas equation. Varies with FiO2. Minimal variability in ARDS with FIO2 > 0.50 and PaO2 < 100.

Respiratory Index (PAO2 - PaO2)/ PaO2 Requires calculation using the alveolar gas equation. Varies with FiO2.

P/F Ratio PaO2 / FIO2

No calculation from the alveolar gas equation required. Does not account for variations in the blood PaCO2. Varies with FiO2. Minimal variability in ARDS with FIO2 > 0.50 and PaO2 < 100

Venous Admixture Shunt EquationMeasure of the contribution of shunt, diffusion, and low V/Q regions to the measured PaO2. Varies with FIO2. Requires calculation from the alveolar gas equation and the measurement or assume mixed venous oxygen content.

Oxygenation Measures ICU Limitations

• Supplemental Oxygen

• Stability Over Range FiO2

Figure 1. Relation between PaO2/FIO2 and FIO2 for patients with <30% true shunt (top) and patients with >30% shunt (bottom). PaO2/FIO2 ratios, characterized by FIO2 values of >or=to0.5 and PaO2 values of <or=to100 torr (<or=to13.3 kPa), are shown by the solid lines. PaO2/FIO2 ratios outside these limits are indicated by the dotted lines.

Copyright © 2011 Critical Care Medicine. Published by Lippincott Williams & Wilkins. 2

Variability  of  indices  of  hypoxemia  in  adult  respiratory  distress  syndrome                              Gowda,  Madhu  S.;  Klocke,  Robert  A.  Cri:cal  Care  Medicine.  25(1):41-­‐45,  January  1997.  doi:  

Closed Circuit Flow Independent Mechanical Ventilation

100% Wall Oxygen (100 psi)Blender

Air (100 psi)

Patient

100% Wall Oxygen (Flow Fixed)

RA - entrainment (variable)

Low Flow

High Flow

Device Flow < Patient Flow

Open Circuit Patient Flow Dependent

Nasal Cannula

Patient

100% Wall Oxygen (Flow Fixed)

Low Flow

High Flow

Device Flow > Patient Flow

RA - entrainment (fixed)

Open Circuit Flow Dependent

Facemask / NIV

PatientDial O2 Flow Total Flow36 14 7440 15 6250 15 4160 15 3080 Flush 5495 Flush 43

100% Wall Oxygen (Flow Fixed)

Low Flow

High Flow

Device Flow < Patient Flow

RA - entrainment (variable)RA - entrainment (fixed)

Open Circuit Flow Dependent

Facemask / NIV

Patient

•FiO2 unknown

•FiO2 comparison difficult

•Flow variable > FiO2 variable

•Know your therapist

• Standard Error = + 4%

• Pigment Dependent

• Light 92%

• Dark 94%

• ? Bilirubin

• Perfusion Dependent

• False Errors

• Carboxyhemoglobin

• Methemoglobin

• V/Q mismatch - titrate O2 up

• chronic obstructive pulmonary disease

• asthma

• obesity / hypoventilation

• pulmonary embolism

• Shunt - titrate O2 down

• pulmonary edema

• atelectasis

• pneumonia

Hypoxemia

• Oxygen diffusion (PaO2)

•Lung (A-a)

•Tissues

• Oxygen transport (SaO2)

•Arterial oxygen content

•Venous oxygen content

PaO2 vs SaO2

FiO2 = 0.70 PEEP = 5.0 cm H2O PaO2 = 352 SaO2 = 95% PaO2/FiO2 = 502

FiO2 = 0.70PEEP = 5.0 cm H2OPaO2 = 82SaO2 = 95%PaO2/FiO2 = 117

Acute Respiratory Failure

A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual

Type Pa02 PaCO2 PA02–Pa 02

Hypoxemic (Type I) ⇓ ⇓ − NL ⇑

Hypercapnic (Type II) ⇓ ⇑ NLMixed

(Type III) ⇓ ⇑ ⇑

Minute Ventilation

VE = ------------------------------------VCO2

pCO2 ( 1 - VD / VT)

hypermetabolism

dyspneahypoxemia

anxiety

bronchospasmmucous plugging

V/Q maldistribution

Equation of Motion

Resistive pressure VT/Ti x resistance

Elastic pressureVT / compliance

Ventilating Pressure

Equation of Motion

Ventilating Pressure(driving force)

Elastic Pressure(volume)

+ Resistive Pressure(flow)

=

Pmus + Pvent(driving force)

Pelastic(volume/compliance) + Presistive

(flow x resistance)=

PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal

•Central Nervous System

•Neuromuscular Transmission

•Thorax and Pleural Space

Hypercapnic Respiratory Failure

Acute Respiratory Failure

A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual

Type Pa02 PaCO2 PA02–Pa 02

Hypercapnic (Type II) ⇓ ⇑ NL

Ventilating Pressure(driving force)

Elastic Pressure(volume) +

Resistive Pressure(flow)

VE = ------------------------------------VCO2

pCO2 ( 1 - VD / VT)

hypermetabolism

dyspneahypoxemia

anxiety

bronchospasmmucous plugging

V/Q maldistribution

METABOLIC

MECHANICAL

PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal

Acute Respiratory Failure

A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual

Type Pa02 PaCO2 PA02– Pa 02

Mixed (Type III) ⇓ ⇑ ⇑

Ventilating Pressure(driving force)

Elastic Pressure(volume) +

Resistive Pressure(flow)

VE = ------------------------------------VCO2

pCO2 ( 1 - VD / VT)

hypermetabolism

dyspneahypoxemia

anxiety

bronchospasmmucous plugging

V/Q maldistribution

METABOLIC

MECHANICAL

PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal

Mechanical Ventilation

• Non-invasive Mechanical Ventilation (NIV)

• Continuous Positive Airway Pressure (CPAP)

• with or without inspiratory support (BIPAP)

• Average Volume Assured Pressure Support (AVAPS)

• Helium-oxygen (Heliox) gas mixtures

• Invasive Mechanical Ventilation (IMV)

Non - Invasive MV

• Indications

• Chronic Obstructive Pulmonary Disease

• Congestive Heart Failure

• Contraindications

• Impending Cardiovascular Collapse

• Unstable upper airway (obstruction/secretions/protection)

• Recent upper airway or GI surgery

Non-invasive MV

Ram FSF et al. Cochran Database 2004

n=14✓Reduced intubation✓Reduced complications✓Reduced hospital LOS✓Reduced mortality

Masip et al. JAMA. 2005;294:3124

n=15✓Reduced intubation✓Reduced complications✓Reduced mortality

Mechanical Ventilation

IMV NIMV

Volume SET Variable

Flow SET Variable

Timing SET Variable

Pressure VARIABLE SET