Chapter 11 Respiratory Failure
§1 Concept and Introduction
Respiration: a process concerned with gas
exchange between a body and its environment.
• External respiration
• Transport of gas in blood
• Internal respiration
External respiration
• Ventilation
• Gas diffusion
• Ventilation / Perfusion=4L/min ∕ 5L/min (VA/ Q = 0.8)
PaO2: 80~100mmHg〔(13.3–0.043×age±0.06)〕
×7.5
PaCO2: 40±0.5mmHg
1mmHg=0.133kPa
1kPa=7.5mmHg
Respiratory Failure & Respiratory
Insufficiency
• Respiratory Failure is a pathological process in which
the external respiration dysfunction leads to an
abnormal decrease of arterial partial pressure of oxygen
with or without retention of carbon dioxide. it is
generally defined as (a resting subject, breathing air at
sea level) a PaO2 of 60mmHg (8KPa) or less with or
without PaCO2 of 50mmHg ( 6.7KPa) or more, or, the
presence of hypoxia with or without hypercapnia.
If FiO2 at sea level is not 20%:
Respiratory failure index(RFI)=PaO2/FiO2≤300
• Respiratory insufficiency
§2 Classification of RF
RF can classified in three ways according to its
duration ,primary site, and blood gas
1. According to duration
(1)Acute RF —developed over minutes to days,
It develops too rapidly to allow prompt, complete
compensation and results in acid- base imbalance
along with hypoxemia.
(2)Chronic FR —developed over months to years,
allowing compensatory mechanisms to improve
oxygen transport and to buffer respiratory acidosis.
.
2.According to primary site
(1) Central RF — caused by disorder of CNS
(2)Peripheral RF — caused by disease of respiratory organs or
cavity.
3.According to blood gas
(1) Hypoxemic RF (GroupⅠ) — characterized by ↓
PaO2 without ↑ PaCO2 (N /↓),often comes from gasexchanging dysfunction
(2) Hypercapnic RF (GroupⅡ) — characterized by
both↓ PaO2 and ↑ PaCO2 ,often comes from the
alveolar ventilation disorder .
§3 Causes & Pathogenesis
• The process of respiration requires the integrated links
of (1)the central control mechanisms and the peripheral
nervous system,(2)the thoracic cage and the respiratory
muscles.(3)the upper airways and lower airways.(4)the
pulmonary gas-exchange units and(5) the pulmonary
circulation. Disruptions of any of these links may be
Causes of respiratory failure.
• The respiratory functions include (1) ventilation ,
(2)gas diffusion,(3)ventilation/perfusion,
Disruptions of any of these functions may be
Pathogenesis of respiratory failure.
Pathogenesis of RF
1.Ventilatory Disorder
(1)Restrictive ventilatory disorder
•
Disorder of activity of respiratory muscles:
– Damage or depression of CNS: Disease of the brain stem ,/ Some drug overdose
– Neuro-muscular disorders: Injury to the spinal cord ,/ Poliomyelitis ,/ Myasthenia gravis
•
Decreased in compliance of chest wall and lungs
compliance≈ distensibility C=dV/dP
Volume ↓, Stiffness ↑ C ↓
Causes: Scoliosis ,/ Fibrothorax ,/ Pneumonectomy , / Pulmonary edema ,/ Surfactant ↓
•
Pneumothorax and Hydrothorax
(2)Obstructive ventilatory disorder
• Central airway obstruction
Diameter>2mm,
– Fixed or variable narrowing or stenosis : Dense scars ,/ Foreign body aspiration ,/
Sputum /external compression.
Variable extrathoracic obstructionInspiratory dyspnea
Variable intrathoracic obstrucdtionExpiratory dyspnia
• Peripheral airway obstruction
Diameter<2mm,
Intrinsic caliber ↓--Chronic bronchitis, / Bronchospasm,
/ Emphysema ( Chronic obstructive pulmonary disease ;COPD).
Equal pressure point shifts up and Dynamic compression
--- Expiratory dyspnea, Small airway closure
• Change of the blood gas in ventilatory disorders
1. PaO2 ↓ + PaCO2↑,(GroupⅡ)
∆PaCO2↑/∆PaO2↓ ≈ respiratory quotient (R) ≈0.8
2. P(A-a)O2=normal (2~5mmHg)
PaCO2=PACO2=0.863 ×VCO2 ∕ VA
VA=4L/min PaCO2=40mmHg
VA=2L/min PaCO2=80mmHg
PaO2≈PAO2=PiO2 - PACO2 / R
VA=4L/min PaO2=150-40 / 0.8=100mmHg
VA=2L/min PaO2=150-80 / 0.8=50mmHg
∆PaCO2↑ ∕ ∆PaO2↓=40 ∕ 50=0.8
2. Diffusion impairment
normal
disorder
• Total alveolar surface
80m2(72)
< 40m2(35)
• Alveolar-capillary membrane
<1μm
↑
thickness
or <5μm
• Diffusing capacity of gas
O2: 0.25s
↓
and Time of blood flow(0.75s) CO2: 0.07s
Diffusing capacity of CO2>Diffusing capacity of O2
(20 times)
Change of blood gas : PaO2 ↓, PaCO2 N ∕ ↓.
(GroupⅠ)
P(A-a)O2>2~5mmHg
3.Ventilation-Perfusion mismatching
VA
Q
VA ∕ Q
apex of lung
base of lung
total lung VA ∕ Q ≈ 0.8  P(A-a)O2=2~5mmHg
3
0.6
(1) VA↓ ∕ Q(N) ﹤ 0.8(0.1), ,Functional shunt , Venous
admixture
Causes: Bronchial asthma,/Chronic bronchitis,/
Obstructive aero- emphysema,/Pulmonary edema,
/Fibroid lung.
(2) VA(N) ∕ Q↓﹥0.8(10),Dead space like ventilation
Causes: Pulmonary artery hypotension,/
Pulmonary artery embolism,/ DIC in lung.
VD ∕ VT=30%60~70%
(3) VA≈0 ∕ Q(N), Anatomical shunt ,True shunt
Causes: Pulmonary artery-vein fistula,/
Pulmonary collapse,/Pulmonary consolidation
Alteration of blood gas:
PaO2↓, PaCO2 N/↓
(GroupⅠ)
[ When total ventilation ↓
PaO2↓+PaCO2↑
(GroupⅡ) ]
Mechanism :
• Intact lung
hyperventilation
• Characteristic of the CO2
dissociation curve ---- line
and the O2 dissociation
curve----- S-shaped
Acute respiratory distress syndrome
(ARDS)
• Concept:
Many etiological factorssevere lung injury (alveolarcapillary membrane ) acute respiratory failure
( groupⅠ)
§3 Alteration of function and metabolism
1.Disorders of acid-base and electrolyte
(1) PaCO2↑Respiratory acidosis
Hyperkalemia, Hypochloremia
(2)PaO2↓ Metabolic acidosis
Hyperkalemia, Hyperchloremia
(3)PaCO2↓Respiratory alkalosis
Hypokalemia, Hyperchloremia
The mixed acid-base disturbances are usually present
in the patient with RF.
2. Alteratino of respiratory system
PaO2↓
hypotonic hypoxia
carotid body
-
+
respiratory centrer
respiratory centrer
(Peripheral chemoreceptor)
PaO2
centrer (-)
centrer(+)
30
PaCO2
60
centrer(+)
50
VA=2L/min
PaO2=50
centrer(-)
80
PaCO2=80
Controlled oxygen therapy
• GroupⅠ oxygen concentration<50%
• GroupⅡ oxygen concentration<30% ,1~2L/min
PaO2: 50~60mmHg
Caused diseases Hypopnea,/Periodic respiration,/
Sighing respiration/Tidal respiration,/ Inspiratory dyspnea ,/
Expiratory dyspnea.
3.Alteration of cardiovascular system
mild
PaO2↓
heart rate↑,CO↑,Bp↑,redistribution of BF
PaCO2↑
severe
heart rate ↓,CO↓,Bp↓,cardiac arrhythmia
pulmonary arteriolar constrictionpulmonary hypertentionRHF
peripheral vasodilationwarm ,sweaty, flushed skin
4.Alteration of CNSPulmonary encephalopathy
PaO2↓hypoxiaATP↓Na-K pump dysfunction
acidosis of cerebral cells
PaCO2↑cerebral vasodilation
intracranial hypertention
cerebral edema
disturbances of cerebral cell
metabolism
5. Altelation of renal function
Acute renal dysfunction / failure
6.Altelation of GI
Dysfunction of GI
§4 Principles of Treatment
1. Treating the causes of respiratory failure
2. Increasing PaO2
3. Decreasing PaCO2
(1) Elimination of airway obstruction
(2) Strengthening of respiratory drive
(3) Mechanical ventilation
4. Treating the consequences of hypoxia and hypercapnia
for example, the acidosis, heart failure etc.