1. Chapter 11 Respiratory Failure

2. §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

3. 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

4. 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 .

5. §2 Classification of RF
RF can classified in three ways according to its duration ,primary site, and blood gas
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. .

6. 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 gas-exchanging dysfunction
(2) Hypercapnic RF (GroupⅡ) — characterized by both↓ PaO2 and ↑ PaCO2 ,often comes from the alveolar ventilation disorder .

7. §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.

8. Causes of Respiratory Failure
Category of impairment
Examples of cause
Impaired Ventilation
1. Damage or depression of CNS
2. Neuro-muscular disorders
3.Disease of the chest and pleura
4.Diseases causing airway obstruction
(1) Obstruction of upper airway
Some drug overdose
Disease of the brain stem
Injury to the spinal cord
Poliomyelitis
Muscular dystrophy, Myasthenia gravis
Rib fracture
Pneumothorax
Massive pleural effusion
Laryngospasm or laryngo edema
Foreign body aspiration
Tumor of the upper airways

9. Causes of Respiratory Failure
Category of impairment
Examples of cause
(2)Obstruction of lower airway
Impaired Diffusion
1.Pulmonary edema
2.Respiratory distress syndrome
(RDS)
Chronic obstructive lung disease(COLD)
Severe pneumonia
Pulmonary edema
Atelectasis
Left heart failure
Inhalation of toxic gas
Shock
Trauma

10. Pathogenesis of RF 1.Ventilatory Disorder Central airway obstruction
(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.

11. Variable extrathoracic obstructionInspiratory dyspnea
Variable intrathoracic obstrucdtionExpiratory dyspnia

12. 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

14. 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= / 0.8=100mmHg
VA=2L/min PaO2= / 0.8=50mmHg
∆PaCO2↑ ∕ ∆PaO2↓=40 ∕ 50=0.8

16. 2. Diffusion impairment
normal disorder
Total alveolar surface m2(72) < 40m2(35)
Alveolar-capillary membrane <1μm ↑
thickness or <5μm
Diffusing capacity of gas O2: s ↓
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

17. 3.Ventilation-Perfusion mismatching
VA Q VA ∕ Q
apex of lung
base of lung 3
0.6
total lung VA ∕ Q ≈ 0.8  P(A-a)O2=2~5mmHg

18. (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

20. 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

23. Acute respiratory distress syndrome (ARDS)
Concept:
Many etiological factorssevere lung injury (alveolar-
capillary membrane ) acute respiratory failure
( groupⅠ)

24. §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.

25. - 2. Alteratino of respiratory system
PaO2↓ hypotonic hypoxia respiratory centrer
carotid body respiratory centrer
(Peripheral chemoreceptor)
PaO centrer (-) centrer(+)
PaCO centrer(+) centrer(-) - + 30 60 50 80
VA=2L/min PaO2= PaCO2=80

26. 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.

27. 3.Alteration of cardiovascular system
PaO2↓ heart rate↑,CO↑,Bp↑,redistribution of BF
PaCO2↑ heart rate ↓,CO↓,Bp↓,cardiac arrhythmia
pulmonary arteriolar constrictionpulmonary hypertentionRHF
peripheral vasodilationwarm ,sweaty, flushed skin
mild
severe

28. 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

29. 5. Altelation of renal function
Acute renal dysfunction / failure
6.Altelation of GI
Dysfunction of GI

30. §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.