Acute respiratory failure

10/2/2012 1

ACUTE RESPIRATORY FAILURE

Ihab B Abdalrahman, MBBS, MD, ABIM, SSBB Ihab Tarawa Consultant of Acute Care Medicine, Soba University Hospital

10/2/2012 Ihab Tarawa 2

BASIC RESPIRATORY PHYSIOLOGY

Respiratory system made ridiculously easy

It is as simple as moving the air in and out.

Allowing gas exchange.


O2 CO2


1,2,3

Bottom line of the respiratory system is to

Let oxygen in,

And carbon dioxide out.


Definitions

acute respiratory failure occurs when:

pulmonary system is no longer able to meet the metabolic demands of the body

hypoxaemic respiratory failure:

PaO2 8 kPa when breathing room air

hypercapnic respiratory failure:

PaCO2 6.7 kPa


Oxygen in Depends on Ventilation

PAO2

Perfusion

Ventilation-perfusion matching

Diffusing capacity


PAO2

The alveolar pressure is equal to the sum of the partial pressures of the gases within the alveolus.

The partial pressure of each gas is proportional to the concentration of the gas.


2A2A2A2A NPOHPCOPOPpressure Alveolar

Nitrogen

Water vapour

Carbon dioxide

Oxygen


Nitrogen

Water vapour

Carbon dioxide

Oxygen



Oxygen in

Depends on PAO2

FIO2

Alveolar pressure

PACO2

Ventilation


Perfusion

Diffusing capacity


Carbon dioxide out

Largely dependent on alveolar ventilation

Anatomical dead space constant but physiological dead space depends on ventilation-perfusion matching


)V-(V xRR nventilatio Alveolar DT

Carbon dioxide out

Respiratory rate

Tidal volume




PATHOPHYSIOLOGY

PAO2=14.74 kPa PACO2=5 kPa

75% 100%

Normal

ventilation &

perfusion


Pathophysiology

Low inspired Po2

Although, in theory, acute respiratory failure may result from a low inspired PO2 this is rarely a problem in Intensive Care except in locations at high altitude.


Pathophysiology

Low inspired oxygen concentration

Hypoventilation


Nitrogen

Water vapour

Carbon dioxide

Oxygen



PAO2=9.74 kPa PACO2=10 kPa

75% 92%

Hypoventilation


Brainstem

Spinal cord

Nerve root Airway

Nerve

Neuromuscular junction

Respiratory muscle

Lung

Pleura

Chest wall

Sites at which disease may cause hypoventilation 10/2/2012 Ihab Tarawa 22

Brainstem

Spinal cord

Nerve root Airway

Nerve

Neuromuscular junction

Respiratory muscle

Lung

Pleura

Chest wall

Sites at which disease may cause hypoventilation 10/2/2012 Ihab Tarawa 23

Pathophysiology


Hypoventilation

Shunting

Dead space ventilation

Diffusion abnormality


Shunt


75% 75%

100% 75%

87.5%

oxygen therapy

has relatively

little effect on

hypoxia due to

shunting.


75% 75%

100% 75%

90%

hypoxic vasoconstriction

↓perfusion to non-ventilated alveoli

↑perfusion to ventilated alveoli,

↓ magnitude of the shunt

↑and increasing the arterial saturation


Shunting

Intra-pulmonary Pneumonia

Pulmonary oedema

Atelectasis

Collapse

Pulmonary haemorrhage or contusion

Intra-cardiac Any cause of right to left shunt eg Fallot’s, Eisenmenger,

Pulmonary hypertension with patent foramen ovale


Pathophysiology


Hypoventilation

Shunting




Dead space ventilated but not perfused


Pathophysiology


Hypoventilation

Shunting




Diffusion abnormalities.

These can result from a failure of diffusion across the alveolar membrane

or a reduction in the number of alveoli resulting in a reduction in the alveolar surface area.

Causes include ARDS and fibrotic lung disease



RESPIRATORY MONITORING

Clinical

Respiratory compensation

Sympathetic stimulation

Tissue hypoxia

Haemoglobin desaturation


Bottom line of the respiratory system is to let oxygen in and CO2 out.

Some sensors will go off

Hypoxia

Acidosis


Clinical

Respiratory compensation Tachypnoea

Accessory muscles

Recession

Nasal flaring


Clinical



Clinical


HR

BP (early)

sweating


Point of making a difference

Coming for help

Or coming with a coffin


Clinical

Tissue hypoxia

Altered mental state

HR and BP (late)


Clinical

Haemoglobin desaturation

cyanosis


If we wait for the patient to become cyanosed


PaO2 (kPa)

Hb s

atu

rati

on (

%)

8

90

Pulse oximetry, notice the sigmoid curve


The oxygen content of blood is mainly dependent on:

the haemoglobin saturation,

with the a very small contribution from dissolved oxygen.


Oxygen delivery


222

2 2

PaO0.0031.37Hbsaturation O content O

10contentOoutput CardiacdeliveryO

Monitoring


123

80 40

87% HR=95


Sources of error

Poor peripheral perfusion

Poorly adherent/positioned probe

False nails or nail varnish

Lipaemia

Bright ambient light

Excessive motion

Carboxyhaemoglobin or methaemoglobin


Thing to remember

Saturation is not a measure of

ventilation


Summary

worry if RR > 30/min (or < 8/min) unable to speak 1/2 sentence without pausing agitated, confused or comatose cyanosed or SpO2 < 90% deteriorating despite therapy

remember normal SpO2 does not mean severe ventilatory problems are not

present



TREATMENT

Treatment

Treat the cause Supportive treatment

Oxygen therapy

CPAP

Mechanical ventilation


Oxygen therapy

Fixed performance devices

Variable performance devices


Nasal canula

Simpler mask

Other


Other devices

Reservoir face mask

Bag valve resuscitator


CPAP

reduces shunt by recruiting partially collapsed alveoli


Pressure

Volu

me

Lung compliance and FRC

reduces work of breathing


Mechanical ventilation

Decision to ventilate

Complex

Multifactorial

No simple rules


Ventilate?

Severity of respiratory failure


Ventilate?


Cardiopulmonary reserve


Ventilate?



Adequacy of compensation

Ventilatory requirement


Ventilate?



Adequacy of compensation

Ventilatory requirement

Expected speed of response

Underlying disease

Treatment already given


Ventilate?



Adequacy of compensation Ventilatory requirement

Expected speed of response Underlying disease


Risks of mechanical ventilation


Ventilate?



Adequacy of compensation Ventilatory requirement

Expected speed of response Underlying disease


Risks of mechanical ventilation

Non-respiratory indication for intubation


Ventilate?

43 year old male

Community acquired pneumonia

Day 1 of antibiotics

PaO2 8 kPa (60 mmHg), PaCO2 4 kPa (30 mmHg), pH 7.15 on 15 l/min O2 via reservoir facemask

Respiratory rate 35/min

Agitated


Yes

43 year old male





Agitated


Yes

43 year old male





Agitated


O2

O2

Yes

43 year old male





Agitated


Ventilate?

24 year old woman

Presents to A&E with acute asthma SOB for 2 days

Salbutamol inhaler, no steroids

PFR 60 L/min, HR 105/min

pH 7.25 PaCO2 6.8 kPa (51 mmHg), PaO2 42 kPa (315 mmHg) on FiO2 0.6

RR 35/min

Alert


No

24 year old woman





RR 35/min

Alert


Pathway

Airway patent

Secure airway

Patient breathing

Ventilat

Is he hypoxic


Hypoxic

Yes No

Acidosis

Shock

PE

Asthma

Pumonary edema

Anxiety


Hypoxic

Pco2

High /normal Low Pco2

pneumonia

ARDS

Pulmonary edema

Aspiration

PE

pneumothorax


Hypoxic

Pco2

High /normal

Normal A-a gradient

Breathing hard

Asthma

COPD

PE

Breathing normally

CND

Drugs

High A-a gradient

Fatigue from hypoxia

Acute on chronic

Low Pco2



QUESTIONS?

Acute respiratory failure

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