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 Introduction
Acute respiratory failure (ARF) is a major cause
of morbidity and mortality in emergency
and intensive care units. Respiratory
failure implies that the patient is not
able to maintain adequate oxygenation and
ventilation either because of impairment
of gas exchange or inability to perform
the required work of breathing. It is
characterized by abnormalities of arterial
blood gas tensions (ABG). Arterial blood
gas criteria for ARF are not absolute but
may be arbitrarily established as acute
hypercapnoea (PaCo2 > 50) or acute
hypoxemia (SaO2 <90% or PaO2 <60)
while breathing an FiO2 of 0.5 (30)
The name encompasses the end stage
of a group of diverse lung pathologies :
acute, chronic and acute on chronic. It is
classified as hypoxemic respiratory
failure (Type 1), when the primary
abnormality is decreased oxygen tension
and hypercapnoeic respiratory failure
(Type 2), when an elevated PaCO2 occurs in
addition to hypoxemia. Pa CO2 level in
Type 1 is normal or low (15).
(Table 1)
Clinical Assessment
As hypoxemia and hypercapnoea are the major
components of ARF, their clinical signs
are important and should be recognized.
Hypoxemia presents as confusion, restlessness,
impaired coordination and may culminate in
coma and death. It is usually associated
with tachycardia (bradycardia often in
children), hypertension and tachypnoea.
With progressive hypoxemia, bradycardia,
hypotension, cardiac arrhythmias,
vasoconstriction and cyanosis ensue.
Cyanosis is not apparent in severely
anemic patients.
Clinical features of hypercapnoea are dependent
on its duration. It presents as
apprehension, confusion, sedation and
coma. Other features are asterixis,
tachycardia and hypertension. These
features may become mild or non apparent
over several days to weeks due to renal
compensation
Severity of respiratory distress can be assessed
clinically on the basis of features
such as respiratory rate, mental status,
increased work of breathing (Table 2).
Knowledge of ABG is useful in diagnosis,
choosing the appropriate mode of
management and monitoring progress of ARF.
There should be a low threshold for
measuring
ABGÕs in patients with
neuromuscular diseases, chest wall
deformity, obesity or acute confusional
states who may be in respiratory failure
without significant breathlessness(15).
Emergency Management
Hypoxemia kills patients and is the primary
cause of death in respiratory failure.
Therefore, the first objective of the
management of respiratory failure is to
reverse and prevent hypoxemia. Secondary
objectives are control of PaCO2 and
respiratory acidosis and appropriate
management of underlying disease (Table 3)
While the initial stabilization of the patient
is progressing, an attempt should be made
to define the existence of ARF on the
basis of clinical assessment, oxymetry and
ABG results. To treat the specific patient
and the underlying problem and not just
the blood gas is a useful rule to
remember.
An array of O2 delivery devices is available to
suit the FiO2 requirements of the specific
patient being
dealt with (Table 4)
TABLE 4: OXYGEN DELIVERY DEVICES
Reversible conditions such as loss of patency of
upper airway (using head tilt-jaw lift),
tension pneumothorax, bronchospasm and
lung infection should be addressed with
specific therapy.
Positive
Pressure Mechanical Ventilation (PPV)
ARF arises when a patient is unable to
adequately oxygenate or ventilate himself.
At times, patients present with pure
hypoxemic respiratory failure and require
ventilatory support primarily to optimize
gas exchange. More commonly, patients
require assisted ventilation because of
either primary or secondary ventilatory
failure represented by an elevation in
PaCO2, low PaO2 and a decrease in arterial
blood pH(12).
The aim of PPV is to provide respiratory support
while definitive therapy for underlying
causes is undertaken. PPV can be provided
by non-invasive or invasive means.
Role of non-invasive
ventilation (NPPV)
There is increasing evidence in medical
literature for the beneficial role of CPAP
and BiPAP in ARF of multiple etiologies
especially in the COPD subgroup. List of
disease states where NPPV has a beneficial
role include COPD, asthma, cardiogenic
pulmonary edema, pneumonia, upper airway
obstruction, obesity hypoventilation
syndrome, immune compromised hosts and
neuromuscular disease (16-22).
However, data regarding its use has
been conflicting in acute pulmonary injury
and ARDS (23, 24).
Rationale for use of NPPV is the reduction of
potential complications associated with ET
intubation such as airway injury and
nosocomial infection. Decreased length of
ICU stays, preservation of speech and
swallowing, and increased patient comfort
are other advantages.
In a step
care approach to the management of ARF;
NPPV can have a role in step up care of
patients failing standard medical therapy.
It should not be used as a substitute for
tracheal intubation and invasive
ventilation when the latter is clearly
more appropriate. Adhering to the CPAP and
BiPAP inclusion criteria (Table 5**) may
help to minimize failure to improve gas
exchange and complications such as
aspiration pneumonia and hypotension.
Exclusion criteria should be looked at
before NPPV is instituted. (Table 6)
TABLE 5: NPPV INCLUSION
CRITERIA
TABLE 6: NPPV
EXCLUSION CRITERIA
While being treated with NPPV, patients should be closely monitored for
clinical deterioration, maintaining
readiness to resort to IPPV if the former
fails.
Role of
Invasive Positive Pressure Ventilation
(IPPV)
Invasive ventilation helps to maintain protection and some degree of
patency of airway in addition to assisting
with oxygenation and ventilation. This is
the only option available for patients who
donÕt respond to, or have
contraindication for, NPPV. Endotracheal
intubation is the commonest form of IPPV
practiced in the emergency
department.
Precise criteria for initiation of IPPV applicable to a broad spectrum of
patients are not available except for the
obvious indication of apnea. (Table +
hemodynamic instability, lack of response
to NPPV) Direct patient assessment, pulse
oximetry, end tidal CO2 (if available) and
spirometry (if patients able to perform)
usually provide adequate support for
decision to intubate(13). Recently there
has been recommendations that decision
making for endotracheal intubation should
include consideration of the patientÕs
underlying disease process and
cardiopulmonary reserve and should not be
restricted to ABG criteria or pulmonary
mechanical parameters(7)
IPPV related problems like infection, barotraumas, and ventilator
associated lung injury (VALI) resulting
from parenchymal damage, and diminished
cardiac output and muscle atrophy should
be borne in mind. Very high tidal volumes
should be avoided in the initial
ventilatory settings to avoid barotraumas
and VALI.
Conclusions
The above approach aims to simplify the assessment, improve management
and reduce the risk of overlooking
problems related to acute respiratory
failure.
In the absence of exclusion criteria, NIPPV should be instituted
concurrently with or following medical
therapy.
References
Other
Topic:
Review Article # 2
- Mobile
Phones and Cardiac Pacemakers
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