Page 411 - ACCCN's Critical Care Nursing

388 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

TABLE 15.1 Physiological indications suggesting the need for mechanical ventilation

Parameter Normal values ARF CRF Associated signs and symptoms
Respiratory 12–20 ≥28 ≥30 Dyspnoea, increased activation of accessory
rate muscles and active expiration
pH 7.35–7.45 <7.30 7.35–7.40 Failure to adequately ventilate:
No compensatory changes May be normal due to Elevated PaCO 2 , acidic pH, headache,
metabolic compensation confusion or other mental status change,
PaCO 2 35–45 mmHg >50 mmHg and rising >50 mmHg and rising tachypnoea (RR >30), flushed skin
80–100 mmHg <65 mmHg and falling <50 mmHg and falling Failure to adequately oxygenate:
PaO 2
Hb/HCT elevated as Decreased PaO 2 and SpO 2 , tachycardia,
compensatory mechanism hyper- or hypotension, dyspnoea, gasping,
nasal flaring, use of accessory muscles,
anxiety, agitation and altered mental
status, cyanosis
−
− 22–28 mmol/L Within normal limits If chronic hypercapnia, then HCO 3 >28 mmol/L is a compensatory mechanism
HCO 3
If CRF is primarily failure to oxygenate then HCO 3 will be within normal limits
−
ARF = acute respiratory failure; CRF = chronic respiratory failure.
MECHANICAL VENTILATION
TABLE 15.2 Equation of motion
As stated in the introduction, 41% of patients in Austra-
lian and New Zealand ICUs received invasive mechanical
ventilation and 8% received non-invasive ventilation Equation: P T (P airway + P muscle ) = V T /Cr + V T /T I × R + PEEP T
1
(NIV) in 2007–08. The median duration of invasive Abbreviations: P T = total pressure: the sum of the
mechanical ventilation for these patients was 2.5 days. pressure in the proximal airway and
the pressure generated by the
In the most recent international study of mechanical- respiratory muscles
ventilation practices, reporting data from 4968 patients V T = tidal volume
in 349 ICUs and 23 countries found the median duration Cr = compliance
of ventilation to be 4 days (interquartile range 2–8 T I = inspiratory time
65
days). In this patient cohort the three most common R = resistance
PEEP T = total positive end expiratory
reasons for mechanical ventilation were postoperative pressure: alveolar pressure at the end
respiratory failure, coma and pneumonia. This interna- of expiration and is the sum of PEEP
tional report did not include data from Australia and applied by the ventilator and any
New Zealand. A study describing ventilation and weaning intrinsic (auto) PEEP.
practices of 55 ICUs in Australia and New Zealand in Notes: V T /Cr: describes the elastic properties the
2005 reported a similar profile for the most common respiratory system
indications for mechanical ventilation. 66 V T /T I : reflects flow in the system
V T /T I × R: resistance of the respiratory
system.
PRINCIPLES OF MECHANICAL VENTILATION
Mechanical ventilation describes the application of posi-
tive or negative pressure breaths using non-invasive or Compliance and Elastance
invasive techniques. Indications for initiation of mechan- Compliance refers to the ease with which lung units
ical ventilation are discussed below. Table 15.1 lists the distend. Elastance is the tendency of the lung units to
patient parameters typically observed in acute and chronic return to their original form once stretched. Compliance
respiratory failure that may be influential in the decision is defined as the change in volume that occurs due to a
to ventilate. During positive pressure ventilation, the type change in pressure.
of ventilation used most commonly in critical care, the
∆
ventilator delivers a flow of gas into the lungs during C = ∆ V/ P
inspiration using a pneumatic system. Expiration is
passive. Lung tissue and the surrounding thoracic structures
contribute to respiratory compliance. Normal compli-
The Equation of Motion ance for a mechanically ventilated patient ranges from
The equation of motion for the respiratory system is a 35–50 mL/cmH 2 O. 68
mathematical model that relates pressure volume and
flow during the delivery of a breath, with the pressure Resistance
required to deliver a volume of gas to the lungs deter- Resistance refers to the forces that oppose airflow. Resis-
mined by the elastic and resistive properties of the respi- tance in the airways is affected by the diameter and length
67
ratory system (see Table 15.2). of the airways, including the artificial airway, the gas flow