Page 354 - ACCCN's Critical Care Nursing

Respiratory Assessment and Monitoring 331

Alveolar Ventilation lungs through to the blood in the adjacent alveolar capil-
Minute volume (MV), often referred to during mechani- lary networks. Similarly, carbon dioxide diffuses from
cal ventilation, is TV multiplied by respiratory frequency capillaries to the alveoli and is then expired.
(e.g. 500 mL × 12 breaths per minute = 6000 mL MV). Oxygen Transport
Importantly, only the first 350 mL of inhaled air in each
breath reaches the alveolar exchange surface, with 150 mL In oxygenated blood transported by the pulmonary capil-
remaining in the conducting airways (called the ‘ana- laries, there is 20 mL of oxygen in each 100 mL of blood.
tomic dead space’). Alveolar ventilation is the amount of Oxygen is transported in two ways; dissolved in plasma
inhaled air that reaches the alveoli each minute (e.g. (about 0.3 mL; 1.5%) with the remainder bound to hae-
8
350 mL × 12 = 4200 mL of alveolar ventilation). 8 moglobin. The 1.5% of oxygen dissolved in the blood is
what constitutes PaO 2 and measured by arterial blood
4
WORK OF BREATHING gases. One gram of haemoglobin carries 1.34 mL oxygen,
and the level of saturation within the total circulating
In a resting state, energy requirements to breathe is haemoglobin can be measured clinically, commonly by
7
minimal (less than 5% of total O 2 consumption). pulse oximetry. The amount of oxygen actually bound to
However, changes in airway resistance and lung compli- haemoglobin compared with the amount of oxygen the
ance affect the work of breathing (WOB), resulting in haemoglobin can carry is commonly reported as SaO 2 .
13
increased oxygen consumption (VO 2 ). As noted earlier, Oxygen is attached to the haemoglobin molecule at four
the lungs are very distensible and expand during inspira- haem sites. As the majority of oxygen transport is via
tion. This expansion is called the elastic or compliance haemoglobin, if all four sites are occupied with oxygen
work and refers to the ease by which lungs expand under molecules the blood is determined to be ‘fully saturated’
pressure. Lung compliance is often monitored when (SaO 2 = 100%). 14
patients are mechanically ventilated, and is calculated by
dividing the change in lung volume by the change in trans- A large reserve of oxygen is available if required, without
3
pulmonary pressure. For the lung to expand, it must the need for any increase in respiratory or cardiac work-
overcome lung viscosity and chest wall tissue (called load. Oxygen extraction is the percentage of oxygen
‘tissue resistance work’). Finally, there is airway resistance extracted and utilised by the tissues. At rest, just 25% of
work – movement of air into the lungs via the airways. the total oxygen delivered to the tissue is extracted,
The work associated with resistance and compliance is although this amount does vary throughout the body,
easily overcome in healthy individuals but in pulmonary with some tissue beds extracting more and others taking
disease, both resistance and compliance work is less. Normally, the oxygen saturation of venous blood is
increased. 3,14 During exertion, when increased muscle 60–75%; values below this indicate that more oxygen
function heightens metabolic rate, oxygen demand rises than normal is being extracted by tissues. This can be due
to match consumption and avoid anaerobic metabolism, to a reduction in oxygen delivery to the tissues, or to an
8,9
and work of breathing is increased. The term ‘work of increase in the tissue consumption of oxygen.
breathing’ is often used in those who are critically ill, when Oxygen delivery (DO 2 ) and oxygen consumption (VO 2 )
basic respiratory processes are challenged and breathing are important aspects to consider in the management of
consumes a far greater proportion of total energy. a critically ill patient. Normal oxygen delivery in a healthy
person at rest is approximately 1000 mL/min. Normal
9
PRINCIPLES OF GAS TRANSPORT AND oxygen consumption is 200–250 mL/min, but this can
EXCHANGE IN ALVEOLI AND TISSUES increase significantly during episodes of sepsis, fever,
hypercatabolism and shivering. The difference between
14
Oxygen and carbon dioxide is transported in the blood- normal delivery and normal consumption highlights the
stream between the alveoli and the tissue cells by the large degree of oxygen reserve available to the body.
cardiac output. Delivery of oxygen to tissues and transfer
of carbon dioxide from the tissues to the capillary occurs Oxygen–Haemoglobin Dissociation Curve
by diffusion and is therefore dependent on the pressure
gradient between the capillary and the cell. Diffusion As blood is transported to the tissues and end-organs, the
involves molecules moving from areas of high concentra- affinity of haemoglobin and oxygen to combine decreases,
tion to low concentration. Other determinants of the rate relative to the surrounding arterial oxygen tension. This
of diffusion include the thickness of the alveolar mem- relationship is illustrated by the oxyhaemoglobin disso-
brane, the amount of surface area of the membrane avail- ciation curve (see Figure 13.9). As oxygen is offloaded at
able for gas transfer and the inherent solubility of the gas. the tissue level, carbon dioxide binds more readily with
Carbon dioxide diffuses about 20 times more rapidly haemoglobin, to be transported back to the lungs for
4
than oxygen because of the much higher solubility of removal.
7
carbon dioxide in blood. At the most distal ends of the In the upper part of the curve (within the lungs), relatively
conducting airways lies an extensive network of approxi- large changes in the PaO 2 cause only small changes in
mately 300 million alveoli. The surface area of the lungs haemoglobin saturation. Therefore, if the PaO 2 drops
2
if spread out flat is about 90 m – about 40 times greater from 100 to 60 mmHg (14–8 kPa), the saturation of hae-
4
than the surface of the skin. Gas exchange occurs through moglobin changes only 7% (from a normal 97%
the exceptionally thin alveolar membranes. Oxygen to 90%). The lower portion (steep component) of the
uptake takes place from the external environment via the oxygen–haemoglobin dissociation curve, when PaO 2 is