Arousal from sleep restores alveolar ventilation
to a variable degree, while resumption of sleep reduces ventilation in a cyclical fashion.
Altered breathing patterns may disrupt the coupling of [f.sub.R] to body movement, so-called 'entrained breathing' which is characteristic of some activities, preventing VE from reaching a mechanical optimum, and limiting alveolar ventilation
. (18) Also, the additional mass and dimensions of a mask and flow sensor system may alter body movement and exercise economy.
Therefore, the transition from one of these states to another involves a shift in the neural and neuromuscular mechanisms that maintain alveolar ventilation
. This modulation includes alterations in the complex feedback loops and reflexes that monitor the demands of the organism and control ventilation to meet its metabolic needs.
The term "noninvasive ventilation" refers to a method of achieving alveolar ventilation
without endotracheal intubation or other invasive respiratory techniques, such as tracheotomy (Hill, 1993).
A pulse oximeter assesses capillary oxygen saturation, but arterial blood gas (ABG) measurement is the best way to assess alveolar ventilation
. Hypoventilation produces an increase in carbon dioxide in the arterial blood ([PaCO.sub.2]), and hyperventilation causes the [PaCO.sub.2] to decrease.
Arterial blood gas determinations of PCO, provide the most accurate determinations of the adequacy of alveolar ventilation
. The [P.sub.a][co.sub.2], concentration in a given patient reflects the balance between metabolic production of [CO.sub.2], and excretion by ventilation.
Hypercapnia occurs commonly in modern critical care practice and is usually due to reduced elimination as an effect of decreased alveolar ventilation
. Decreased alveolar ventilation
may be due to a reduction in minute ventilation or increased dead space ventilation.
Using on-line, breath-by-breath analysis of lung function, the ventilator is driven by a programmed computer to provide optimal alveolar ventilation
, according to the patient's changing requirements (Fig.
Ventilator dyssynchrony can cause increased intrathoracic pressure, decreased alveolar ventilation
, and increased work of breathing for the patient.
Of the studied physiologic factors, the blood:air partition coefficient and alveolar ventilation
were most significant in determining the respiratory uptake (p < 0.001 for each).
The NICO 2 provides a standard, expired single-breath C[O.sub.2] waveform that can be divided into three phases (Figure 1); Phase I represents C[O.sub.2]-free gas expired from the large airway and tracheal tube dead space, Phase H represents a mixture of gas from both the large airway dead space and alveolar dead space and Phase III (or alveolar plateau) represents alveolar ventilation
They likewise stimulate increase alveolar ventilation
and acute respiratory alkalosis.
Assuming the anatomical deadspace in intubated patients to be approximately 70 ml (12), the reduction of alveolar ventilation
during PC ventilation would be close to 40% which could be expected to increase the [P.sub.a] C[O.sub.2] by close to 50%13.