Page 430 - Clinical Small Animal Internal Medicine

398 Section 5 Critical Care Medicine

The thorax is next auscultated bilaterally to ensure breath FiO 2 until it is <60%. Once FiO 2 is less than 60%,
VetBooks.ir sounds are audible. Next, other monitoring parameters emphasis shifts to reducing PEEP and peak airway
pressure. If the PaO 2 is lower than desired (<80 mmHg),
are evaluated (e.g., blood pressure, ECG, pulse oximeter,
ETCO 2 , etc.). Once the patient appears stable, serial arte-
acute and severe drop in PaO 2 , placing the animal on
rial blood gas analysis is ideal for accurate titration of ven- the first option is to increase the FiO 2 . If there is an
tilator settings. If an arterial blood sample is not available, 100% oxygen is appropriate until the issue can be
a venous blood gas can be used to assess PCO 2 if cardiac resolved and more definitive therapy can be provided.
output is not excessively low. The evaluation of oxygena- Ultimately, it is hoped that manipulation of ventilator
tion by pulse oximetry is the standard of care in some set- settings will increase the oxygenating efficiency of the
tings (e.g., human neonatology) and may serve as both a lung and allow lowering of the FiO 2 once again. All
noninvasive alternative and a means of reducing iatro- increases in oxygenation are obtained by raising FiO 2 ,
genic blood loss. mean airway pressure, or both. Mean airway pressure
may be increased by increasing peak pressure, respira-
tory rate, PEEP, inspiratory time, or the format of flow
Goals of Mechanical Ventilation
delivery (e.g., block versus sine versus descending
The goal of IPPV therapy is to maintain acceptable ramp flow patterns). Acute hypoxemic episodes are
blood gas values with the lowest level of ventilator sup- life‐threatening complications that require immediate
port possible. The optimal ventilator settings for each intervention.
individual patient cannot be predicted and are deter-
mined through a careful process of trial and error. The
patient should be fully evaluated after every change in Adjustments to Improve PaCO 2
ventilator settings, including blood gas analysis when- The PaCO 2 is dependent on alveolar minute ventila-
ever possible. It is advisable to make only one change in tion, which is the product of the effective tidal volume
ventilator settings at a time in order to accurately evalu- (excluding dead space volume) and the respiratory rate.
ate what effect each change alone has on gas exchange. If the PaCO 2 is higher than the desired range then the
Whenever feasible, it is advised to allow 15 minutes to respiratory rate, tidal volume, or both should be
pass before reevaluating PaCO 2 . Whole‐body stores of increased. If the PaCO 2 is too low, the respiratory rate
carbon dioxide are quite large and a quarter of an hour and/or tidal volume should be decreased. Excessive
or more may be required for a new steady state to be gas trapping may lead to carbon dioxide retention.
reached. Common blood gas goals of mechanical venti- Increasing expiratory time or reducing I:E ratio may
lation include a PaO 2 of 80–120 mmHg (or SpO 2 >95%) increase alveolar ventilation in this circumstance. In
and a PaCO 2 of 35–55 mmHg (35–40 mmHg in patients patients with lower airway or parenchymal disease,
with intracranial hypertension). inspiratory times that are too short may lead to mini-
In some settings, permissive hypercapnia may be mal ventilation of alveoli with high time constants
employed. In this approach, PaCO 2 is allowed to climb (product of resistance and compliance). In this instance,
higher to 60–70 mmHg as long as the resultant acidemia prolongation of the inspiratory time may enhance alve-
does not become too severe (pH <7.1). This approach is olar ventilation.
often taken when the clinician fears that maintaining An abrupt increase in PaCO 2 in the ventilator patient
carbon dioxide tensions within the normal target range may indicate a life‐threatening complication. Similarly,
will require injurious ventilator settings. Permissive an abrupt drop in end‐tidal carbon dioxide may reflect a
hypoxemia approaches may also be employed at times. sudden increase in physiologic dead space (e.g., pulmo-
With this approach, the clinician focuses on maintaining nary thromboembolism) or an abrupt reduction in car-
central venous hemoglobin saturations (S CV O 2 ) above diac output.
65–68%. Hypoxemia (PaO 2 <80 mmHg) is tolerated as
long as tissue oxygen delivery appears adequate based on
S CV O 2 readings. This approach reflects a school of Nursing Care for Ventilator Patients
thought in which it is felt that clinicians should focus
more on maintaining adequate global oxygen delivery Nursing care of the ventilator patient is one of the most
rather than arterial blood gas values. labor‐intensive and important aspects of ventilator
patient management. Each ventilator patient usually
requires a dedicated caregiver and two or more people
Adjustments to Improve PaO 2
are needed for many of the patient care procedures.
If the PaO 2 is higher than the desired range (>120– Nursing care includes airway management, oral and eye
150 mmHg), the first priority should be to decrease the care, catheter care, and recumbent patient care.