Using blood gases in practice (Proceedings)


Using blood gases in practice (Proceedings)

Nov 01, 2009

Blood gas, electrolyte and lactate analysis are very useful in management of the ill or injured dog or cat. Knowledge of normal values and what they indicate can help improve patient care and understanding of the pathophysiological changes accompanying critical illness.

Blood gas analysis

Blood gas analysis is a specialized method of evaluating the acid base and oxygenation indices in the blood stream. Blood gas analysis helps the clinician to better care for the individual patient through careful titration of fluids and supplemental oxygen AND imparts a better knowledge of underlying pathophysiology accompanying various diseases. Blood gases can be measured on venous or arterial blood. In fact, gas concentrations can be evaluated in any fluid that is run through a blood gas analyzer.

Historically, blood gas analysis has only been available at large referral or university hospitals, but in the last decade a variety of portable point-of care (POC) machines have become affordable and available in practices. The ISTAT (Heska-Fort Collins CO) model has been widely used and is generally considered accurate and user-friendly; the IRMA (ITC, Edison, NJ) is also popular. These devices (and other similar products) are able to measure blood gas values and other chemistries on small volumes of blood. Disposable cartridges make each test relatively economical and the turn-around time is excellent. Larger veterinary hospitals often used the Nova Biomedical Analyzer ( Nova- Waltham, MA). Other alternatives may include local human hospital or regional veterinary laboratories.

However, practically the only way an individual practitioner (or hospital)will be able to gain comfort in interpretation of blood gases is through frequent analysis of samples from a variety of patients. This is much less likely to be done if a machine is not available in-house. Most practice managers and clinicians rapidly determine that despite the initial purchase cost, the benefits of better patient care and improved revenue are quickly realized. Additionally, patient care is often optimized and there appears to be improved job satisfaction!

Blood gas samples can be either from a central vein (or rarely the pulmonary artery) or a peripheral vein. Arterial samples are typically collected from the dorsal pedal artery or the femoral artery. Samples may also be collected from indwelling arterial or venous catheters. In healthy pets, there is little variation from samples collected from various venous sites, but in patients with cardiovascular compromise the differences can be outstanding (eg. And clinically significant!) . (due to differences in oxygen extraction).

Arterial samples will "gush" from the catheter!

Using blood gas analysis
The first area in which blood gases are useful are in the assessment of acid-base balance. The following guidelines have been adapted from the references and serve as the recommendation for interpretation from the American College of Veterinary Emergency and Critical Care.

Normal values
pH= 7.36-7.44
PC02= 36-40mmHg
HCO3=20-24 mEq/l
BE = ± 4
PO2= 90-100 mmHg

Disturbance                              Clinical guidelines for compensation
Metabolic Acidosis                      Each 1 mEq/L decrease in HCO3 will decrease PCO2 by 0.7 mmHg
Metabolic Alkalosis                      Each 1 mEq/L increase in HCO3 will increase PCO2 by 0.7 mmHg

Respiratory ccidosis
Acute: Each 1 mmHg increase in PCO2 will increase HCO3 by 0.15 mEq/l
      Chronic: Each 1 mmHg increase in PCO2 will increase HCO3 by 0.35 mEq/l