A NOVEL DIAGRAM FOR THE INTERPRETATION OF ACID-BASE DISORDERS IN CRITICAL ILL PATIENTS.
ABSTRACT
BACKGROUND: Although acid base disturbances are common clinical problems especially in the ICU environment, comparison studies between the various acid-base diagnostic methods have not been contacted yet.
OBJECTIVES: To propose a new pH-logPaCO2 diagram and to investigate its diagnostic efficacy in comparison to various diagrammatic approaches in the diagnosis of acid-base disorders.
METHODS: Two thousands, one hundred and forty three arterial blood gas samples were drawn from 75 critical ill surgical and trauma patients and interpreted with: 1) The new, proposed, pH-PaCO2 diagram, 2) the “Grogono” diagram, 3) The “Siggaard-Andersen” (S-A) chart, 4) The “Behrakis” diagram and 5) One physician with more than 20 years of experience in ICU, considered expert in acid-base disorders. Sixty-seven patients were submitted to general, cardio-thoracic and/or trauma surgery, while twenty-eight patients presented multiple trauma. Statistical analysis included AC1 test which is considered as a more powerful statistical tool and an improvement of K-statistics.
RESULTS: 1) The new diagram presents the highest diagnostic agreement with the other methods (AC1=0.58 - 0.76). 2) The diagnostic agreement between the “Grogono” diagram and the physician (AC1=0.61) is the highest among the comparisons of the “Grogono” diagram with the other diagnostic methods, except the “new” diagram. 3) The diagnostic agreement between the “Grogono” diagram and the “S-A” chart is extremely low (AC1=0.47). 4) The “Behrakis” diagram shows low to moderate (AC1=0.48 - 0.57) diagnostic agreement with the other diagnostic methods.
CONCLUSIONS: The main conclusions were: 1) the various acid-base diagrams present inadequate diagnostic efficacy, 2) the “Grogono” diagram, although superior to the other previous diagnostic methods, cannot be safely used for the diagnosis of acid-base balance disorders. 3) The “new” proposed diagram presents the higher diagnostic agreement with the other methods and the physician and can be a reliable and useful diagnostic technique for the diagnosis of acid-base disorders in the every day clinical practice.
BACKGROUND
Acid base disturbances are common clinical problems especially in the ICU environment(1,2). In such troublesome conditions, where integrated interpretation is important, skilled diagnostic approach becomes necessary(3).
In 1931 Hastings and Steinhaus described a tri-axial chart for studying acid-base displacement changes. Henderson-Hasselbalch, at the beginning of 20th century(4), expressed the quantitative relationship between the acid-base parameters. Such a relationship can be shown graphically by a nomogram. Some of the earliest studies that attempted to describe acid-base nomograms were performed in the mid 50s by Davenport and in the late 60s by R.W. Winters(5) who introduced bands which determined each acid-base disturbance on the quantitative description of the primary and compensatory mechanism. Later, in the 70’s, Siggaard-Andersen described a two-axial pH-logPCO2 chart (S-A chart) based on Henderson-Hasselbalch equation(6). Recently, A.W. Grogono(7) introduced an acid-base diagram which was derived from redrawing the “S-A chart” on the two clinical axes, respiratory (PCO2) and metabolic (BE, Base Excess).
It is surprising that comparison studies between various acid-base diagnostic methods have not been contacted yet. The aim of this study was to develop and describe a new diagram(8) focused especially on simple and mixed acid-base disorders and to investigate its diagnostic efficacy in comparison to various diagrammatic approaches.
METHODS
The new diagram
The new diagram is two-dimensional (Figure 1) and was designed for redrawing the “S-A chart” on the clinical axis of pH indicated on the abscissa and logPaCO2 on the ordinate. It includes 17 zones corresponding to simple and mixed respiratory and metabolic disturbances. The zones were obtained from the graphical representation of the most accurate and reliable equations(9), as zones published by different authors vary considerably. The definitions of acid-base conditions are classified as the following:
1. Normal condition: This area indicates the acid-base values for normal resting subjects(10). Any point in this area has values ranging from 7.35 to 7.45 for pH and between 35 mmHg to 45 mmHg for PaCO2. The center of the area represents the ideal point corresponding to pH=7.4 and PaCO2=40 mmHg values. Noteworthy, sometimes the presence of two opposing primary acid-base disturbances may result in little or no deviation of the blood pH from normal.
2. Uncompensated respiratory acidosis: This term is identical to “acute respiratory acidosis” and “acute hypercapnia”(11). The zone represents a simple acid-base disturbance characterized of an increased PaCO2>45 mmHg and a decreased plasma pH<7.35 value. The design of this zone is based on the graphical solution of equations proposed by Van-Slyke(12), S-A(13) and Brackett(11).
3. Partially compensated respiratory acidosis: The term describes a mixed acid base disturbance and includes the action of incomplete renal compensatory response. Any point is this zone is characterized by an increased PaCO2> 45 mmHg and a pH value higher than that defined for uncompensated but lower than that corresponding to maximal compensated respiratory acidosis for the particular PaCO2 value.
4. Maximal compensated respiratory acidosis: This term is identical to “chronic respiratory acidosis” and “chronic hypercapnia” terms, as, renal compensation has been completed several days after induction of hypercapnia(5). The zone constitutes a mixed acid-base disturbance and is characterized by an increased PaCO2>45 mmHg value and a pH value which tends to but not exceed 7.40. The design of this zone is based on the graphical solution of equation proposed by Engel(5) and Swartch(14).
5. Respiratory acidosis and metabolic alkalosis: This zone indicates a mixed acid-base disturbance which is characterized of an increased PaCO2>45 mmHg and is associated with a pH value higher than that defined for the maximal compensated respiratory acidosis but lower than that corresponding to maximal compensated metabolic alkalosis for the particular PaCO2 value(15,16).
6. Maximal compensated metabolic alkalosis: This term is identical to “chronic metabolic alkalosis” as respiratory compensatory response has been completed. The zone indicates a mixed acid-base disturbance which is characterized of an increased pH>7.45 value and is associated with a PaCO2 value ranging between 45-60 mmHg(15). The sketch of this zone is based on graphical solution of equations proposed by Van-Slyke(12), S-A(13) and Javaheri(15).
7. Partially compensated metabolic alkalosis: The term describes a mixed acid base disturbance and includes the action of incomplete respiratory compensatory response. The zone is characterized of an increased PaCO2>45 mmHg and is associated with an increased pH value, higher than that defined for maximal compensated metabolic alkalosis for the particular PaCO2 value.
8. Uncompensated metabolic alkalosis(15,16): The zone indicates an acute acid-base disturbance which is characterized by an increased plasma pH>7.45 value and is associated with an unaltered PaCO2 value (35 mmHg <PaCO2< 45 mmHg). Acute, in this term means less than an hour duration, before the respiratory compensation begins. In this zone real patients rarely maybe found.
9. Combined respiratory and metabolic alkalosis: The triangular zone represents a combined alkalosis in which both respiratory (hypocapnia) and metabolic alkalosis coexist. Any point in this zone is characterized of a decreased PaCO2<35 mmHg value and is associated with a pH value higher than that defined for uncompensated respiratory alkalosis for the particular PaCO2 value.
10. Uncompensated respiratory alkalosis: This term is identical to «acute respiratory alkalosis» and «acute hypocapnia». The zone constitutes a simple acid-base disturbance and is characterized by a decreased PaCO2<35 mmHg and an increased plasma pH>7.45 value. The design of this zone is based on the graphical solution of equations proposed by Arbus(17,18).
11. Partially compensated respiratory alkalosis: The term describes a mixed acid base disturbance and includes the action of incomplete renal compensatory response. Any point in this zone is characterized by a decreased PaCO2< 35 mmHg and a pH value lower than that defined for uncompensated and higher than that corresponding to maximal compensated respiratory alkalosis for the particular PaCO2 value.
12. Maximal compensated respiratory alkalosis: This term is identical to “chronic respiratory alkalosis” and “chronic hypocapnia” terms, as, renal compensation has been completed several days after induction of hypocapnia(18). The zone consists a mixed acid-base disturbance and is characterized of a decreased PaCO2<35 mmHg value associated with a pH value which closes to but is higher than 7.40. The design of this zone is based on the graphical solution of a equation proposed by Schlichtig(7).
13. Respiratory alkalosis(18) and metabolic acidosis: This zone indicates a mixed acid-base disturbance which is characterized of a decreased PaCO2<35 mmHg and is associated with a pH value lower than that defined by the maximal compensated respiratory alkalosis but higher than that corresponding to maximal compensated metabolic acidosis for the particular PaCO2 value.
14. Maximal compensated metabolic acidosis: This term is identical to “chronic metabolic acidosis” as respiratory compensatory response has been completed. The zone represents a mixed acid-base disturbance characterized by a decreased pH<7.4 value associated with a decreased PaCO2<35 mmHg value. The design of this zone is based on the graphical solution of equations proposed by Pierce(19).
15. Partially compensated metabolic acidosis: The term describes a mixed acid base disturbance and includes the action of incomplete respiratory compensatory response. The zone is characterized of a decreased PaCO2<35 mmHg and is associated with a decreased pH value, lower than that defined for maximal compensated metabolic acidosis for the particular PaCO2 value.
16. Uncompensated metabolic acidosis: The zone indicates an acute acid-base disturbance which is characterized by a decreased plasma pH<7.35 value and is associated with an unaltered PaCO2 value (35 mmHg <PaCO2< 45 mmHg). Acute, in this term means less than an hour duration, before the respiratory compensation begins. In this zone real patients rarely maybe found.
17. Combined respiratory and metabolic acidosis: The triangular zone indicates a combined acidosis in which respiratory (hypercapnia)(20) and metabolic acidosis(19) co-exist. Any point in the zone is characterized by an increased PaCO2>45 mmHg value and is associated with a pH value lower than that defined for uncompensated respiratory acidosis for the particular PaCO2 value.
Clinical Application of the proposed diagram
For the clinical application of the diagram 2143 arterial blood samples of a total of 75 critical ill patients were studied over a six month period. All patients were hospitalized in the I.C.U for a mean duration of 11.2±4.6 days (5-41 days). The main causes for admission of the patients to the ICU are listed in table 1. Sixty-seven patients were submitted to general(21,22), cardio-thoracic(23,24) and/or trauma surgery(25,26) (Table 2), while twenty-eight patients presented multiple trauma(27,28,29,30,31) (Table 3). The study was conducted in Athens, in the General adult I.C.U of Red Cross Hospital and in “Evagelismos Adult Cardiac Surgery I.C.U”.
The conventional technique of arterial blood sampling was performed(32,33) and a code number was assigned to each sample. A Radiometer-Copenhagenâ ABL 700 analyzer was used for pH, PaCO2 and PaO2 measurements(34,35,36). The [HCO3-], st[HCO3-], ABE, SBE, SaO2 and O2cont values were calculated(9). Blood gases drawn at temperatures above normal were corrected for temperature (37o C)(37).
The laboratory data (pair PaCO2 and pH values) were plotted on the diagram and the acid-base status was readily evaluated with reference to simple and mixed acid-base disturbances. The diagnostic efficacy of the diagram was compared with the “S-A chart”(6), the Grogono diagram(7), the “Behrakis” diagram(8,38) and an physician with more than 20 years of experience in ICU, considered to be an expert in acid-base disorders.
Statistical Analysis
Direct comparisons of the agreement between all the diagnostic methods were performed. However, these comparisons are not biased since there does not exist a “gold standard” among the diagnostic methods being compared. Statistical analysis included AC1 test(39) which is considered as a more powerful statistical tool and an improvement of K-statistics. When the AC1-coefficient is lower than 0.5, there is no statistical significant agreement between the diagnostic methods. When the AC1-coefficient value range between 0.5-0.75, there is moderate agreement, while when it is greater than 0.75, the agreement is considered high.
RESULTS
Table 4 lists the AC1-coefficients of all possible comparisons between the methods used for the diagnostic evaluation of all arterial blood gas samples in this study. An initial and principal remark regarding the data presented in this table is that AC1-coefficients between the “Grogono” diagram and all other methods, although higher than all the rest except the “new” diagram, range from (0.5-0.61). Interestingly, the lower agreement ratio of the “Grogono” diagram is that with the “S-A chart” (AC1=0.47) and the higher that with the expert physician (AC1=0.61).
AC1-coefficients of diagnostic agreement between “S-A chart” and the other methods are generally low (0.34-0-58). It should be stressed that “S-A chart” shows higher agreement with the “new” diagram (AC1=0.58). Moreover, it is noted that the “S-A chart” presents the lower AC1-coefficient with the expert physician (AC1=0.34) compared to the respective coefficients with the “new” diagram, the “Grogono” diagram and the “Behrakis” diagram (AC1=0.58, 0.47 and 0.52 respectively). In addition, there was a significant difference in the diagnostic agreement between the physician and the “S-A chart” (AC1=0.34) and between the physician and the “Behrakis” diagram (AC1=0.48), while the highest diagnostic agreement of the physician was recorded with the “new” diagram (AC1=0.64). Table 4 also records the AC1-coefficients between each one of the three study methods and the physician combined them for all the arterial blood gas samples. Relevant data point out AC1-coefficients between the “new” diagram, the “Grogono” diagram and the “Behrakis” diagram and that of the expert physician’s (AC1=0.34-0.76). On the other hand, the “S-A chart” has extremely low AC1-coefficients combined that hardly exceeds 0.58 and are significantly different from the respective AC1-coefficients of the “new”, “Grogono” and “Behrakis” diagrams.
DISCUSSION
Acid-base disturbances remain a common problem frequently encountered in the intensive care medicine. One of the earliest tools for addressing diagnosis in acid-base disturbances is that of acid-base mapping which even in its simplest form help in elucidating the nature of these disturbances(40). Currently, there are not available studies to highlight the efficacy of acid-base diagrams in assessing arterial blood gases disturbances.
Acid-base charts have been widely used in the management of patients with acid-base disturbances. The main role of these charts(35) is to support ICU personnel in their interpretation process(40,41). A drawback of such charts is that the diagnostic zones vary considerably, are not universally valid and cannot be considered as true reference indexes. This study was designed to: 1) answer the following question: “How effective are diagrams in assessing acid-base disturbances?” and 2) to propose a new diagnostic diagram and assess the efficacy among various diagnostic methods and experienced physicians. The results obtained from our study suggest that the new diagram is the most reliable diagnostic tool (AC1=0.57-0.76), while the “S-A chart” constitutes the method with the lowest diagnostic potential (AC1=0.34-0.58).